KEYBOARDTYPE=“pc”
KEYTABLE=“us”
| | We will discuss networking files in this directory in the networking chapter**.** **data** directories ### /home Users can store personal or project data under **/home.** It is common **(**but not mandatory by the fhs**)** practice to name the users home directory after the user name in the format **/home/ $USERNAME.** For example**:** Besides giving every user **(**or every project or group**)** a location to store personal files**,** the home directory of a user also serves as a location to store the user profile**.** A typical Unix user profile contains many hidden files **(**files whose file name starts with a dot**).** The hidden files of the Unix user profiles contain settings specific for that user**.** user**@Predator-pc::\~$** ls **-**d **/**home**/**user**/.\*** **/**home**/**user**/.** **/**home**/**user**/.**bash**\_**profile **/**home**/**user**/.**ssh **/**home**/**user**/..** **/**home**/**user**/.**bashrc **/**home**/**user**/.**viminfo **/**home**/**user**/.**bash**\_**history **/**home**/**user**/.**lesshst ### /root On many systems **/root** is the default location for personal data and profile of the **root user.** If it does not exist by default**,** then some administrators create it**.** ### /srv You may use **/srv** for data that is **served by your system.** The FHS allows locating cvs**,** rsync**,** ftp and www data in this location**.** The FHS also approves administrative naming in **/** srv**,** like **/**srv**/**project55**/**ftp and **/**srv**/**sales**/**www**.** On Sun Solaris **(**or Oracle Solaris**)** **/export** is used for this purpose**.** ### /media The **/media** directory serves as a mount point for **removable media devices** such as CDROM**’**s**,** digital cameras**,** and various usb**-**attached devices**.** Since **/media** is rather new in the Unix world**,** you could very well encounter systems running without this directory**.** Solaris 9 does not have it**,** Solaris 10 does**.** Most Linux distributions today mount all removable media in **/media.** user\@predator:\~$ ls /media/ cdrom cdrom0 usbdisk ### /mnt The **/mnt** directory should be empty and should only be used for temporary mount points **(**according to the FHS**).** Unix and Linux administrators used to create many directories here**,** like **/**mnt**/**something**/.** You likely will encounter many systems with more than one directory created and**/**or mounted inside **/mnt** to be used for various local and remote filesystems**.** ### /tmp Applications and users should use **/tmp** to store temporary data when needed**.** Data stored in **/tmp** may use either disk space or RAM**.** Both of which are managed by the operating system**.** Never use **/tmp** to store data that is important or which you wish to archive**.** ### in memory directories ### /dev Device files in **/dev** appear to be ordinary files but are not actually located on the hard disk**.** The **/dev** directory is populated with files as the kernel is recognising hardware**.** **common physical devices** Common hardware such as hard disk devices are represented by device files in **/dev.** Below a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop**.** **(**The detailed meaning of these devices will be discussed later**.)** **#** **#** SATA or SCSI or USB **#** user**@**predator**-**pc**\~$** ls **/**dev**/**sd**\*** **/**dev**/**sda **/**dev**/**sda1 **/**dev**/**sda2 **/**dev**/**sda3 **/**dev**/**sdb **/**dev**/**sdb1 **/**dev**/**sdb2 **#** **#** IDE or ATAPI **#** user**@**barry**:\~$** ls **/**dev**/**hd**\*** **/**dev**/**hda **/**dev**/**hda1 **/**dev**/**hda2 **/**dev**/**hdb **/**dev**/**hdb1 **/**dev**/**hdb2 **/**dev**/**hdc Besides representing physical hardware**,** some device files are special**.** These special devices can be very useful**.** ### /dev/tty and /dev/pts For example**,** **/dev/tty1** represents a terminal or console attached to the system**.** **(**Don**’**t break your head on the exact terminology of **‘**terminal**’** or **‘**console**’,** what we mean here is a command line interface**.)** When typing commands in a terminal that is part of a graphical interface like Gnome or KDE**,** then your terminal will be represented as **/dev/pts/1** **(**1 can be another number**).** ### /dev/null On Linux you will find other special devices such as **/dev/null** which can be considered a black hole**;** it has unlimited storage**,** but nothing can be retrieved from it**.** Technically speaking**,** anything written to **/**dev**/**null will be discarded**.** **/**dev**/**null can be useful to discard unwanted output from commands**.** ***/**dev**/**null is not a good location to store your backups **;-)*****.** ### /proc conversation with the kernel **/proc** is another special directory**,** appearing to be ordinary files**,** but not taking up disk space**.** It is actually a view of the kernel**,** or better**,** what the kernel manages**,** and is a means to interact with it directly**.** **/proc** is a proc filesystem**.** **user\@Predator-pc:\~$** mount **-**t proc none on /proc type proc (rw) When listing the **/**proc directory you will see many numbers **(**on any Unix**)** and some interesting files **(**on Linux**)** user**@**predator**-**pc**\~$** ls **/**proc 1 2339 4724 5418 6587 7201 cmdline mounts 10175 2523 4729 5421 6596 7204 cpuinfo mtrr 10211 2783 4741 5658 6599 7206 crypto net 10239 2975 4873 5661 6638 7214 devices pagetypeinfo 141 29775 4874 5665 6652 7216 diskstats partitions 15045 29792 4878 5927 6719 7218 dma sched**\_**debug 1519 2997 4879 6 6736 7223 driver scsi 1548 3 4881 6032 6737 7224 execdomains self 1551 30228 4882 6033 6755 7227 fb slabinfo 1554 3069 5 6145 6762 7260 filesystems stat 1557 31422 5073 6298 6774 7267 fs swaps 1606 3149 5147 6414 6816 7275 ide sys 1. 31507 5203 6418 6991 7282 interrupts sysrq**-**trigger 2. 3189 5206 6419 6993 7298 iomem sysvipc 3. 3193 5228 6420 6996 7319 ioports timer**\_**list 18898 3246 5272 6421 7157 7330 irq timer**\_**stats 19799 3248 5291 6422 7163 7345 kallsyms tty 1. 3253 5294 6423 7164 7513 kcore uptime 2. 3372 5356 6424 7171 7525 key**-**users version 1987 4 5370 6425 7175 7529 kmsg version**\_**signature 1989 42 5379 6426 7188 9964 loadavg vmcore 2 45 5380 6430 7189 acpi locks vmnet 20845 4542 5412 6450 7191 asound meminfo vmstat 221 46 5414 6551 7192 buddyinfo misc zoneinfo 2338 4704 5416 6568 7199 bus modules Let**’**s investigate the file properties inside **/proc.** Looking at the date and time will display the current date and time showing the files are constantly updated **(**a view on the kernel**).** **user\@Predator-pc:\~$** date Mon Jan 29 18**:**06**:**32 EST 2007 **user\@Predator-pc:\~$** ls **-**al **/**proc**/**cpuinfo **-**r**--**r**--**r**--** 1 root root 0 Jan 29 18**:**06 **/**proc**/**cpuinfo **user\@Predator-pc:\~$** **user\@Predatorpc:\~$** **...**time passes**...** **user\@Predator-pc:\~$** **user\@Predatorpc:\~$** date Mon Jan 29 18**:**10**:**00 EST 2007 **user\@Predator-pc:\~$** ls **-**al **/**proc**/**cpuinfo **-**r**--**r**--**r**--** 1 root root 0 Jan 29 18**:**10 **/**proc**/**cpuinfo Most files in **/**proc are 0 bytes**,** yet they contain data**--**sometimes a lot of data**.** You can see this by executing cat on files like **/proc/cpuinfo,** which contains information about the CPU**.** **user\@Predator-pc:\~$** file **/**proc**/**cpuinfo **/**proc**/**cpuinfo**:** empty **user\@Predator-pc:\~$** cat **/**proc**/**cpuinfo processor **:** 0 vendor**\_**id **:** AuthenticAMD cpu family **:** 15 model **:** 43 Just for fun**,** here is **/**proc**/**cpuinfo on a Sun Sunblade 1000**...** user**@**pasha**:\~$** cat **/**proc**/**cpuinfo cpu **:** TI UltraSparc III **(**Cheetah**)** fpu **:** UltraSparc III integrated FPU promlib **:** Version 3 Revision 2 prom **:** 4**.**2**.**2 type **:** sun4u ncpus probed **:** 2 ncpus active **:** 2 Cpu0Bogo **:** 498**.**68 Cpu0ClkTck **:** 000000002cb41780 Cpu1Bogo **:** 498**.**68 Cpu1ClkTck **:** 000000002cb41780 MMU Type **:** Cheetah State**:** CPU0**:** online CPU1**:** online Most of the files in **/**proc are read only**,** some require root privileges**,** some files are writable**,** and many files in **/proc/sys** are writable**.** Let**’**s discuss some of the files in **/**proc**.** ### /proc/interrupts On the x86 architecture**,** **/proc/interrupts** displays the interrupts**.** **user\@Predator-pc:\~$** cat **/**proc**/**interrupts CPU0 0**:** 13876877 IO**-**APIC**-**edge timer 1**:** 15 IO**-**APIC**-**edge i8042 8**:** 1 IO**-**APIC**-**edge rtc 9**:** 0 IO**-**APIC**-**level acpi 12**:** 67 IO**-**APIC**-**edge i8042 14**:** 128 IO**-**APIC**-**edge ide0 15**:** 124320 IO**-**APIC**-**edge ide1 169**:** 111993 IO**-**APIC**-**level ioc0 177**:** 2428 IO**-**APIC**-**level eth0 NMI**:** 0 LOC**:** 13878037 ERR**:** 0 MIS**:** 0 On a machine with two CPU**’**s**,** the file looks like this**.** user**@**predator**-**pc**\~$** cat **/**proc**/**interrupts CPU0 CPU1 0**:** 860013 0 IO**-**APIC**-**edge timer 1**:** 4533 0 IO**-**APIC**-**edge i8042 7**:** 0 0 IO**-**APIC**-**edge parport0 8**:** 6588227 0 IO**-**APIC**-**edge rtc 10**:** 2314 0 IO**-**APIC**-**fasteoi acpi 12**:** 133 0 IO**-**APIC**-**edge i8042 14**:** 0 0 IO**-**APIC**-**edge libata 15**:** 72269 0 IO**-**APIC**-**edge libata 18**:** 1 0 IO**-**APIC**-**fasteoi yenta 19**:** 115036 0 IO**-**APIC**-**fasteoi eth0 20**:** 126871 0 IO**-**APIC**-**fasteoi libata**,** ohci1394 21**:** 30204 0 IO**-**APIC**-**fasteoi ehci**\_**hcd**:**usb1**,** uhci**\_**hcd**:**usb2 22**:** 1334 0 IO**-**APIC**-**fasteoi saa7133\[0]**,** saa7133\[0] 24**:** 234739 0 IO**-**APIC**-**fasteoi nvidia NMI**:** 72 42 LOC**:** 860000 859994 ERR**:** 0 ### /proc/kcore The physical memory is represented in **/proc/kcore.** Do not try to cat this file instead use a debugger**.** The size of **/**proc**/**kcore is the same as your physical memory plus four bytes**.** user**@**predator**-**pc**\~$** ls **-**lh **/**proc**/**kcore **-**r**--------** 1 root root 2**.**0G 2007**-**01**-**30 08**:**57 **/**proc**/**kcore user**@**predator**-**pc**\~$** ### /sys Linux hot plugging The **/sys** directory was created for the Linux 2**.**6 kernel**.** Since 2**.**6**,** Linux uses **sysfs** to support **usb** and **IEEE 1394** **(FireWire)** hot plug devices**.** See the manual pages of udev**(**8**)** **(**the successor of **devfs)** and hotplug**(**8**)** for more info **(**or visit http**://**linuxhotplug**.**sourceforge**.**net**/** **).** Basically the **/sys** directory contains kernel information about hardware**.** ### /usr Unix System Resources Although **/usr** is pronounced like user remember that it stands for **Unix System Resources.** The **/usr** hierarchy should contain **shareable read only** data**.** Some people choose to mount **/usr** as read only**.** This can be done from its own partition or from a read only NFS share **(**NFS is discussed later**).** ### /usr/bin The **/usr/bin** directory contains a lot of commands**.** user**@**deb508**:\~$** ls **/**usr**/**bin | wc **-**l 1395 **(**On Solaris the **/bin** directory is a symbolic link to **/usr/bin.)** ### /usr/include The **/usr/include** directory contains general use include files for C**.** user**@**ubu1010**:\~$** ls **/**usr**/**include**/** aalib**.**h expat**\_**config**.**h math**.**h search**.**h af**\_**vfs**.**h expat**\_**external**.**h mcheck**.**h semaphore**.**h aio**.**h expat**.**h memory**.**h setjmp**.** fcntl**.**h menu**.**h sgtty**.**h aliases**.** features**.**h mntent**.**h shadow**.**h **...** h AL h ### /usr/lib The **/usr/lib** directory contains libraries that are not directly executed by users or scripts**.** user**@**deb508**:\~$** ls **/**usr**/**lib | head **-**7 4Suite ao apt arj aspell avahi bonobo ### /usr/local The **/usr/local** directory can be used by an administrator to install software locally**.** user**@**deb508**:\~$** ls **/**usr**/**local**/** bin etc games include lib man sbin share src user**@**deb508**:\~$** du **-**sh **/**usr**/**local**/** 128K **/**usr**/**local**/** ### /usr/share The **/usr/share** directory contains architecture independent data**.** As you can see**,** this is a fairly large directory**.** | user**@**deb508**:\~$** ls **/**usr**/**share**/** \| wc **-**l | | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------- | - | | 263 user**@**deb508**:\~$** du **-**sh **/**usr**/**share**/** 1**.**3G **/**usr**/**share**/** | | This directory typically contains **/usr/share/man** for manual pages**.** user**@**deb508**:\~$** ls **/**usr**/**share**/**man cs fr hu it**.**UTF**-**8 man2 man6 pl**.**ISO8859**-**2 sv de fr**.**ISO8859**-**1 id ja man3 man7 pl**.**UTF**-**8 tr es fr**.**UTF8 it ko man4 man8 pt**\_**BR zh**\_**CN fi gl it**.**ISO8859**-** 1 man1 man5 pl ru zh**\_**TW And it contains **/usr/share/games** for all static game data **(**so no high**-**scores or play logs**).** user**@**ubu1010**:\~$** ls **/**usr**/**share**/**games**/** ### /usr/src The **/usr/src** directory is the recommended location for kernel source files**.** user**@**deb508**:\~$** ls **-**l **/**usr**/**src**/** ### /var variable data Files that are unpredictable in size such as log**,** cache and spool files**,** should be located in **/var.** ### /var/log The **/var/log** directory serves as a central point to contain all log files**.** ### set You can use the **set** command to display a list of environment variables**.** On Debian stable and Debian systems**,** the set command will also list shell functions after the shell variables**.** Use set | more to see the variables then**.** ### unset Use the **unset** command to remove a variable from your shell environment**.** \[user**@**RHEL4b **\~**]**$** MyVar=8472 \[user**@**RHEL4b **\~**]**$** echo **$**MyVar 8472 \[user**@**RHEL4b **\~**]**$** unset MyVar \[user**@**RHEL4b **\~**]**$** echo **$**MyVar \[user**@**RHEL4b **\~**]**$** ### PS1 The **$PS1** variable determines your shell prompt**.** You can use backslash escaped special characters like **\u** for the username or **\w** for the working directory**.** The **bash** manual has a complete reference**.** In this example we change the value of **$PS1** a couple of times**.** user**@**deb503**:\~$** PS1=prompt prompt promptPS1=**‘**prompt **‘** prompt prompt PS1=**‘>** **‘** **>** **>** PS1=**‘\\**u**@\\**h**$** **‘** user**@**deb503**$** user**@**deb503**$** PS1=**‘\\**u**@\\**h**:\\**W**$’** user**@**deb503**:\~$** To avoid unrecoverable mistakes**,** you can set normal user prompts to green and the root prompt to red**.** Add the following to your **.bashrc** for a green user prompt**:** **#** color prompt by user RED=**‘\\**\[**\\**033\[01**;**31m**\\**]**’** WHITE=**‘\\**\[**\\**033\[01**;**00m**\\**]**’** GREEN=**‘\\**\[**\\**033\[01**;**32m**\\**]**’** BLUE=**‘\\**\[**\\**033\[01**;**34m**\\**]**’** export PS1=**“$**{debian**\_**chroot**:+($**debian**\_**chroot**)**}**$**GREEN**\\**u**$**WHITE**@$**BLUE**\\**h**$**WHITE**\\**w**\\$** **“** ### $PATH The **$PATH** variable is determines where the shell is looking for commands to execute **(**unless the command is builtin or aliased**).** This variable contains a list of directories**,** separated by colons**.** \[\[user**@**RHEL4b **\~**]**$** echo **$**PATH **/**usr**/**kerberos**/**bin**:/**usr**/**local**/**bin**:/**bin**:/**usr**/**bin**:** The shell will not look in the current directory for commands to execute**!** **(**Looking for executables in the current directory provided an easy way to hack PC**-**DOS computers**).** If you want the shell to look in the current directory then add a **. (dot)** at the end of your **$**PATH**.** \[user**@**RHEL4b **\~**]**$** PATH=**$**PATH**:.** \[user**@**RHEL4b **\~**]**$** echo **$**PATH **/**usr**/**kerberos**/**bin**:/**usr**/**local**/**bin**:/**bin**:/**usr**/**bin**:.** \[user**@**RHEL4b **\~**]**$** Your path might be different when using su instead of **su -** because the latter will take on the environment of the target user**.** The root user typically has **/sbin** directories added to the **$**PATH variable**.** \[user**@**RHEL3 **\~**]**$** su Password**:** \[root**@**RHEL3 user]**#** echo **$**PATH **/**usr**/**local**/**bin**:/**bin**:/**usr**/**bin**:/**usr**/**X11R6**/**bin \[root**@**RHEL3 user]**#** exit \[user**@**RHEL3 **\~**]**$** su Password**:** \[root**@**RHEL3 **\~**]**#** echo **$**PATH **/**usr**/**local**/**sbin**:/**usr**/**local**/**bin**:/**sbin**:/**bin**:/**usr**/**sbin**:/**usr**/**bin**:** \[root**@**RHEL3 **\~**]**#** ### env The **env** command without options will display a list of **exported variables.** The difference with **set** with options is that **set** lists all variables**,** including those not exported to child shells**.** But **env** can also be used to start a clean shell **(**a shell without any inherited environment**).** The **env -i** command clears the environment for the subshell**.** Notice in this screenshot that **bash** will set the **$SHELL** variable on startup**.** \[user**@**RHEL4b **\~**]**$** bash **-**c **‘**echo **$**SHELL **$**HOME **$**USER**’** **/**bin**/**bash **/**home**/**user user \[user**@**RHEL4b **\~**]**$** env **-**i bash **-**c **‘**echo **$**SHELL **$**HOME **$**USER**’** **/**bin**/**bash \[user**@**RHEL4b **\~**]**$** You can use the **env** command to set the **$LANG,** or any other**,** variable for just one instance of **bash** with one command**.** The example below uses this to show the influence of the **$LANG** variable on file globbing **(**see the chapter on file globbing**).** \[user**@**RHEL4b test]**$** env LANG=C bash **-**c **‘**ls File\[a**-**z]**’** Filea Fileb \[user**@**RHEL4b test]**$** env LANG=en**\_**US**.**UTF**-**8 bash **-**c **‘**ls File\[a**-**z]**’** Filea FileA Fileb FileB \[user**@**RHEL4b test]**$** ### export You can export shell variables to other shells with the **export** command**.** This will export the variable to child shells**.** \[user**@**RHEL4b **\~**]**$** var3=three \[user**@**RHEL4b **\~**]**$** var4=four \[user**@**RHEL4b **\~**]**$** export var4 \[user**@**RHEL4b **\~**]**$** echo **$**var3 **$**var4 three four \[user**@**RHEL4b **\~**]**$** bash \[user**@**RHEL4b **\~**]**$** echo **$**var3 **$**var4 four But it will not export to the parent shell **(**previous screenshot continued**).** \[user**@**RHEL4b **\~**]**$** export var5=five \[user**@**RHEL4b **\~**]**$** echo **$**var3 **$**var4 **$**var5 four five \[user**@**RHEL4b **\~**]**$** exit exit \[user**@**RHEL4b **\~**]**$** echo **$**var3 **$**var4 **$**var5 three four \[user**@**RHEL4b **\~**]**$** ### Licensing ### About software licenses There are two predominant software paradigms**:** **Free and Open Source Software** **(**FOSS**)** and **proprietary software.** The criteria for differentiation between these two approaches is based on control over the software**.** With **proprietary software** control tends to lie more with the vendor**,** while with **Free and Open Source Software** tends to be more weighted towards the end user**.** But even though the paradigms differ**,** they use the same **copyright laws** to reach and enforce their goals**.** From a legal perspective**,** **Free and Open Source Software** can be considered as software to which users generally receive more rights via their license agreement than they would have with a **proprietary software license,** yet the underlying license mechanisms are the same**.** Legal theory states that the author of FOSS**,** contrary to the author of **public domain** software**,** has in no way whatsoever given up his rights on his work**.** FOSS supports on the rights of the author **(**the **copyright)** to impose FOSS license conditions**.** The FOSS license conditions need to be respected by the user in the same way as proprietary license conditions**.** Always check your license carefully before you use third party software**.** Examples of proprietary software are **AIX** from IBM**,** **HP-UX** from HP and **Oracle Database 11g.** You are not authorised to install or use this software without paying a licensing fee**.** You are not authorised to distribute copies and you are not authorised to modify the closed source code**.** ### Public domain software and freeware Software that is original in the sense that it is an intellectual creation of the author benefits **copyright** protection**.** Non**-**original software does not come into consideration for **copyright** protection and can**,** in principle**,** be used freely**.** Public domain software is considered as software to which the author has given up all rights and on which nobody is able to enforce any rights**.** This software can be used**,** reproduced or executed freely**,** without permission or the payment of a fee**.** Public domain software can in certain cases even be presented by third parties as own work**,** and by modifying the original work**,** third parties can take certain versions of the public domain software out of the public domain again**.** **Freeware** is not public domain software or FOSS**.** It is proprietary software that you can use without paying a license cost**.** However**,** the often-strict license terms need to be respected**.** Examples of freeware are **Adobe Reader,** **Skype** and **Command and Conquer: Tiberian Sun** **(**this game was sold as proprietary in 1999 and is since 2011 available as freeware**).** ### Free Software or Open Source Software Both the **Free Software** **(**translates to **vrije software** in Dutch and to **Logiciel Libre** in French**)** and the **Open-Source Software** movement largely pursue similar goals and endorse similar software licenses**.** But historically**,** there has been some perception of differentiation due to different emphases**.** Where the **Free Software** movement focuses on the rights **(**the four freedoms**)** which Free Software provides to its users**,** the **Open Source Software** movement points to its Open Source Definition and the advantages of peer**-**to**-**peer software development**.** Recently**,** the term free and open source software or FOSS has arisen as a neutral alternative**.** A lesser**-**used variant is free**/**libre**/**open source software **(**FLOSS**),** which uses **libre** to clarify the meaning of free as in **freedom** rather than as in **at no charge.** Examples of **free software** are **gcc,** **MySQL** and **gimp.** Detailed information about the **four freedoms** can be found here**:** http**://**www**.**gnu**.**org**/**philosophy**/**free**-**sw**.**html The **open-source definition** can be found at**:** http**://**www**.**opensource**.**org**/**docs**/**osd The above definition is based on the **Debian Free Software Guidelines** available here**:** http**://**www**.**debian**.**org**/**social**\_**contract**#**guidelines ### GNU General Public License More and more software is being released under the **GNU GPL** **(**in 2006 Java was released under the GPL**).** This license **(**v2 and v3**)** is the main license endorsed by the Free Software Foundation**.** It**’**s main characteristic is the **copyleft** principle**.** This means that everyone in the chain of consecutive users**;** in return for the right of use that is assigned**,** needs to distribute the improvements he makes to the software and his derivative works under the same conditions to other users**,** if he chooses to distribute such improvements or derivative works**.** In other words**;** software which incorporates GNU GPL software**,** needs to be distributed in turn as GNU GPL software **(**or compatible**,** see below**).** It is not possible to incorporate copyright protected parts of GNU GPL software in a proprietary licensed work**.** The GPL has been upheld in court**.** ### Using GPLv3 software You can use **GPLv3 software** almost without any conditions**.** If you solely run the software**,** you even don**’**t have to accept the terms of the GPLv3**.** However**,** any other use **-** such as modifying or distributing the software **-** implies acceptance**.** In case you use the software internally **(**including over a network**),** you may modify the software without being obliged to distribute your modification**.** You may hire third parties to work on the software exclusively for you and under your direction and control**.** But if you modify the software and use it otherwise than merely internally**,** this will be considered as distribution**.** You must distribute your modifications under GPLv3 **(**the copyleft principle**).** Several more obligations apply if you distribute GPLv3 software**.** Check the GPLv3 license carefully**.** You create output with GPLv3 software**:** The GPLv3 does not automatically apply to the output**.** ### BSD license There are several versions of the original Berkeley Distribution License**.** The most common one is the 3**-**clause license **(“**New BSD License**”** or **“**Modified BSD License**”).** This is a permissive free software license**.** The license places minimal restrictions on how the software can be redistributed**.** This is in contrast to copyleft licenses such as the GPLv**.** 3 discussed above**,** which have a copyleft mechanism**.** This difference is of less importance when you merely use the software**,** but kicks in when you start redistributing verbatim copies of the software or your own modified versions**.** ### Other licenses FOSS or not**,** there are many kind of licenses on software**.** You should read and understand them before using any software**.** ### Combination of software licenses When you use several sources or wishes to redistribute your software under a different license**,** you need to verify whether all licenses are compatible**.** Some FOSS licenses **(**such as BSD**)** are compatible with proprietary licenses**,** but most are not**.** If you detect a license incompatibility**,** you must contact the author to negotiate different license conditions or refrain from using the incompatible software**.** ### First steps on the command line man pages This chapter will explain the use of **man** pages **(**also called **manual pages)** on your Unix or Linux computer**.** You will learn the **man** command together with related commands like **whereis,** **whatis** and **mandb.** Most Unix files and commands have pretty good man pages to explain their use**.** Man pages also come in handy when you are using multiple flavours of Unix or several Linux distributions since options and parameters sometimes vary**.** ### man $command Type **man** followed by a command **(**for which you want help**)** and start reading**.** Press **q** to quit the manpage**.** Some man pages contain examples **(**near the end**).** user**@**predator**-**pc**\~$** man whois Reformatting whois**(**1**),** please wait**...** ### man $configfile Most **configuration files** have their own manual**.** user**@**predator**-**pc**\~$** man syslog**.**conf Reformatting syslog**.**conf**(**5**),** please wait**...** ### man $daemon This is also true for most **daemons** **(**background programs**)** on your system**.** user**@**predator**-**pc**\~$** man syslogd Reformatting syslogd**(**8**),** please wait**...** man -k (apropos) man -k (or apropos) shows a list of man pages containing a string**.** user**@**predator**-**pc**\~$** man **-**k syslog lm**-**syslog**-**setup **(**8**)** **-** configure laptop mode to switch syslog**.**conf **...** logger **(**1**)** **-** a shell command interface to the syslog**(**3**)** **...** syslog**-**facility **(**8**)** **-** Setup and remove LOCALx facility for sysklogd syslog**.**conf **(**5**)** **-** syslogd**(**8**)** configuration file syslogd **(**8**)** **-** Linux system logging utilities**.** syslogdlistfiles **(**8**)** **-** list system logfiles ### whatis To see just the description of a manual page**,** use **whatis** followed by a string**.** user**@**u810**:\~$** whatis route route **(**8**)** **-** show **/** manipulate the IP routing table ### whereis The location of a manpage can be revealed with **whereis.** user**@**predator**-**pc**\~$** whereis **-**m whois whois**:** **/**usr**/**share**/**man**/**man1**/**whois**.**1**.**gz This file is directly readable by **man.** user**@**predator**-**pc**\~$** man **/**usr**/**share**/**man**/**man1**/**whois**.**1**.**gz ### man $section $file Therefor**,** when referring to the man page of the passwd command**,** you will see it written as **passwd(1);** when referring to the **passwd file,** you will see it written as **passwd(5).** The screenshot explains how to open the man page in the correct section**.** \[user**@**RHEL52 **\~**]**$** man passwd **#** opens the first manual found \[user**@**RHEL52 **\~**]**$** man 5 passwd **#** opens a page from section 5 ### man man If you want to know more about **man,** then Read The Fantastic Manual **(**RTFM**).** *Unfortunately**,** manual pages do not have the answer to everything**...*** user**@**predator**-**pc**\~$** man woman No manual entry for woman ### mandb Should you be convinced that a man page exists**,** but you can**’**t access it**,** then try running **mandb** on Debian**/**Mint**.** root**@**predator**-**pc**\~#** mandb 0 man subdirectories contained newer manual pages**.** 0 manual pages were added**.** 0 stray cats were added**.** 0 old database entries were purged**.** Or run **makewhatis** on CentOS**/**Redhat**.** \[root**@**centos65 **\~**]**#** apropos scsi scsi**:** nothing appropriate \[root**@**centos65 **\~**]**#** makewhatis \[root**@**centos65 **\~**]**#** apropos scsi hpsa **(**4**)** **-** HP Smart Array SCSI driver lsscsi **(**8**)** **-** list SCSI devices **(**or hosts**)** and their attributes sd **(**4**)** **-** Driver for SCSI Disk Drives st **(**4**)** **-** SCSI tape device ### Working with directories This module is a brief overview of the most common commands to work with directories**:** **pwd,** **cd,** **ls,** **mkdir** and **rmdir.** These commands are available on any Linux **(**or Unix**)** system**.** This module also discusses **absolute** and **relative paths** and **path completion** in the **bash** shell**.** ### pwd The **you are here** sign can be displayed with the **pwd** command **(**Print Working Directory**).** Go ahead**,** try it**:** Open a command line interface **(**also called a terminal**,** console or xterm**)** and type **pwd.** The tool displays your **current directory.** user**@**debian8**:\~$** **pwd /**home**/**user ### cd You can change your current directory with the **cd** command **(**Change Directory**).** user**@**debian8**$** **cd /etc** user**@**debian8**$** **pwd** **/**etc user**@**debian8**$** **cd /bin** user**@**debian8**$** **pwd** **/**bin user**@**debian8**$** **cd /home/user/** user**@**debian8**$** **pwd /**home**/**user ### cd \~ The **cd** is also a shortcut to get back into your home directory**.** Just typing **cd** without a target directory**,** will put you in your home directory**.** Typing **cd \~** has the same effect**.** user**@**debian8**$** **cd /etc** user**@**debian8**$** **pwd** **/**etc user**@**debian8**$** **cd** user**@**debian8**$** **pwd** **/**home**/**user user**@**debian8**$** **cd \~** user**@**debian8**$** **pwd** **/**home**/**user ### cd .. To go to the **parent directory** **(**the one just above your current directory in the directory tree**),** type **cd ..** **.** user**@**debian8**$** **pwd /**usr**/**share**/**games user**@**debian8**$** **cd ..** user**@**debian8**$** **pwd /**usr**/**share *To stay in the current directory**,** type **cd .*** ***;-)*** We will see useful use of the **.** character representing the current directory later**.** ### cd - Another useful shortcut with **cd** is to just type **cd -** to go to the previous directory**.** user**@**debian8**$** **pwd** **/**home**/**user user**@**debian8**$** **cd /etc** user**@**debian8**$** **pwd** **/**etc user**@**debian8**$** **cd -** **/**home**/**user user**@**debian8**$** **cd /**etc ### absolute and relative paths You should be aware of **absolute and relative paths** in the file tree**.** When you type a path starting with a **slash (/),** then the **root** of the file tree is assumed**.** If you don**’**t start your path with a slash**,** then the current directory is the assumed starting point**.** The screenshot below first shows the current directory **/home/user.** From within this directory**,** you have to type **cd /home** instead of **cd home** to go to the **/home** directory**.** user**@**debian8**$** **pwd /**home**/**user user**@**debian8**$** **cd home** bash**:** cd**:** home**:** No such file or directory user**@**debian8**$** **cd /home** user**@**debian8**$** **pwd** **/**home When inside **/home,** you have to type **cd user** instead of **cd /user** to enter the subdirectory **user** of the current directory **/home.** user**@**debian8**$** **pwd** **/**home user**@**debian8**$** **cd /user** bash**:** cd**:** **/**user**:** No such file or directory user**@**debian8**$** **cd user** user**@**debian8**$** **pwd** **/**home**/**user In case your current directory is the **root directory /,** then both **cd /home** and **cd home** will get you in the **/home** directory**.** user**@**debian8**$** **pwd** **/** user**@**debian8**$** **cd home** user**@**debian8**$** **pwd** **/**home user**@**debian8**$** **cd /** user**@**debian8**$** **cd /home** user**@**debian8**$** **pwd /**home This was the last screenshot with **pwd** statements**.** From now on**,** the current directory will often be displayed in the prompt**.** Later in this book we will explain how the shell variable **$PS1** can be configured to show this**.** ### path completion The **tab key** can help you in typing a path without errors**.** Typing **cd /et** followed by the **tab key** will expand the command line to **cd /etc/.** When typing **cd /Et** followed by the **tab key,** nothing will happen because you typed the wrong **path** **(**upper case E**).** You will need fewer key strokes when using the **tab key,** and you will be sure your typed **path** is correct**!** ### ls You can list the contents of a directory with **ls.** user**@**debian8**:\~$** **ls** allfiles**.**txt dmesg**.**txt services stuff summer**.**txt user**@**debian8**:\~$** ### ls -a A frequently used option with ls is **-a** to show all files**.** Showing all files means including the **hidden files.** When a file name on a Linux file system starts with a dot**,** it is considered a **hidden file** and it does not show up in regular file listings**.** user**@**debian8**:\~$** **ls** allfiles**.**txt dmesg**.**txt services stuff summer**.**txt user**@**debian8**:\~$** **ls -a** **.** allfiles**.**txt **.**bash**\_**profile dmesg**.**txt **.**lesshst stuff **..** **.**bash**\_**history **.**bashrc services **.**ssh summer**.**txt user**@**debian8**:\~$** ### ls -l Many times you will be using options with **ls** to display the contents of the directory in different formats or to display different parts of the directory**.** Typing just **ls** gives you a list of files in the directory**.** Typing **ls -l** **(**that is a letter L**,** not the number 1**)** gives you a long listing**.** user**@**debian8**:\~$** **ls -l** total 17296 **-**rw**-**r**--**r**--** 1 user user 17584442 Sep 17 00**:**03 allfiles**.**txt **-**rw**-**r**--**r**--** 1 user user 96650 Sep 17 00**:**03 dmesg**.**txt **-**rw**-**r**--**r**--** 1 user user 19558 Sep 17 00**:**04 services drwxr**-**xr**-**x 2 user user 4096 Sep 17 00**:**04 stuff **-**rw**-**r**--**r**--** 1 user user 0 Sep 17 00**:**04 summer**.**txt ### ls -lh Another frequently used ls option is **-h.** It shows the numbers **(**file sizes**)** in a more human readable format**.** Also shown below is some variation in the way you can give the options to **ls.** We will explain the details of the output later in this book**.** *Note that we use the letter L as an option in this screenshot**,** not the number 1**.*** |user@debian8:~~$ ls -l -h total 17M
-rw-r--r-- 1 user user 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 user user 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 user user 20K Sep 17 00:04 services drwxr-xr-x 2 user user 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 user user 0 Sep 17 00:04 summer.txt user@debian8:~~$ ls -lh total 17M
-rw-r--r-- 1 user user 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 user user 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 user user 20K Sep 17 00:04 services drwxr-xr-x 2 user user 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 user user 0 Sep 17 00:04 summer.txt user@debian8:~~$ ls -hl total 17M
-rw-r--r-- 1 user user 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 user user 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 user user 20K Sep 17 00:04 services drwxr-xr-x 2 user user 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 user user 0 Sep 17 00:04 summer.txt user@debian8:~~$ ls -h -l total 17M
-rw-r--r-- 1 user user 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 user user 95K Sep 17 00:03 dmesg.txt
| | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | |-rw-r--r-- 1 user user 20K Sep 17 00:04 services drwxr-xr-x 2 user user 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 user user 0 Sep 17 00:04 summer.txt user@debian8:\~$
| ### mkdir Walking around the Unix file tree is fun**,** but it is even more fun to create your own directories with **mkdir.** You have to give at least one parameter to **mkdir,** the name of the new directory to be created**.** Think before you type a leading **/** **.** user**@**debian8**:\~$** **mkdir mydir** user**@**debian8**:\~$** **cd mydir** user**@**debian8**:\~/**mydir**$** **ls -al** total 8 drwxr**-**xr**-**x 2 user user 4096 Sep 17 00**:**07 **.** drwxr**-**xr**-**x 48 user user 4096 Sep 17 00**:**07 **..** user**@**debian8**:\~/**mydir**$** **mkdir stuff** user**@**debian8**:\~/**mydir**$** **mkdir otherstuff** user**@**debian8**:\~/**mydir**$** **ls -l** total 8 drwxr**-**xr**-**x 2 user user 4096 Sep 17 00**:**08 otherstuff drwxr**-**xrx 2 user user 4096 Sep 17 00**:**08 stuff user**@**debian8**:\~/**mydir**$** ### mkdir -p The following command will fail**,** because the **parent directory** of **threedirsdeep** does not exist**.** user**@**debian8**:\~$** **mkdir mydir2/mysubdir2/threedirsdeep** mkdir**:** cannot create directory **‘**mydir2**/**mysubdir2**/**threedirsdeep**’:** No such fi**\\** le or directory When given the option **-p,** then **mkdir** will create **parent directories** as needed**.** ### rmdir When a directory is empty**,** you can use **rmdir** to remove the directory**.** user**@**debian8**:\~/**mydir**$** **ls -l** total 8 drwxr**-**xr**-**x 2 user user 4096 Sep 17 00**:**08 otherstuff drwxr**-**xrx 2 user user 4096 Sep 17 00**:**08 stuff user**@**debian8**:\~/**mydir**$** **rmdir otherstuff** user**@**debian8**:\~/**mydir**$** **cd ..** user**@**debian8**:\~$** **rmdir mydir** rmdir**:** failed to remove **‘**mydir**’:** Directory not empty user**@**debian8**:\~$** **rmdir mydir/stuff** user**@**debian8**:\~$** **rmdir mydir** ### rmdir -p And similar to the **mkdir -p** option**,** you can also use **rmdir** to recursively remove directories**.** user**@**debian8**:\~$** **mkdir -p test42/subdir** user**@**debian8**:\~$** **rmdir -p** **test42/subdir** ### working with files In this chapter we learn how to recognise**,** create**,** remove**,** copy and move files using commands like **file, touch, rm, cp, mv** and **rename.** ### all files are case sensitive Files on Linux **(**or any Unix**)** are **case sensitive.** This means that **FILE1** is different from **file1,** and **/etc/hosts** is different from **/etc/Hosts** **(**the latter one does not exist on a typical Linux computer**).** This screenshot shows the difference between two files**,** one with upper case **W,** the other with lower case **w.** user**@**predator**-**pc**\~/**Linux**$** **ls** winter**.**txt Winter**.**txt user**@**predator**-**pc**\~/**Linux**$** **cat winter.txt** It is cold**.** user**@**predatorpc**\~/**Linux**$** **cat Winter.txt** It is very cold**!** ### Everything is a file A **directory** is a special kind of **file,** but it is still a **(**case sensitive**!)** **file.** Each terminal window **(**for example **/dev/pts/4),** any hard disk or partition **(**for example **/dev/sdb1)** and any process are all represented somewhere in the **file system** as a **file.** It will become clear throughout this course that everything on Linux is a **file.** **file** The **file** utility determines the file type**.** Linux does not use extensions to determine the file type**.** The command line does not care whether a file ends in **.**txt or **.**pdf**.** As a system administrator**,** you should use the **file** command to determine the file type**.** Here are some examples on a typical Linux system**.** user**@**predator**-**pc**\~$** **file pic33.png** pic33**.**png**:** PNG image data**,** 3840 x 1200**,** 8**-**bit**/**color RGBA**,** non**-**interlaced user**@**predator**-**pc**\~$** **file /etc/passwd** **/**etc**/**passwd**:** ASCII text user**@**predator**-**pc**\~$** **file HelloWorld.c** HelloWorld**.**c**:** ASCII C program text The file command uses a magic file that contains patterns to recognise file types**.** The magic file is located in **/usr/share/file/magic.** Type **man 5 magic** for more information**.** It is interesting to point out **file -s** for special files like those in **/dev** and **/proc.** root**@**debian6**\~#** **file /dev/sda** **/**dev**/**sda**:** block special root**@**debian6**\~#** **file -s /dev/sda** **/**dev**/**sda**:** x86 boot sector**;** partition 1**:** ID=0x83**,** active**,** starthead**...** root**@**debian6**\~#** **file /proc/cpuinfo** **/**proc**/**cpuinfo**:** empty root**@**debian6**\~#** **file -s /proc/cpuinfo /**proc**/**cpuinfo**:** ASCII C**++** program text ### touch create an empty file One easy way to create an empty file is with **touch.** **(**We will see many other ways for creating files later in this book**.)** This screenshot starts with an empty directory**,** creates two files with **touch** and the lists those files**.** user**@**debian7**:\~$** **ls -l** total 0 user**@**debian7**:\~$** **touch file42** user**@**debian7**:\~$** **touch file33** user**@**debian7**:\~$** **ls l** total 0 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 08**:**57 file33 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 08**:**56 file42 user**@**debian7**:\~$** ### touch -t The **touch** command can set some properties while creating empty files**.** Can you determine what is set by looking at the next screenshot? If not**,** check the manual for **touch.** user**@**debian7**:\~$** **touch -t 200505050000 SinkoDeMayo** user**@**debian7**:\~$** **touch -t** **130207111630 BigBattle.txt** user**@**debian7**:\~$** **ls -l** total 0 **-**rw**-**r**--**r**--** 1 user user 0 Jul 11 1302 BigBattle**.**txt **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 08**:**57 file33 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 08**:**56 file42 **-**rw**-**r**--**r**--** 1 user user 0 May 5 2005 SinkoDeMayo user**@**debian7**:\~$** ### rm ### remove forever When you no longer need a file**,** use **rm** to remove it**.** Unlike some graphical user interfaces**,** the command line in general does not have a **waste bin** or **trash can** to recover files**.** When you use **rm** to remove a file**,** the file is gone**.** Therefore**,** be careful when removing files**!** user**@**debian7**:\~$** **ls** BigBattle**.**txt file33 file42 SinkoDeMayo user**@**debian7**:\~$** **rm BigBattle.txt** user**@**debian7**:\~$** **ls** file33 file42 SinkoDeMayo user**@**debian7**:\~$** ### rm -i To prevent yourself from accidentally removing a file**,** you can type **rm -i.** user**@**debian7**:\~$** **ls** file33 file42 SinkoDeMayo user**@**debian7**:\~$** **rm -i file33** rm**:** remove regular empty file \`file33**’**? yes user**@**debian7**:\~$** **rm -i SinkoDeMayo** rm**:** remove regular empty file \`SinkoDeMayo**’**? n user**@**debian7**:\~$** **ls** file42 SinkoDeMayo user**@**debian7**:\~$** ### rm -rf By default**,** **rm -r** will not remove non**-**empty directories**.** However, **rm** accepts several options that will allow you to remove any directory**.** The **rm -rf** statement is famous because it will erase anything **(**providing that you have the permissions to do so**).** When you are logged on as root**,** be very careful with **rm -rf** **(**the **f** means **force** and the **r** means **recursive)** since being root implies that permissions don**’**t apply to you**.** You can literally erase your entire file system by accident**.** user**@**debian7**:\~$** **mkdir test** user**@**debian7**:\~$** **rm test** rm**:** cannot remove \`test**’:** Is a directory user**@**debian7**:\~$** **rm -rf test** user**@**debian7**:\~$** **ls test** ls**:** cannot access test**:** No such file or directory user**@**debian7**:\~$** ### cp copy one file To copy a file**,** use **cp** with a source and a target argument**.** user**@**debian7**:\~$** **ls** file42 SinkoDeMayo user**@**debian7**:\~$** **cp file42 file42.copy** user**@**debian7**:\~$** **ls** file42 file42**.**copy SinkoDeMayo ### copy to another directory If the target is a directory**,** then the source files are copied to that target directory**.** user**@**debian7**:\~$** **mkdir dir42** user**@**debian7**:\~$** **cp SinkoDeMayo dir42** user**@**debian7**:\~$** **ls dir42/** SinkoDeMayo ### cp -r To copy complete directories**,** use **cp -r** **(**the **-r** option forces **recursive** copying of all files in all subdirectories**).** user**@**debian7**:\~$** **ls** dir42 file42 file42**.**copy SinkoDeMayo user**@**debian7**:\~$** **cp -r dir42/ dir33** user**@**debian7**:\~$** **ls** dir33 dir42 file42 file42**.**copy SinkoDeMayo user**@**debian7**:\~$** **ls dir33/** SinkoDeMayo ### copy multiple files to directory You can also use cp to copy multiple files into a directory**.** In this case**,** the last argument **(**a**.**k**.**a**.** the target**)** must be a directory**.** user**@**debian7**:\~$** **cp file42 file42.copy SinkoDeMayo dir42/** user**@**debian7**:\~$** **ls dir42/** file42 file42**.**copy SinkoDeMayo ### cp -i To prevent **cp** from overwriting existing files**,** use the **-i** **(**for interactive**)** option**.** user**@**debian7**:\~$** **cp SinkoDeMayo file42** user**@**debian7**:\~$** **cp SinkoDeMayo file42** user**@**debian7**:\~$** **cp -i SinkoDeMayo file42** cp**:** overwrite \`file42**’**? **n** user**@**debian7**:\~$** ### mv ### rename files with mv Use **mv** to rename a file or to move the file to another directory**.** user**@**debian7**:\~$** **ls** dir33 dir42 file42 file42**.**copy SinkoDeMayo user**@**debian7**:\~$** **mv file42 file33** user**@**debian7**:\~$** **ls** dir33 dir42 file33 file42**.**copy SinkoDeMayo user**@**debian7**:\~$** When you need to rename only one file then **mv** is the preferred command to use**.** ### rename directories with mv The same **mv** command can be used to rename directories**.** user**@**debian7**:\~$** **ls -l** total 8 drwxr**-**xr**-**x 2 user user 4096 Oct 15 09**:**36 dir33 drwxr**-**xr**-**x 2 user user 4096 Oct 15 09**:**36 dir42 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 09**:**38 file33 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 09**:**16 file42**.**copy **-**rw**-**r**--**r**--** 1 user user 0 May 5 2005 SinkoDeMayo user**@**debian7**:\~$** **mv dir33 backup** user**@**debian7**:\~$** **ls -l** total 8 drwxr**-**xr**-**x 2 user user 4096 Oct 15 09**:**36 backup drwxr**-**xr**-**x 2 user user 4096 Oct 15 09**:**36 dir42 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 09**:**38 file33 **-**rw**-**r**--**r**--** 1 user user 0 Oct 15 09**:**16 file42**.**copy **-**rw**-**r**--**r**--** 1 user user 0 May 5 2005 SinkoDeMayo user**@**debian7**:\~$** **mv -i** The **mv** also has a **-i** switch similar to **cp** and **rm.** this screenshot shows that **mv -i** will ask permission to overwrite an existing file**.** user**@**debian7**:\~$** **mv -i file33 SinkoDeMayo** mv**:** overwrite \`SinkoDeMayo**’**? **no** user**@**debian7**:\~$** **Rename** The **rename** command is one of the rare occasions where the Linux Fundamentals book has to make a distinction between Linux distributions**.** Almost every command in the **Fundamentals** part of this book works on almost every Linux computer**.** But **rename** is different**.** Try to use **mv** whenever you need to rename only a couple of files**.** ### Rename on Debian/Debian stable The **rename** command on Debian uses regular expressions **(**regular expression or shor regex are explained in a later chapter**)** to rename many files at once**.** Below a **rename** example that switches all occurrences of txt to png for all file names ending in **.**txt**.** user**@**debian7**:\~/**test42**$** **ls** abc**.**txt file33**.**txt file42**.**txt user**@**debian7**:\~/**test42**$** **rename ‘s/\\.txt/\\.png/’ \*.txt** user**@**debian7**:\~/**test42**$** **ls** abc**.**png file33**.**png file42**.**png This second example switches all **(**first**)** occurrences of **file** into **document** for all file names ending in **.**png**.** user**@**debian7**:\~/**test42**$** **ls** abc**.**png file33**.**png file42**.**png user**@**debian7**:\~/**test42**$** **rename ‘s/file/document/’ \*.png** user**@**debian7**:\~/**test42**$** **ls** abc**.**png document33**.**png document42**.**png user**@**debian7**:\~/**test42**$** ### Rename on CentOS/RHEL/Fedora On Red Hat Enterprise Linux**,** the syntax of **rename** is a bit different**.** The first example below renames all **\*.**conf files replacing any occurrence of **.**conf with **.**backup**.** \[user**@**centos7 **\~**]**$** **touch one.conf two.conf three.conf** \[user**@**centos7 **\~**]**$** **rename .conf .backup \*.conf** \[user**@**centos7 **\~**]**$** **ls** one**.**backup three**.**backup two**.**backup \[user**@**centos7 **\~**]**$** The second example renames all **(\*)** files replacing one with ONE**.** \[user**@**centos7 **\~**]**$** **ls** one**.**backup three**.**backup two**.**backup \[user**@**centos7 **\~**]**$** **rename one ONE \*** \[user**@**centos7 **\~**]**$** **ls** ONE**.**backup three**.**backup two**.**backup \[user**@**centos7 **\~**]**$** ### Head You can use **head** to display the first ten lines of a file**.** user**@**debian7**\~$** **head /etc/passwd** root**:**x**:**0**:**0**:**root**:/**root**:/**bin**/**bash daemon**:**x**:**1**:**1**:**daemon**:/**usr**/**sbin**:/**bin**/**sh bin**:**x**:**2**:**2**:**bin**:/**bin**:/**bin**/**sh sys**:**x**:**3**:**3**:**sys**:/**dev**:/**bin**/**sh sync**:**x**:**4**:**65534**:**sync**:/**bin**:/**bin**/**sync games**:**x**:**5**:**60**:**games**:/**usr**/**games**:/**bin**/**sh man**:**x**:**6**:**12**:**man**:/**var**/**cache**/**man**:/**bin**/**sh lp**:**x**:**7**:**7**:**lp**:/**var**/**spool**/**lpd**:/**bin**/**sh mail**:**x**:**8**:**8**:**mail**:/**var**/**mail**:/**bin**/**sh news**:**x**:**9**:**9**:**news**:/**var**/**spool**/**news**:/**bin**/**sh root**@**debian7**\~#** The **head** command can also display the first **n** lines of a file**.** user**@**debian7**\~$** **head -4 /etc/passwd** root**:**x**:**0**:**0**:**root**:/**root**:/**bin**/**bash daemon**:**x**:**1**:**1**:**daemon**:/**usr**/**sbin**:/**bin**/**sh bin**:**x**:**2**:**2**:**bin**:/**bin**:/**bin**/**sh sys**:**x**:**3**:**3**:**sys**:/**dev**:/**bin**/**sh user**@**debian7**\~$** And **head** can also display the first **n bytes.** user**@**debian7**\~$** **head -c14 /etc/passwd** root**:**x**:**0**:**0**:**roouser**@**debian7**\~$** ### tail Similar to **head,** the **tail** command will display the last ten lines of a file**.** user**@**debian7**\~$** **tail /etc/services** vboxd 20012**/**udp binkp 24554**/**tcp **#** binkp fidonet protocol asp 27374**/**tcp **#** Address Search Protocol asp 27374**/**udp csync2 30865**/**tcp **#** cluster synchronization tool dircproxy 57000**/**tcp **#** Detachable IRC Proxy tfido 60177**/**tcp **#** fidonet EMSI over telnet fido 60179**/**tcp **#** fidonet EMSI over TCP **#** Local services user**@**debian7**\~$** You can give **tail** the number of lines you want to see**.** user**@**debian7**\~$** **tail -3 /etc/services** fido 60179**/**tcp **#** fidonet EMSI over TCP **#** Local services user**@**debian7**\~$** The **tail** command has other useful options**,** some of which we will use during this course**.** **cat** The **cat** command is one of the most universal tools**,** yet all it does is copy **standard input** to **standard output.** In combination with the shell this can be very powerful and diverse**.** Some examples will give a glimpse into the possibilities**.** The first example is simple**,** you can use cat to display a file on the screen**.** If the file is longer than the screen**,** it will scroll to the end**.** user**@**debian8**:\~$** **cat /etc/resolv.conf** domain linux**-**training**.**be search linux**-**training**.**be nameserver 192**.**168**.**1**.**42 ### concatenate **cat** is short for **concatenate.** One of the basic uses of **cat** is to concatenate files into a bigger **(**or complete**)** file**.** user**@**debian8**:\~$** **echo one >part1** user**@**debian8**:\~$** **echo two >part2** user**@**debian8**:\~$** **echo three** **>part3** user**@**debian8**:\~$** **cat part1** one user**@**debian8**:\~$** **cat part2** two user**@**debian8**:\~$** **cat part3** three user**@**debian8**:\~$** **cat part1 part2 part3** one two three user**@**debian8**:\~$** **cat part1 part2 part3 >all** user**@**debian8**:\~$** **cat all** one two three user**@**debian8**:\~$** ### create files You can use **cat** to create flat text files**.** Type the **cat > winter.txt** command as shown in the screenshot below**.** Then type one or more lines**,** finishing each line with the enter key**.** After the last line**,** type and hold the Control **(**Ctrl**)** key and press d**.** user**@**debian8**:\~$** **cat > winter.txt** It is very cold today**!** user**@**debian8**:\~$** **cat winter.txt** It is very cold today**!** user**@**debian8**:\~$** The **Ctrl d** key combination will send an **EOF** **(**End of File**)** to the running process ending the **cat** command**.** ### custom end marker You can choose an end marker for **cat** with **<<** as is shown in this screenshot**.** This construction is called a **here directive** and will end the **cat** command**.** user**@**debian8**:\~$** **cat > hot.txt <\user@debian5:~~/test42$ cat
user@debian5:~~/test42$ !to touch file42 user@debian5:\~/test42$
| **file42** | ### history To see older commands**,** use **history** to display the shell command history **(**or use **history n** to see the last n commands**).** user**@**debian5**:\~/**test**$** **history 10** 1. mkdir test 2. cd test 3. touch file1 4. echo hello **>** file2 5. echo It is very cold today **>** winter**.**txt 6. ls 7. ls **-**l 8. cp winter**.**txt summer**.**txt 9. ls **-**l 10. history 10 ### !n When typing **!** followed by the number preceding the command you want repeated**,** then the shell will echo the command and execute it**.** user**@**debian5**:\~/**test**$** **!43** ls file1 file2 summer**.**txt winter**.**txt ### Ctrl-r Another option is to use **ctrl-r** to search in the history**.** In the screenshot below i only typed **ctrl-r** followed by four characters **apti** and it finds the last command containing these four consecutive characters**.** user**@**debian5**:\~$** **(**reverse**-**i**-**search**)**\`apti**’:** sudo aptitude install screen ### HISTSIZE The **$**HISTSIZE variable determines the number of commands that will be remembered in your current environment**.** Most distributions default this variable to 500 or 1000**.** user**@**debian5**:\~$** **echo $HISTSIZE** 500 You can change it to any value you like**.** user**@**debian5**:\~$** **HISTSIZE=15000** user**@**debian5**:\~$** **echo $HISTSIZE** 15000 ### HISTFILE The **$**HISTFILE variable points to the file that contains your history**.** The **bash** shell defaults this value to **\~/.bash\_history.** user**@**debian5**:\~$** **echo $HISTFILE /**home**/**user**/.**bash**\_**history A session history is saved to this file when you **exit** the session**!** *Closing a gnome**-**terminal with the mouse**,** or typing **reboot** as root will NOT save your terminal**’**s history**.*** ### HISTFILESIZE The number of commands kept in your history file can be set using **$**HISTFILESIZE**.** user**@**debian5**:\~$** **echo $HISTFILESIZE** 15000 ### prevent recording a command You can prevent a command from being recorded in **history** using a space prefix**.** user**@**debian8**:\~/**github**$** **echo abc** abc user**@**debian8**:\~/**github**$** **echo def** def user**@**debian8**:\~/**github**$** **echo ghi** ghi user**@**debian8**:\~/**github**$** **history 3** 1. echo abc 2. echo ghi ### (optional)regular expressions It is possible to use **regular expressions** when using the **bang** to repeat commands**.** The screenshot below switches 1 into 2**.** |user@debian5:~~/test$ cat
user@debian5:~~/test$ !c:s/1/2 cat file2
hello
user@debian5:\~/test$
| | **file1** | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | - | --------- | ### (optional) Korn shell history Repeating a command in the **Korn shell** is very similar**.** The Korn shell also has the **history** command**,** but uses the letter **r** to recall lines from history**.** This screenshot shows the history command**.** Note the different meaning of the parameter**.** **$** **history 17** 1. clear 2. echo hoi 3. history 12 4. echo world 5. history 17 Repeating with **r** can be combined with the line numbers given by the history command**,** or with the first few letters of the command**.** **$** **r e** echo world world **$** **cd /etc** **$** **r** cd **/**etc **$** ### Introduction to users This little chapter will teach you how to identify your user account on a Unix computer using commands like **who am i,** **id,** and more**.** In a second part you will learn how to become another user with the **su** command**.** And you will learn how to run a program as another user with **sudo.** ### whoami The **whoami** command tells you your username**.** \[user**@**centos7 **\~**]**$** **whoami** user \[user**@**centos7 **\~**]**$** ### who The **who** command will give you information about who is logged on the system**.** \[user**@**centos7 **\~**]**$** **who** root pts**/**0 2014**-**10**-**10 23**:**07 **(**10**.**104**.**33**.**101**)** user pts**/**1 2014**-**10**-**10 23**:**30 **(**10**.**104**.**33**.**101**)** laura pts**/**2 2014**-**10**-**10 23**:**34 **(**10**.**104**.**33**.**96**)** tania pts**/**3 2014**-**10**-**10 23**:**39 **(**10**.**104**.**33**.**91**)** \[user**@**centos7 **\~**]**$** ### who am i With **who am i** the **who** command will display only the line pointing to your current session**.** \[user**@**centos7 **\~**]**$** **who am i** user pts**/**1 2014**-**10**-**10 23**:**30 **(**10**.**104**.**33**.**101**)** \[user**@**centos7 **\~**]**$** ### w The **w** command shows you who is logged on and what they are doing**.** \[user**@**centos7 **\~**]**$** **w** 23**:**34**:**07 up 31 min**,** 2 users**,** load average**:** 0**.**00**,** 0**.**01**,** 0**.**02 USER TTY LOGIN**@** IDLE JCPU PCPU WHAT root pts**/**0 23**:**07 15**.**00s 0**.**01s 0**.**01s top user pts**/**1 23**:**30 7**.**00s 0**.**00s 0**.**00s w \[user**@**centos7 **\~**]**$** ### id The **id** command will give you your user id**,** primary group id**,** and a list of the groups that you belong to**.** user**@**debian7**:\~$** **id** uid=1000**(**user**)** gid=1000**(**user**)** groups=1000**(**user**)** On RHEL**/**CentOS you will also get **SELinux** context information with this command**.** \[root**@**centos7 **\~**]**#** **id** uid=0**(**root**)** gid=0**(**root**)** groups=0**(**root**)** context=unconfined**\_**u**:**unconfined**\_**r**\\** **:**unconfined**\_**t**:**s0**-**s0**:**c0**.**c1023 ### su to another user The **su** command allows a user to run a shell as another user**.** laura**@**debian7**:\~$** **su tania** Password**:** tania**@**debian7**:/**home**/**laura**$** ### su to root Yes you can also **su** to become **root,** when you know the **root password.** laura**@**debian7**:\~$** **su root** Password**:** root**@**debian7**:/**home**/**laura**#** ### su as root You need to know the password of the user you want to substitute to**,** unless your are logged in as **root.** The **root** user can become any existing user without knowing that user**’**s password**.** root**@**debian7**:\~#** **id** uid=0**(**root**)** gid=0**(**root**)** groups=0**(**root**)** root**@**debian7**:\~#** **su -** **valentina** valentina**@**debian7**:\~$** ### su - $username By default**,** the **su** command maintains the same shell environment**.** To become another user and also get the target user**’**s environment**,** issue the **su -** command followed by the target username**.** root**@**debian7**:\~#** **su laura** laura**@**debian7**:/**root**$** **exit** exit root**@**debian7**:\~#** **su - laura** laura**@**debian7**:\~$** **pwd /**home**/**laura ### su - When no username is provided to **su** or **su -,** the command will assume **root** is the target**.** tania**@**debian7**:\~$** **su -** Password**:** root**@**debian7**:\~#** ### run a program as another user The sudo program allows a user to start a program with the credentials of another user**.** Before this works**,** the system administrator has to set up the **/etc/sudoers** file**.** This can be useful to delegate administrative tasks to another user **(**without giving the root password**).** The screenshot below shows the usage of **sudo.** User **user** received the right to run **useradd** with the credentials of **root.** This allows **user** to create new users on the system without becoming **root** and without knowing the **root password.** First the command fails for **user.** user**@**debian7**:\~$** **/usr/sbin/useradd -m valentina** useradd**:** Permission denied**.** useradd**:** cannot lock **/**etc**/**passwd**;** try again later**.** But with **sudo** it works**.** user**@**debian7**:\~$** **sudo /usr/sbin/useradd -m valentina** \[sudo] password for user**:** user**@**debian7**:\~$** ### visudo Check the man page of **visudo** before playing with the **/etc/sudoers** file**.** Editing the **sudoers** is out of scope for this fundamentals book**.** user**@**rhel65**:\~$** **apropos visudo** visudo **(**8**)** **-** edit the sudoers file user**@**rhel65**:\~$** ### sudo su - On some Linux systems like Debian stable and XDebian stable**,** the **root** user does not have a password set**.** This means that it is not possible to login as **root** **(**extra security**).** To perform tasks as **root,** the first user is given all **sudo rights** via the **/etc/sudoers.** In fact all users that are members of the admin group can use sudo to run all commands as root**.** root**@**predator**-**pc**\~#** **grep admin /etc/sudoers** **#** Members of the admin group may gain root privileges **%**admin ALL=**(**ALL**)** ALL The end result of this is that the user can type **sudo su -** and become root without having to enter the root password**.** The sudo command does require you to enter your own password**.** Thus the password prompt in the screenshot below is for sudo**,** not for su**.** user**@**predator**-**pc**\~$** **sudo su -** Password**:** root**@**predator**-**pc**\~#** ### sudo logging Using **sudo** without authorization will result in a severe warning**:** user**@**rhel65**:\~$** **sudo su -** We trust you have received the usual lecture from the local System Administrator**.** It usually boils down to these three things**:** **#**1**)** Respect the privacy of others**.** **#**2**)** Think before you type**.** **#**3**)** With great power comes great responsibility**.** \[sudo] password for user**:** user is not in the sudoers file**.** This incident will be reported**.** user**@**rhel65**:\~$** The root user can see this in the **/var/log/secure** on Red Hat and in **/var/log/auth.log** on Debian**).** root**@**rhel65**:\~#** **tail /var/log/secure | grep sudo | tr -s ‘ ‘** Apr 13 16**:**03**:**42 rhel65 sudo**:** user **:** user NOT in sudoers **;** TTY=pts**/**0 **;** PWD=**\\** **/**home**/**user **;** USER=root **;** COMMAND=**/**bin**/**su **-** root**@**rhel65**:\~#** ### user management This chapter will teach you how to use **useradd,** **usermod** and **userdel** to create**,** modify and remove user accounts**.** You will need **root** access on a Linux computer to complete this chapter**.** User management on Linux can be done in three complementary ways**.** You can use the **graphical** tools provided by your distribution**.** These tools have a look and feel that depends on the distribution**.** If you are a novice Linux user on your home system**,** then use the graphical tool that is provided by your distribution**.** This will make sure that you do not run into problems**.** Another option is to use **command line tools** like useradd**,** usermod**,** gpasswd**,** passwd and others**.** Server administrators are likely to use these tools**,** since they are familiar and very similar across many different distributions**.** This chapter will focus on these command line tools**.** A third and rather extremist way is to **edit the local configuration files** directly using vi **(**or vipw**/**vigr**).** Do not attempt this as a novice on production systems**!** ### /etc/passwd The local user database on Linux **(**and on most Unixes**)** is **/etc/passwd.** \[root**@**RHEL5 **\~**]**#** **tail /etc/passwd** inge**:**x**:**518**:**524**:**art dealer**:/**home**/**inge**:/**bin**/**ksh ann**:**x**:**519**:**525**:**flute player**:/**home**/**ann**:/**bin**/**bash frederik**:**x**:**520**:**526**:**rubius poet**:/**home**/**frederik**:/**bin**/**bash steven**:**x**:**521**:**527**:**roman emperor**:/**home**/**steven**:/**bin**/**bash pascale**:**x**:**522**:**528**:**artist**:/**home**/**pascale**:/**bin**/**ksh geert**:**x**:**524**:**530**:**kernel developer**:/**home**/**geert**:/**bin**/**bash wim**:**x**:**525**:**531**:**master damuti**:/**home**/**wim**:/**bin**/**bash sandra**:**x**:**526**:**532**:**radish stresser**:/**home**/**sandra**:/**bin**/**bash As you can see**,** this file contains seven columns separated by a colon**.** The columns contain the username**,** an x**,** the user id**,** the primary group id**,** a description**,** the name of the home directory**,** and the login shell**.** More information can be found by typing **man 5 passwd.** \[root**@**RHEL5 **\~**]**#** **man 5 passwd** ### root The **root** user also called the **superuser** is the most powerful account on your Linux system**.** This user can do almost anything**,** including the creation of other users**.** The root user always has userid 0 **(**regardless of the name of the account**).** \[root**@**RHEL5 **\~**]**#** **head -1 /etc/passwd** root**:**x**:**0**:**0**:**root**:/**root**:/**bin**/**bash ### Useradd You can add users with the **useradd** command**.** The example below shows how to add a user named yanina **(**last parameter**)** and at the same time forcing the creation of the home directory **(-**m**),** setting the name of the home directory **(-**d**),** and setting a description **(-**c**).** \[root**@**RHEL5 **\~**]**#** **useradd -m -d /home/yanina -c “yanina wickmayer” yanina** \[root**@**RHEL5 **\~**]**#** **tail -1 /etc/passwd** yanina**:**x**:**529**:**529**:**yanina wickmayer**:/**home**/**yanina**:/**bin**/**bash The user named yanina received userid 529 and **primary group** id 529**.** ### /etc/default/useradd Both Red Hat Enterprise Linux and Debian**/**Debian stable have a file called **/etc/default/useradd** that contains some default user options**.** Besides using cat to display this file**,** you can also use **useradd -D.** \[root**@**RHEL4 **\~**]**#** **useradd -D** GROUP=100 HOME=**/**home INACTIVE=**-**1 EXPIRE= SHELL=**/**bin**/**bash SKEL=**/**etc**/**skel ### userdel You can delete the user yanina with **userdel.** The **-**r option of userdel will also remove the home directory**.** \[root**@**RHEL5 **\~**]**#** **userdel -r yanina** ### usermod You can modify the properties of a user with the **usermod** command**.** This example uses **usermod** to change the description of the user harry**.** \[root**@**RHEL4 **\~**]**#** **tail -1 /etc/passwd** harry**:**x**:**516**:**520**:**harry potter**:/**home**/**harry**:/**bin**/**bash \[root**@**RHEL4 **\~**]**#** **usermod -c ‘wizard’ harry** \[root**@**RHEL4 **\~**]**#** **tail -1 /etc/passwd** harry**:**x**:**516**:**520**:**wizard**:/**home**/**harry**:/**bin**/**bash ### creating home directories The easiest way to create a home directory is to supply the **-m** option with **useradd** **(**it is likely set as a default option on Linux**).** A less easy way is to create a home directory manually with **mkdir** which also requires setting the owner and the permissions on the directory with **chmod** and **chown** **(**both commands are discussed in detail in another chapter**).** \[root**@**RHEL5 **\~**]**#** **mkdir /home/laura** \[root**@**RHEL5 **\~**]**#** **chown laura:laura /home/laura** \[root**@**RHEL5 **\~**]**#** **chmod 700 /home/laura** \[root**@**RHEL5 **\~**]**#** **ls -ld /home/laura/** drwx**------** 2 laura laura 4096 Jun 24 15**:**17 **/**home**/**laura**/** ### /etc/skel/ When using **useradd** the **-m** option**,** the **/etc/skel/** directory is copied to the newly created home directory**.** The **/etc/skel/** directory contains some **(**usually hidden**)** files that contain profile settings and default values for applications**.** In this way **/etc/skel/** serves as a default home directory and as a default user profile**.** \[root**@**RHEL5 **\~**]**#** **ls -la /etc/skel/** total 48 drwxr**-**xr**-**x 2 root root 4096 Apr 1 00**:**11 **.** drwxr**-**xr**-**x 97 root root 12288 Jun 24 15**:**36 **..** **-**rw**-**r**--**r**--** 1 root root 24 Jul 12 2006 **.**bash**\_**logout **-**rw**-**r**--**r**--** 1 root root 176 Jul 12 2006 **.**bash**\_**profile **-**rw**-**r**--**r**--** 1 root root 124 Jul 12 2006 **.**bashrc ### deleting home directories The **-**r option of **userdel** will make sure that the home directory is deleted together with the user account**.** \[root**@**RHEL5 **\~**]**#** **ls -ld /home/wim/** drwx**------** 2 wim wim 4096 Jun 24 15**:**19 **/**home**/**wim**/** \[root**@**RHEL5 **\~**]**#** **userdel -r wim** \[root**@**RHEL5 **\~**]**#** **ls -ld /home/wim/** ls**:** **/**home**/**wim**/:** No such file or directory ### login shell The **/etc/passwd** file specifies the **login shell** for the user**.** In the screenshot below you can see that user annelies will log in with the **/bin/bash** shell**,** and user laura with the **/bin/ksh** shell**.** \[root**@**RHEL5 **\~**]**#** **tail -2 /etc/passwd** annelies**:**x**:**527**:**533**:**sword fighter**:/**home**/**annelies**:/**bin**/**bash laura**:**x**:**528**:**534**:**art dealer**:/**home**/**laura**:/**bin**/**ksh You can use the usermod command to change the shell for a user**.** |\[root@RHEL5 ~~]# usermod -s /bin/bash laura
\[root@RHEL5 ~~]# tail -1 /etc/passwd dealer:/home/laura:/bin/bash
| laura**:**x**:**528**:**534**:**art | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------- | ### chsh Users can change their login shell with the **chsh** command**.** First**,** user harry obtains a list of available shells **(**he could also have done a **cat /etc/shells)** and then changes his login shell to the **Korn shell** **(/**bin**/**ksh**).** At the next login**,** harry will default into ksh instead of bash**.** \[laura**@**centos7 **\~**]**$** **chsh -l** **/**bin**/**sh **/**bin**/**bash **/**sbin**/**nologin **/**usr**/**bin**/**sh **/**usr**/**bin**/**bash **/**usr**/**sbin**/**nologin **/**bin**/**ksh **/**bin**/**tcsh **/**bin**/**csh \[laura**@**centos7 **\~**]**$** Note that the **-l** option does not exist on Debian and that the above screenshot assumes that **ksh** and **csh** shells are installed**.** The screenshot below shows how **laura** can change her default shell **(**active on next login**).** \[laura**@**centos7 **\~**]**$** **chsh -s /bin/ksh** Changing shell for laura**.** Password**:** Shell changed**.** ### user passwords This chapter will tell you more about passwords for local users**.** Three methods for setting passwords are explained**;** using the **passwd** command**,** using **openssel passwd,** and using the **crypt** function in a C program**.** The chapter will also discuss password settings and disabling**,** suspending or locking accounts**.** ### passwd Passwords of users can be set with the **passwd** command**.** Users will have to provide their old password before twice entering the new one**.** \[tania**@**centos7 **\~**]**$** **passwd** Changing password for user tania**.** Changing password for tania**.** **(**current**)** UNIX password**:** New password**:** BAD PASSWORD**:** The password is shorter than 8 characters New password**:** BAD PASSWORD**:** The password is a palindrome New password**:** BAD PASSWORD**:** The password is too similar to the old one passwd**:** Have exhausted maximum number of retries for service As you can see**,** the passwd tool will do some basic verification to prevent users from using too simple passwords**.** The **root** user does not have to follow these rules **(**there will be a warning though**).** The **root** user also does not have to provide the old password before entering the new password twice**.** root**@**debian7**:\~#** passwd tania Enter new UNIX password**:** Retype new UNIX password**:** passwd**:** password updated successfully ### shadow file User passwords are encrypted and kept in **/etc/shadow.** The **/**etc**/**shadow file is read only and can only be read by root**.** We will see in the file permissions section how it is possible for users to change their password**.** For now**,** you will have to know that users can change their password with the **/usr/bin/passwd** command**.** \[root**@**centos7 **\~**]**#** **tail -4 /etc/shadow** user**:$**6**$**ikp2Xta5BT**.**Tml**.**p**$**2TZjNnOYNNQKpwLJqoGJbVsZG5**/**Fti8ovBRd**.**VzRbiDSl7TEq**\\** IaSMH**.**TeBKnTS**/**SjlMruW8qffC0JNORW**.**BTW1**:**16338**:**0**:**99999**:**7**:::** tania**:$**6**$**8Z**/**zovxj**$**9qvoqT8i9KIrmN**.**k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu**.**z4DlpvMREH**\\** eQpQzvRnqFdKkVj17H5ST**.**c79HDZw0**:**16356**:**0**:**99999**:**7**:::** laura**:$**6**$**glDuTY5e**$/**NYYWLxfHgZFWeoujaXSMcR**.**Mz**.**lGOxtcxFocFVJNb98nbTPhWFXfKWG**\\** SyYh1WCv6763Wq54**.**w24Yr3uAZBOm**/:**16356**:**0**:**99999**:**7**:::** valentina**:$**6**$**jrZa6PVI**$**1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh**.**v4iqUk9MVreui1lv7**\\** GxHOUDSKA0N55ZRNhGHa6T2ouFnVno**/**0o1**:**16356**:**0**:**99999**:**7**:::** \[root**@**centos7 **\~**]**#** The **/etc/shadow** file contains nine colon separated columns**.** The nine fields contain **(**from left to right**)** the user name**,** the encrypted password **(**note that only inge and laura have an encrypted password**),** the day the password was last changed **(**day 1 is January 1**,** 1970**),** number of days the password must be left unchanged**,** password expiry day**,** warning number of days before password expiry**,** number of days after expiry before disabling the account**,** and the day the account was disabled **(**again**,** since 1970**).** The last field has no meaning yet**.** All the passwords in the screenshot above are hashes of **hunter2.** ### Encryption with passwd Passwords are stored in an encrypted format**.** This encryption is done by the **crypt** function**.** The easiest **(**and recommended**)** way to add a user with a password to the system is to add the user with the **useradd -m user** command**,** and then set the user**’**s password with **passwd.** \[root**@**RHEL4 **\~**]**#** **useradd -m xavier** \[root**@**RHEL4 **\~**]**#** **passwd xavier** Changing password for user xavier**.** New UNIX password**:** Retype new UNIX password**:** passwd**:** all authentication tokens updated successfully**.** \[root**@**RHEL4 **\~**]**#** ### encryption with openssl Another way to create users with a password is to use the **-**p option of useradd**,** but that option requires an encrypted password**.** You can generate this encrypted password with the **openssl passwd** command**.** The **openssl passwd** command will generate several distinct hashes for the same password**,** for this it uses a **salt.** user**@**rhel65**:\~$** **openssl passwd hunter2** 86jcUNlnGDFpY user**@**rhel65**:\~$** **openssl passwd hunter2** Yj7mDO9OAnvq6 user**@**rhel65**:\~$** **openssl passwd hunter2** YqDcJeGoDbzKA user**@**rhel65**:\~$** This **salt** can be chosen and is visible as the first two characters of the hash**.** user**@**rhel65**:\~$** **openssl passwd -salt 42 hunter2** 42ZrbtP1Ze8G**.** user**@**rhel65**:\~$** **openssl passwd -salt 42 hunter2** 42ZrbtP1Ze8G**.** user**@**rhel65**:\~$** **openssl passwd -salt 42 hunter2** 42ZrbtP1Ze8G**.** user**@**rhel65**:\~$** This example shows how to create a user with password**.** root**@**rhel65**:\~#** **useradd -m -p $(openssl passwd hunter2) mohamed** ### groups Users can be listed in **groups.** Groups allow you to set permissions on the group level instead of having to set permissions for every individual user**.** Every Unix or Linux distribution will have a graphical tool to manage groups**.** Novice users are advised to use this graphical tool**.** More experienced users can use command line tools to manage users**,** but be careful**:** Some distributions do not allow the mixed use of GUI and CLI tools to manage groups **(**YaST in Novell Suse**).** Senior administrators can edit the relevant files directly with **vi** or **vigr.** ### groupadd Groups can be created with the **groupadd** command**.** The example below shows the creation of five **(**empty**)** groups**.** | root**@**predator**-**pc**\~#** | groupadd | tennis | | ----------------------------------------------------------------------- | -------- | -------- | | root**@**predator**-**pc**\~#** | groupadd | football | | root**@**predator**-**pc**\~#** | groupadd | snooker | | root**@**predator**-**pc**\~#** | groupadd | formula1 | | root**@**predator**-**pc**\~#** groupadd salsa | | | ### group file Users can be a member of several groups**.** Group membership is defined by the **/etc/group** file**.** root**@**predator**-**pc**\~#** tail **-**5 **/**etc**/**group tennis**:**x**:**1006**:** football**:**x**:**1007**:** snooker**:**x**:**1008**:** formula1**:**x**:**1009**:** salsa**:**x**:**1010**:** root**@**predator**-**pc**\~#** The first field is the group**’**s name**.** The second field is the group**’**s **(**encrypted**)** password **(**can be empty**).** The third field is the group identification or **GID.** The fourth field is the list of members**,** these groups have no members**.** ### groups A user can type the **groups** command to see a list of groups where the user belongs to**.** \[harry**@**RHEL4b **\~**]**$** groups harry sports \[harry**@**RHEL4b **\~**]**$** ### usermod Group membership can be modified with the useradd or **usermod** command**.** root**@**predator**-**pc**\~#** usermod **-**a **-**G tennis inge root**@**predator**-**pc**\~#** usermod **-**a **-**G tennis katrien root**@**predator**-**pc**\~#** usermod **-**a **-**G salsa katrien root**@**predator**-**pc**\~#** usermod **-**a **-**G snooker sandra root**@**predator**-**pc**\~#** usermod **-**a **-**G formula1 annelies root**@**predator**-**pc**\~#** tail **-**5 **/**etc**/**group tennis**:**x**:**1006**:**inge**,**katrien football**:**x**:**1007**:** snooker**:**x**:**1008**:**sandra formula1**:**x**:**1009**:**annelies salsa**:**x**:**1010**:**katrien root**@**predator**-**pc**\~#** Be careful when using **usermod** to add users to groups**.** By default**,** the **usermod** command will **remove** the user from every group of which he is a member if the group is not listed in the command**!** Using the **-a** **(**append**)** switch prevents this behaviour**.** ### groupmod You can change the group name with the **groupmod** command**.** root**@**predator**-**pc**\~#** groupmod **-**n darts snooker root**@**predator**-**pc**\~#** tail **-**5 **/**etc**/**group tennis**:**x**:**1006**:**inge**,**katrien football**:**x**:**1007**:** formula1**:**x**:**1009**:**annelies salsa**:**x**:**1010**:**katrien darts**:**x**:**1008**:**sandra ### gpasswd You can delegate control of group membership to another user with the **gpasswd** command**.** In the example below we delegate permissions to add and remove group members to serena for the sports group**.** Then we **su** to serena and add harry to the sports group**.** \[root**@**RHEL4b **\~**]**#** gpasswd **-**A serena sports \[root**@**RHEL4b **\~**]**#** su **-** serena \[serena**@**RHEL4b **\~**]**$** id harry uid=516**(**harry**)** gid=520**(**harry**)** groups=520**(**harry**)** \[serena**@**RHEL4b **\~**]**$** gpasswd **-**a harry sports Adding user harry to group sports \[serena**@**RHEL4b **\~**]**$** id harry uid=516**(**harry**)** gid=520**(**harry**)** groups=520**(**harry**),**522**(**sports**)** \[serena**@**RHEL4b **\~**]**$** tail **-**1 **/**etc**/**group sports**:**x**:**522**:**serena**,**venus**,**harry \[serena**@**RHEL4b **\~**]**$** Group administrators do not have to be a member of the group**.** They can remove themselves from a group**,** but this does not influence their ability to add or remove members**.** \[serena**@**RHEL4b **\~**]**$** gpasswd **-**d serena sports Removing user serena from group sports \[serena**@**RHEL4b **\~**]**$** exit Information about group administrators is kept in the **/etc/gshadow** file**.** \[root**@**RHEL4b **\~**]**#** tail **-**1 **/**etc**/**gshadow sports**:!:**serena**:**venus**,**harry \[root**@**RHEL4b **\~**]**#** To remove all group administrators from a group**,** use the **gpasswd** command to set an empty administrators list**.** \[root**@**RHEL4b **\~**]**#** gpasswd **-**A **““** sports ### newgrp You can start a **child shell** with a new temporary **primary group** using the **newgrp** command**.** root**@**rhel65**:\~#** mkdir prigroup root**@**rhel65**:\~#** cd prigroup**/** root**@**rhel65**:\~/**prigroup**#** touch standard**.**txt root**@**rhel65**:\~/**prigroup**#** ls **-**l total 0 **-**rw**-**r**--**r**--.** 1 root root 0 Apr 13 17**:**49 standard**.**txt root**@**rhel65**:\~/**prigroup**#** echo **$**SHLVL 1 root**@**rhel65**:\~/**prigroup**#** newgrp tennis root**@**rhel65**:\~/**prigroup**#** echo **$**SHLVL 2 root**@**rhel65**:\~/**prigroup**#** touch newgrp**.**txt root**@**rhel65**:\~/**prigroup**#** ls **-**l total 0 **-**rw**-**r**--**r**--.** 1 root tennis 0 Apr 13 17**:**49 newgrp**.**txt **-**rw**-**r**--**r**--.** 1 root root 0 Apr 13 17**:**49 standard**.**txt root**@**rhel65**:\~/**prigroup**#** exit exit root**@**rhel65**:\~/**prigroup**#** ### user profiles Logged on users have a number of preset **(**and customized**)** aliases**,** variables**,** and functions**,** but where do they come from ? The **shell** uses a number of startup files that are executed **(**or rather **sourced)** whenever the shell is invoked**.** What follows is an overview of startup scripts**.** ### system profile Both the **bash** and the **ksh** shell will verify the existence of **/etc/profile** and **source** it if it exists**.** When reading this script**,** you will notice **(**both on Debian and on Red Hat Enterprise Linux**)** that it builds the PATH environment variable **(**among others**).** The script might also change the PS1 variable**,** set the HOSTNAME and execute even more scripts like **/etc/inputrc** This screenshot uses grep to show PATH manipulation in **/etc/profile** on Debian**.** root**@**debian7**:\~#** **grep PATH /etc/profile** PATH=**“/**usr**/**local**/**sbin**:/**usr**/**local**/**bin**:/**usr**/**sbin**:/**usr**/**bin**:/**sbin**:/**bin**”** PATH=**“/**usr**/**local**/**bin**:/**usr**/**bin**:/**bin**:/**usr**/**local**/**games**:/**usr**/**games**”** export PATH root**@**debian7**:\~#** This screenshot uses grep to show PATH manipulation in **/etc/profile** on RHEL7**/**CentOS7**.** \[root**@**centos7 **\~**]**#** **grep PATH /etc/profile** case **“:$**{PATH}**:”** in PATH=**$**PATH**:$**1 PATH=**$**1**:$**PATH export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL \[root**@**centos7 **\~**]**#** The **root user** can use this script to set aliases**,** functions**,** and variables for every user on the system**.** ### bash\_profile When this file exists in the home directory**,** then **bash** will source it**.** On Debian Linux 5**/**6**/**7 this file does not exist by default**.** RHEL7**/**CentOS7 uses a small **\~/.bash\_profile** where it checks for the existence of **\~/.bashrc** and then sources it**.** It also adds **$**HOME**/**bin to the **$**PATH variable**.** \[root**@**rhel7 **\~**]**#** **cat /home/user/.bash\_profile** **#** **.**bash**\_**profile **#** Get the aliases and functions if \[ **-**f **\~/.**bashrc ]**;** then **.** **\~/.**bashrc fi **#** User specific environment and startup programs PATH=**$**PATH**:$**HOME**/.**local**/**bin**:$**HOME**/**bin export PATH \[root**@**rhel7 **\~**]**#** ### bash\_login When **.bash\_profile** does not exist**,** then **bash** will check for **\~/.bash\_login** and source it**.** Neither Debian nor Red Hat have this file by default**.** ### profile When neither **\~/.bash\_profile** and **\~/.bash\_login** exist**,** then bash will verify the existence of **\~/.profile** and execute it**.** This file does not exist by default on Red Hat**.** On Debian this script can execute **\~/.bashrc** and will add **$**HOME**/**bin to the **$**PATH variable**.** root**@**debian7**:\~#** **tail -11 /home/user/.profile** if \[ **-**n **“$**BASH**\_**VERSION**”** ]**;** then **#** include **.**bashrc if it exists if \[ **-**f **“$**HOME**/.**bashrc**”** ]**;** then **.** **“$**HOME**/.**bashrc**”** fi fi **#** set PATH so it includes user**’**s private bin if it exists if \[ **-**d **“$**HOME**/**bin**”** ] **;** then PATH=**“$**HOME**/**bin**:$**PATH**”** fi RHEL**/**CentOS does not have this file by default**.** ### bashrc The **\~/.bashrc** script is often sourced by other scripts**.** Let us take a look at what it does by default**.** Red Hat uses a very simple **\~/.bashrc,** checking for **/etc/bashrc** and sourcing it**.** It also leaves room for custom aliases and functions**.** \[root**@**rhel7 **\~**]**#** **cat /home/user/.bashrc** **#** **.**bashrc **#** Source global definitions if \[ **-**f **/**etc**/**bashrc ]**;** then **.** **/**etc**/**bashrc fi **#** Uncomment the following line if you don**’**t like systemctl**’**s auto**-**paging feature**:** **#** export SYSTEMD**\_**PAGER= **#** User specific aliases and functions On Debian this script is quite a bit longer and configures **$**PS1**,** some history variables and a number af active and inactive aliases**.** root**@**debian7**:\~#** wc **-**l **/**home**/**user**/.**bashrc 110 **/**home**/**user**/.**bashrc ### bash\_logout When exiting **bash,** it can execute **\~/.bash\_logout.** Debian use this opportunity to clear the console screen**.** serena**@**deb503**:\~$** **cat .bash\_logout #** **\~/.**bash**\_**logout**:** executed by bash**(**1**)** when login shell exits**.** **#** when leaving the console clear the screen to increase privacy if \[ **“$**SHLVL**”** = 1 ]**;** then \[ **-**x **/**usr**/**bin**/**clear**\_**console ] **&&** **/**usr**/**bin**/**clear**\_**console **-**q fi Red Hat Enterprise Linux 5 will simple call the **/usr/bin/clear** command in this script**.** \[serena**@**rhel53 **\~**]**$** cat **.**bash**\_**logout **#** **\~/.**bash**\_**logout **/**usr**/**bin**/**clear Red Hat Enterprise Linux 6 and 7 create this file**,** but leave it empty **(**except for a comment**).** user**@**rhel65**:\~$** **cat .bash\_logout** **#** **\~/.**bash**\_**logout