Linux: Useful commands for beginners

1. Command: ls

The command “ls” stands for (List Directory Contents), List the contents of the folder, be it file or folder, from which it runs.

root@tecmint:~# ls

Android-Games                     Music
Pictures                          Public
Desktop                           Tecmint.com
Documents                         TecMint-Sync
Downloads                         Templates

The command “ls -l” list the content of folder, in long listingfashion.

root@tecmint:~# ls -l

total 40588
drwxrwxr-x 2 ravisaive ravisaive     4096 May  8 01:06 Android Games
drwxr-xr-x 2 ravisaive ravisaive     4096 May 15 10:50 Desktop
drwxr-xr-x 2 ravisaive ravisaive     4096 May 16 16:45 Documents
drwxr-xr-x 6 ravisaive ravisaive     4096 May 16 14:34 Downloads
drwxr-xr-x 2 ravisaive ravisaive     4096 Apr 30 20:50 Music
drwxr-xr-x 2 ravisaive ravisaive     4096 May  9 17:54 Pictures
drwxrwxr-x 5 ravisaive ravisaive     4096 May  3 18:44 Tecmint.com
drwxr-xr-x 2 ravisaive ravisaive     4096 Apr 30 20:50 Templates

Command “ls -a“, list the content of folder, including hiddenfiles starting with ‘.’.

root@tecmint:~# ls -a

.   .gnupg   .dbus   .goutputstream-PI5VVW  .mission-control
.adobe                  deja-dup                .grsync                 .mozilla                  .themes
.gstreamer-0.10         .mtpaint                .thumbnails             .gtk-bookmarks           .thunderbird
.HotShots               .mysql_history          .htaccess  .apport-ignore.xml       .ICEauthority           
.profile                .bash_history           .icons                  .bash_logout                    .fbmessenger
.jedit                  .pulse                  .bashrc                 .liferea_1.8              .pulse-cookie            
.Xauthority  .gconf                  .local                  .Xauthority.HGHVWW  .cache
.gftp                   .macromedia             .remmina                .cinnamon                       .gimp-2.8
.ssh                    .xsession-errors  .compiz                 .gnome                          teamviewer_linux.deb          
.xsession-errors.old .config                 .gnome2                 .zoncolor

Note: In Linux file name starting with ‘.‘ is hidden. In Linux every file/folder/device/command is a file. The output of ls -lis:

1. d (stands for directory).
2. rwxr-xr-x is the file permission of the file/folder for owner, group and world.
3. The 1st ravisaive in the above example means that file is owned by user ravisaive.
4. The 2nd ravisaive in the above example means file belongs to user group ravisaive.
5. 4096 means file size is 4096 Bytes.
6. May 8 01:06 is the date and time of last modification.
7. And at the end is the name of the File/Folder.

For more “ls” command examples read 15 ‘ls’ Command Examples in Linux.

2. Command: lsblk

The “lsblk” stands for (List Block Devices), print block devices by their assigned name (but not RAM) on the standard output in a tree-like fashion.

root@tecmint:~# lsblk

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 232.9G  0 disk 
├─sda1   8:1    0  46.6G  0 part /
├─sda2   8:2    0     1K  0 part 
├─sda5   8:5    0   190M  0 part /boot
├─sda6   8:6    0   3.7G  0 part [SWAP]
├─sda7   8:7    0  93.1G  0 part /data
└─sda8   8:8    0  89.2G  0 part /personal
sr0     11:0    1  1024M  0 rom

The “lsblk -l” command list block devices in ‘list‘ structure (not tree like fashion).

root@tecmint:~# lsblk -l

NAME MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda    8:0    0 232.9G  0 disk 
sda1   8:1    0  46.6G  0 part /
sda2   8:2    0     1K  0 part 
sda5   8:5    0   190M  0 part /boot
sda6   8:6    0   3.7G  0 part [SWAP]
sda7   8:7    0  93.1G  0 part /data
sda8   8:8    0  89.2G  0 part /personal
sr0   11:0    1  1024M  0 rom

Note: lsblk is very useful and easiest way to know the name of New Usb Device you just plugged in, especially when you have to deal with disk/blocks in terminal.

3. Command: md5sum

The “md5sum” stands for (Compute and Check MD5 Message Digest), md5 checksum (commonly called hash) is used to match or verify integrity of files that may have changed as a result of a faulty file transfer, a disk error or non-malicious interference.

root@tecmint:~# md5sum teamviewer_linux.deb 

47790ed345a7b7970fc1f2ac50c97002  teamviewer_linux.deb

Note: The user can match the generated md5sum with the one provided officially. Md5sum is considered less secure than sha1sum, which we will discuss later.

4. Command: dd

Command “dd” stands for (Convert and Copy a file), Can be used to convert and copy a file and most of the times is used to copy a iso file (or any other file) to a usb device (or any other location), thus can be used to make a ‘Bootlable‘ Usb Stick.

root@tecmint:~# dd if=/home/user/Downloads/debian.iso of=/dev/sdb1 bs=512M; sync

Note: In the above example the usb device is supposed to be sdb1 (You should Verify it using command lsblk, otherwise you will overwrite your disk and OS), use name of disk very Cautiously!!!.

dd command takes some time ranging from a few seconds to several minutes in execution, depending on the size and type of file and read and write speed of Usb stick.

5. Command: uname

The “uname” command stands for (Unix Name), print detailed information about the machine name, Operating System and Kernel.

root@tecmint:~# uname -a

Linux tecmint 3.8.0-19-generic #30-Ubuntu SMP Wed May 1 16:36:13 UTC 2013 i686 i686 i686 GNU/Linux

Note: uname shows type of kernel. uname -a output detailed information. Elaborating the above output of uname -a.

1. “Linux“: The machine’s kernel name.
2. “tecmint“: The machine’s node name.
3. “3.8.0-19-generic“: The kernel release.
4. “#30-Ubuntu SMP“: The kernel version.
5. “i686“: The architecture of the processor.
6. “GNU/Linux“: The operating system name.

6. Command: history

The “history” command stands for History (Event) Record, it prints the history of long list of executed commands in terminal.

root@tecmint:~# history

 1  sudo add-apt-repository ppa:tualatrix/ppa
 2  sudo apt-get update
 3  sudo apt-get install ubuntu-tweak
 4  sudo add-apt-repository ppa:diesch/testing
 5  sudo apt-get update
 6  sudo apt-get install indicator-privacy
 7  sudo add-apt-repository ppa:atareao/atareao
 8  sudo apt-get update
 9  sudo apt-get install my-weather-indicator
 10 pwd
 11 cd && sudo cp -r unity/6 /usr/share/unity/
 12 cd /usr/share/unity/icons/
 13 cd /usr/share/unity

Note: Pressing “Ctrl + R” and then search for already executed commands which lets your command to be completed with auto completion feature.

(reverse-i-search)`if': ifconfig

7. Command: sudo

The “sudo” (super user do) command allows a permitted user to execute a command as the superuser or another user, as specified by the security policy in the sudoers list.

root@tecmint:~# sudo add-apt-repository ppa:tualatrix/ppa

Note: sudo allows user to borrow superuser privileged, while a similar command ‘su‘ allows user to actually log in as superuser. Sudo is safer than su.
It is not advised to use sudo or su for day-to-day normal use, as it can result in serious error if accidentally you did something wrong, that’s why a very popular saying in Linux community is:

“To err is human, but to really foul up everything, you need root password.”

8. Command: mkdir

The “mkdir” (Make directory) command create a new directory with name path. However is the directory already exists, it will return an error message “cannot create folder, folder already exists”.

root@tecmint:~# mkdir tecmint

Note: Directory can only be created inside the folder, in which the user has write permission. mkdir: cannot create directory `tecmint‘: File exists
(Don’t confuse with file in the above output, you might remember what i said at the beginning – In Linux every file, folder, drive, command, scripts are treated as file).

9. Command: touch

The “touch” command stands for (Update the access and modification times of each FILE to the current time). touchcommand creates the file, only if it doesn’t exist. If the file already exists it will update the timestamp and not the contents of the file.

root@tecmint:~# touch tecmintfile

Note: touch can be used to create file under directory, on which the user has write permission, only if the file don’t exist there.

10. Command: chmod

The Linux “chmod” command stands for (change file mode bits). chmod changes the file mode (permission) of each given file, folder, script, etc.. according to mode asked for.

There exist 3 types of permission on a file (folder or anything but to keep things simple we will be using file).

Read (r)=4
Write(w)=2
Execute(x)=1

So if you want to give only read permission on a file it will be assigned a value of ‘4‘, for write permission only, a value of ‘2‘ and for execute permission only, a value of ‘1‘ is to be given. For read and write permission 4+2 = ‘6‘ is to be given, ans so on.

Now permission need to be set for 3 kinds of user and usergroup. The first is owner, then usergroup and finally world.

rwxr-x--x   abc.sh

Here the root’s permission is rwx (read, write and execute).
usergroup to which it belongs, is r-x (read and execute only, no write permission) and
for world is –x (only execute).

To change its permission and provide read, write and executepermission to owner, group and world.

root@tecmint:~# chmod 777 abc.sh

only read and write permission to all three.

root@tecmint:~# chmod 666 abc.sh

read, write and execute to owner and only execute to groupand world.

root@tecmint:~# chmod 711 abc.sh

Note: one of the most important command useful for sysadmin and user both. On a multi-user environment or on a server, this command comes to rescue, setting wrong permission will either makes a file inaccessible or provide unauthorized access to someone.

11. Command: chown

The Linux “chown” command stands for (change file owner and group). Every file belongs to a group of user and a owner. It is used Do ‘ls -l‘ into your directory and you will see something like this.

root@tecmint:~# ls -l 

drwxr-xr-x 3 server root 4096 May 10 11:14 Binary 
drwxr-xr-x 2 server server 4096 May 13 09:42 Desktop

Here the directory Binary is owned by user “server” and it belongs to usergroup “root” where as directory “Desktop” is owned by user “server” and belongs to user group “server“.

This “chown” command is used to change the file ownership and thus is useful in managing and providing file to authorised user and usergroup only.

root@tecmint:~# chown server:server Binary

drwxr-xr-x 3 server server 4096 May 10 11:14 Binary 
drwxr-xr-x 2 server server 4096 May 13 09:42 Desktop

Note: “chown” changes the user and group ownership of each given FILE to NEW-OWNER or to the user and group of an existing reference file.

12. Command: apt

The Debian based “apt” command stands for (Advanced Package Tool). Apt is an advanced package manager forDebian based system (Ubuntu, Kubuntu, etc.), that automatically and intelligently search, install, update andresolves dependency of packages on Gnu/Linux system from command line.

root@tecmint:~# apt-get install mplayer

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  java-wrappers
Use 'apt-get autoremove' to remove it.
The following extra packages will be installed:
  esound-common libaudiofile1 libesd0 libopenal-data libopenal1 libsvga1 libvdpau1 libxvidcore4
Suggested packages:
  pulseaudio-esound-compat libroar-compat2 nvidia-vdpau-driver vdpau-driver mplayer-doc netselect fping
The following NEW packages will be installed:
  esound-common libaudiofile1 libesd0 libopenal-data libopenal1 libsvga1 libvdpau1 libxvidcore4 mplayer
0 upgraded, 9 newly installed, 0 to remove and 8 not upgraded.
Need to get 3,567 kB of archives.
After this operation, 7,772 kB of additional disk space will be used.
Do you want to continue [Y/n]? y

root@tecmint:~# apt-get update

Hit http://ppa.launchpad.net raring Release.gpg                                           
Hit http://ppa.launchpad.net raring Release.gpg                                           
Hit http://ppa.launchpad.net raring Release.gpg                      
Hit http://ppa.launchpad.net raring Release.gpg                      
Get:1 http://security.ubuntu.com raring-security Release.gpg [933 B] 
Hit http://in.archive.ubuntu.com raring Release.gpg                                                   
Hit http://ppa.launchpad.net raring Release.gpg                      
Get:2 http://security.ubuntu.com raring-security Release [40.8 kB]   
Ign http://ppa.launchpad.net raring Release.gpg                                                  
Get:3 http://in.archive.ubuntu.com raring-updates Release.gpg [933 B]                            
Hit http://ppa.launchpad.net raring Release.gpg                                                                
Hit http://in.archive.ubuntu.com raring-backports Release.gpg

Note: The above commands results into system-wide changes and hence requires root password (Check ‘#‘ and not ‘$’ as prompt). Apt is considered more advanced and intelligent as compared to yum command.

As the name suggest, apt-cache search for package containing sub package mpalyer. apt-get install, update all the packages, that are already installed, to the newest one.

Read more about apt-get and apt-cache commands at 25 APT-GET and APT-CACHE Commands

13. Command: tar

The “tar” command is a Tape Archive is useful in creation of archive, in a number of file format and their extraction.

root@tecmint:~# tar -zxvf abc.tar.gz (Remember 'z' for .tar.gz)

root@tecmint:~# tar -jxvf abc.tar.bz2 (Remember 'j' for .tar.bz2)

root@tecmint:~# tar -cvf archieve.tar.gz(.bz2) /path/to/folder/abc

Note: A ‘tar.gz‘ means gzipped. ‘tar.bz2‘ is compressed with bzip which uses a better but slower compression method.

Read more about “tar command” examples at 18 Tar Command Examples

14. Command: cal

The “cal” (Calendar), it is used to displays calendar of the present month or any other month of any year that is advancing or passed.

root@tecmint:~# cal 

May 2013        
Su Mo Tu We Th Fr Sa  
          1  2  3  4  
 5  6  7  8  9 10 11  
12 13 14 15 16 17 18  
19 20 21 22 23 24 25  
26 27 28 29 30 31

Show calendar of year 1835 for month February, that already has passed.

root@tecmint:~# cal 02 1835

   February 1835      
Su Mo Tu We Th Fr Sa  
 1  2  3  4  5  6  7  
 8  9 10 11 12 13 14  
15 16 17 18 19 20 21  
22 23 24 25 26 27 28

Shows calendar of year 2145 for the month of July, that will advancing

root@tecmint:~# cal 07 2145

     July 2145        
Su Mo Tu We Th Fr Sa  
             1  2  3  
 4  5  6  7  8  9 10  
11 12 13 14 15 16 17  
18 19 20 21 22 23 24  
25 26 27 28 29 30 31

Note: You need not to turn the calendar of 50 years back, neither you need to make complex mathematical calculation to know what day you were worn or your coming birthday will fall on which day.

15. Command: date

The “date” (Date) command print the current date and time on the standard output, and can further be set.

root@tecmint:~# date

Fri May 17 14:13:29 IST 2013

root@tecmint:~# date --set='14 may 2013 13:57' 

Mon May 13 13:57:00 IST 2013

Note: This Command will be very use-full in scripting, time and date based scripting, to be more perfect. Moreover changing date and time using terminal will make you feelGEEK!!!. (Obviously you need to be root to perform this operation, as it is a system wide change).

16. Command: cat

The “cat” stands for (Concatenation). Concatenate (join) two or more plain file and/or print contents of a file on standard output.

root@tecmint:~# cat a.txt b.txt c.txt d.txt abcd.txt

root@tecmint:~# cat abcd.txt
....
contents of file abcd
...

Note: “>>” and “>” are called append symbol. They are used to append the output to a file and not on standard output. “>” symbol will delete a file already existed and create a new file hence for security reason it is advised to use “>>” that will write the output without overwriting or deleting the file.

Before Proceeding further, I must let you know about wildcards (you would be aware of wildcard entry, in most of the Television shows) Wildcards are a shell feature that makes the command line much more powerful than any GUIfile managers. You see, if you want to select a big group of files in a graphical file manager, you usually have to select them with your mouse. This may seem simple, but in some cases it can be very frustrating.

For example, suppose you have a directory with a huge amount of all kinds of files and subdirectories, and you decide to move all the HTML files, that have the word “Linux” somewhere in the middle of their names, from that big directory into another directory. What’s a simple way to do this? If the directory contains a huge amount of differently named HTML files, your task is everything but simple!

In the Linux CLI that task is just as simple to perform as moving only one HTML file, and it’s so easy because of the shell wildcards. These are special characters that allow you to select file names that match certain patterns of characters. This helps you to select even a big group of files with typing just a few characters, and in most cases it’s easier than selecting the files with a mouse.

Here’s a list of the most commonly used wildcards :

Wildcard   Matches
   *   zero or more characters
   ?   exactly one character
[abcde]   exactly one character listed
 [a-e]   exactly one character in the given range
[!abcde]  any character that is not listed
 [!a-e]   any character that is not in the given range
{debian,linux}  exactly one entire word in the options given

! is called not symbol, and the reverse of string attached with ‘!’ is true.

Read more examples of Linux “cat command” at 13 Cat Command Examples in Linux

17. Command: cp

The “copy” stands for (Copy), it copies a file from one location to another location.

root@tecmint:~# cp /home/user/Downloads abc.tar.gz /home/user/Desktop (Return 0 when sucess)

Note: cp is one of the most commonly used command in shell scripting and it can be used with wildcard characters (Describe in the above block), for customised and desired file copying.

18. Command: mv

The “mv” command moves a file from one location to another location.

root@tecmint:~# mv /home/user/Downloads abc.tar.gz /home/user/Desktop (Return 0 when sucess)

Note: mv command can be used with wildcard characters. mvshould be used with caution, as moving of system/unauthorised file may lead to security as well as breakdown of system.

19. Command: pwd

The command “pwd” (print working directory), prints the current working directory with full path name from terminal.

root@tecmint:~# pwd 

/home/user/Desktop

Note: This command won’t be much frequently used in scripting but it is an absolute life saver for newbie who gets lost in terminal in their early connection with nux. (Linux is most commonly referred as nux or nix).

20. Command: cd

Finally, the frequently used “cd” command stands for (change directory), it change the working directory to execute, copy, move write, read, etc. from terminal itself.

root@tecmint:~# cd /home/user/Desktop

server@localhost:~$ pwd

/home/user/Desktop

Note: cd comes to rescue when switching between directories from terminal. “Cd ~” will change the working directory to user’s home directory, and is very useful if a user finds himself lost in terminal. “Cd ..” will change the working directory to parent directory (of current working directory).

These commands will surely make you comfortable withLinux. But it’s not the end. Very soon I will be coming with other commands which will be useful for ‘Middle Level User‘ i.e., You! No don’t exclaim, if you get used-to these commands, You will notice promotion in user-level fromnewbie to Middle-level-user. In the next article, I will be coming up with commands like ‘Kill‘, ‘Ps‘, ‘grep‘,….Wait for the article and I don’t want to spoil your interest.

21. Command: Find

Search for files in the given directory, hierarchically starting at the parent directory and moving to sub-directories.

root@tecmint:~# find -name *.sh 

./Desktop/load.sh 
./Desktop/test.sh 
./Desktop/shutdown.sh 
./Binary/firefox/run-mozilla.sh 
./Downloads/kdewebdev-3.5.8/quanta/scripts/externalpreview.sh 
./Downloads/kdewebdev-3.5.8/admin/doxygen.sh 
./Downloads/kdewebdev-3.5.8/admin/cvs.sh 
./Downloads/kdewebdev-3.5.8/admin/ltmain.sh 
./Downloads/wheezy-nv-install.sh

Note: The `-name‘ option makes the search case sensitive. You can use the `-iname‘ option to find something regardless of case. (* is a wildcard and searches all the file having extension ‘.sh‘ you can use filename or a part of file name to customise the output).

root@tecmint:~# find -iname *.SH ( find -iname *.Sh /  find -iname *.sH)

./Desktop/load.sh 
./Desktop/test.sh 
./Desktop/shutdown.sh 
./Binary/firefox/run-mozilla.sh 
./Downloads/kdewebdev-3.5.8/quanta/scripts/externalpreview.sh 
./Downloads/kdewebdev-3.5.8/admin/doxygen.sh 
./Downloads/kdewebdev-3.5.8/admin/cvs.sh 
./Downloads/kdewebdev-3.5.8/admin/ltmain.sh 
./Downloads/wheezy-nv-install.sh

root@tecmint:~# find -name *.tar.gz 

/var/www/modules/update/tests/aaa_update_test.tar.gz 
./var/cache/flashplugin-nonfree/install_flash_player_11_linux.i386.tar.gz 
./home/server/Downloads/drupal-7.22.tar.gz 
./home/server/Downloads/smtp-7.x-1.0.tar.gz 
./home/server/Downloads/noreqnewpass-7.x-1.2.tar.gz 
./usr/share/gettext/archive.git.tar.gz 
./usr/share/doc/apg/php.tar.gz 
./usr/share/doc/festival/examples/speech_pm_1.0.tar.gz 
./usr/share/doc/argyll/examples/spyder2.tar.gz 
./usr/share/usb_modeswitch/configPack.tar.gz

Note: The above command searches for all the file having extension ‘tar.gz‘ in root directory and all the sub-directories including mounted devices.

Read more examples of Linux ‘find‘ command at 35 Find Command Examples in Linux

22. Command: grep

The ‘grep‘ command searches the given file for lines containing a match to the given strings or words. Search ‘/etc/passwd‘ for ‘tecmint‘ user.

root@tecmint:~# grep tecmint /etc/passwd 

tecmint:x:1000:1000:Tecmint,,,:/home/tecmint:/bin/bash

Ignore word case and all other combination with ‘-i‘ option.

root@tecmint:~# grep -i TECMINT /etc/passwd 

tecmint:x:1000:1000:Tecmint,,,:/home/tecmint:/bin/bash

Search recursively (-r) i.e. read all files under each directory for a string “127.0.0.1“.

root@tecmint:~# grep -r "127.0.0.1" /etc/ 

/etc/vlc/lua/http/.hosts:127.0.0.1
/etc/speech-dispatcher/modules/ivona.conf:#IvonaServerHost "127.0.0.1"
/etc/mysql/my.cnf:bind-address  = 127.0.0.1
/etc/apache2/mods-available/status.conf:    Allow from 127.0.0.1 ::1
/etc/apache2/mods-available/ldap.conf:    Allow from 127.0.0.1 ::1
/etc/apache2/mods-available/info.conf:    Allow from 127.0.0.1 ::1
/etc/apache2/mods-available/proxy_balancer.conf:#    Allow from 127.0.0.1 ::1
/etc/security/access.conf:#+ : root : 127.0.0.1
/etc/dhcp/dhclient.conf:#prepend domain-name-servers 127.0.0.1;
/etc/dhcp/dhclient.conf:#  option domain-name-servers 127.0.0.1;
/etc/init/network-interface.conf: ifconfig lo 127.0.0.1 up || true
/etc/java-6-openjdk/net.properties:# localhost & 127.0.0.1).
/etc/java-6-openjdk/net.properties:# http.nonProxyHosts=localhost|127.0.0.1
/etc/java-6-openjdk/net.properties:# localhost & 127.0.0.1).
/etc/java-6-openjdk/net.properties:# ftp.nonProxyHosts=localhost|127.0.0.1
/etc/hosts:127.0.0.1 localhost

Note: You can use these following options along with grep.

1. -w for word (egrep -w ‘word1|word2‘ /path/to/file).
2. -c for count (i.e., total number of times the pattern matched) (grep -c ‘word‘ /path/to/file).
3. –color for coloured output (grep –color server /etc/passwd).

23. Command: man

The ‘man‘ is the system’s manual pager. Man provides online documentation for all the possible options with a command and its usages. Almost all the command comes with their corresponding manual pages. For example,

root@tecmint:~# man man

MAN(1)                                                               Manual pager utils                                                              MAN(1)

NAME
       man - an interface to the on-line reference manuals

SYNOPSIS
       man  [-C  file]  [-d]  [-D]  [--warnings[=warnings]]  [-R  encoding]  [-L  locale]  [-m  system[,...]]  [-M  path]  [-S list] [-e extension] [-i|-I]
       [--regex|--wildcard] [--names-only] [-a] [-u] [--no-subpages] [-P pager] [-r prompt] [-7] [-E encoding] [--no-hyphenation] [--no-justification]  [-p
       string] [-t] [-T[device]] [-H[browser]] [-X[dpi]] [-Z] [[section] page ...] ...
       man -k [apropos options] regexp ...
       man -K [-w|-W] [-S list] [-i|-I] [--regex] [section] term ...
       man -f [whatis options] page ...
       man -l [-C file] [-d] [-D] [--warnings[=warnings]] [-R encoding] [-L locale] [-P pager] [-r prompt] [-7] [-E encoding] [-p string] [-t] [-T[device]]
       [-H[browser]] [-X[dpi]] [-Z] file ...
       man -w|-W [-C file] [-d] [-D] page ...
       man -c [-C file] [-d] [-D] page ...
       man [-hV]

Manual page for man page itself, similarly ‘man cat‘ (Manual page for cat command) and ‘man ls‘ (Manual page forcommand ls).

Note: man page is intended for command reference and learning.

24. Command: ps

ps (Process) gives the status of running processes with a unique Id called PID.

root@tecmint:~# ps

 PID TTY          TIME CMD
 4170 pts/1    00:00:00 bash
 9628 pts/1    00:00:00 ps

To list status of all the processes along with process id andPID, use option ‘-A‘.

root@tecmint:~# ps -A

 PID TTY          TIME CMD
    1 ?        00:00:01 init
    2 ?        00:00:00 kthreadd
    3 ?        00:00:01 ksoftirqd/0
    5 ?        00:00:00 kworker/0:0H
    7 ?        00:00:00 kworker/u:0H
    8 ?        00:00:00 migration/0
    9 ?        00:00:00 rcu_bh
....

Note: This command is very useful when you want to know which processes are running or may need PID sometimes, for process to be killed. You can use it with ‘grep‘ command to find customised output. For example,

root@tecmint:~# ps -A | grep -i ssh

 1500 ?        00:09:58 sshd
 4317 ?        00:00:00 sshd

Here ‘ps‘ is pipelined with ‘grep‘ command to find customised and relevant output of our need.

25. Command: kill

OK, you might have understood what this command is for, from the name of the command. This command is used to kill process which is not relevant now or is not responding. It is very useful command, rather a very very useful command. You might be familiar with frequent windows restarting because of the fact that most of the time a running process can’t be killed, and if killed it needs windows to get restart so that changes could be taken into effect but in the world of Linux, there is no such things. Here you can kill a process and start it without restarting the whole system.

You need a process’s pid (ps) to kill it.

Let suppose you want to kill program ‘apache2‘ that might not be responding. Run ‘ps -A‘ along with grep command.

root@tecmint:~# ps -A | grep -i apache2

1285 ?        00:00:00 apache2

Find process ‘apache2‘, note its pid and kill it. For example, in my case ‘apache2‘ pid is ‘1285‘.

root@tecmint:~# kill 1285 (to kill the process apache2)

Note: Every time you re-run a process or start a system, a new pid is generated for each process and you can know about the current running processes and its pid using command ‘ps‘.

Another way to kill the same process is.

root@tecmint:~# pkill apache2

Note: Kill requires job id / process id for sending signals, where as in pkill, you have an option of using pattern, specifying process owner, etc.

26. Command: whereis

The ‘whereis‘ command is used to locate the Binary, Sourcesand Manual Pages of the command. For example, to locate the Binary, Sources and Manual Pages of the command ‘ls‘ and ‘kill‘.

root@tecmint:~# whereis ls 

ls: /bin/ls /usr/share/man/man1/ls.1.gz

root@tecmint:~# whereis kill

kill: /bin/kill /usr/share/man/man2/kill.2.gz /usr/share/man/man1/kill.1.gz

Note: This is useful to know where the binaries are installed for manual editing sometimes.

27. Command: service

The ‘service‘ command controls the Starting, Stopping orRestarting of a ‘service‘. This command make it possible tostart, restart or stop a service without restarting the system, for the changes to be taken into effect.

Startting an apache2 server on Ubuntu

root@tecmint:~# service apache2 start

 * Starting web server apache2                                                                                                                                 apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName
httpd (pid 1285) already running      [ OK ]

Restarting a apache2 server on Ubuntu

root@tecmint:~# service apache2 restart

* Restarting web server apache2                                                                                                                               apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName
 ... waiting .apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName  [ OK ]

Stopping a apache2 server on Ubuntu

root@tecmint:~# service apache2 stop

 * Stopping web server apache2                                                                                                                                 apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName
 ... waiting                                                             [ OK ]

Note: All the process script lies in ‘/etc/init.d‘, and the path might needs to be included on certain system, i.e., in spite of running “service apache2 start” you would be asked to run “/etc/init.d/apache2 start”.

28. Command: alias

alias is a built in shell command that lets you assign name for a long command or frequently used command.

I uses ‘ls -l‘ command frequently, which includes 5 characters including space. Hence I created an alias for this to ‘l‘.

root@tecmint:~# alias l='ls -l'

check if it works or not.

root@tecmint:~# l

total 36 
drwxr-xr-x 3 tecmint tecmint 4096 May 10 11:14 Binary 
drwxr-xr-x 3 tecmint tecmint 4096 May 21 11:21 Desktop 
drwxr-xr-x 2 tecmint tecmint 4096 May 21 15:23 Documents 
drwxr-xr-x 8 tecmint tecmint 4096 May 20 14:56 Downloads 
drwxr-xr-x 2 tecmint tecmint 4096 May  7 16:58 Music 
drwxr-xr-x 2 tecmint tecmint 4096 May 20 16:17 Pictures 
drwxr-xr-x 2 tecmint tecmint 4096 May  7 16:58 Public 
drwxr-xr-x 2 tecmint tecmint 4096 May  7 16:58 Templates 
drwxr-xr-x 2 tecmint tecmint 4096 May  7 16:58 Videos

To remove alias ‘l‘, use the following ‘unalias‘ command.

root@tecmint:~# unalias l

check, if ‘l‘ still is alias or not.

root@tecmint:~# l

bash: l: command not found

Making a little fun out of this command. Make alias of certain important command to some other important command.

alias cd='ls -l' (set alias of ls -l to cd)
alias su='pwd' (set alias of pwd to su)
....
(You can create your own)
....

Now when your friend types ‘cd‘, just think how funny it would be when he gets directory listing and not directory changing. And when he tries to be ‘su‘ the all he gets is the location of working directory. You can remove the alias later using command ‘unalias‘ as explained above.

29. Command: df

Report disk usages of file system. Useful for user as well as System Administrator to keep track of their disk usages. ‘df‘ works by examining directory entries, which generally are updated only when a file is closed.

root@tecmint:~# df

Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/sda1       47929224 7811908  37675948  18% /
none                   4       0         4   0% /sys/fs/cgroup
udev             1005916       4   1005912   1% /dev
tmpfs             202824     816    202008   1% /run
none                5120       0      5120   0% /run/lock
none             1014120     628   1013492   1% /run/shm
none              102400      44    102356   1% /run/user
/dev/sda5         184307   79852     94727  46% /boot
/dev/sda7       95989516   61104  91045676   1% /data
/dev/sda8       91953192   57032  87218528   1% /personal

For more examples of ‘df‘ command, read the article 12 df Command Examples in Linux.

30. Command: du

Estimate file space usage. Output the summary of disk usages by ever file hierarchically, i.e., in recursive manner.

root@tecmint:~# du

8       ./Daily Pics/wp-polls/images/default_gradient
8       ./Daily Pics/wp-polls/images/default
32      ./Daily Pics/wp-polls/images
8       ./Daily Pics/wp-polls/tinymce/plugins/polls/langs
8       ./Daily Pics/wp-polls/tinymce/plugins/polls/img
28      ./Daily Pics/wp-polls/tinymce/plugins/polls
32      ./Daily Pics/wp-polls/tinymce/plugins
36      ./Daily Pics/wp-polls/tinymce
580     ./Daily Pics/wp-polls
1456    ./Daily Pics
36      ./Plugins/wordpress-author-box
16180   ./Plugins
12      ./May Articles 2013/Xtreme Download Manager
4632    ./May Articles 2013/XCache

Note: ‘df‘ only reports usage statistics on file systems, while ‘du‘, on the other hand, measures directory contents. For more ‘du‘ command examples and usage, read 10 du (Disk Usage) Commands.

31. Command: rm

The command ‘rm‘ stands for remove. rm is used to remove files (s) and directories.

Removing a directory

root@tecmint:~# rm PassportApplicationForm_Main_English_V1.0

rm: cannot remove `PassportApplicationForm_Main_English_V1.0': Is a directory

The directory can’t be removed simply by ‘rm‘ command, you have to use ‘-rf‘ switch along with ‘rm‘.

root@tecmint:~# rm -rf PassportApplicationForm_Main_English_V1.0

Warning: “rm -rf” command is a destructive command if accidently you make it to the wrong directory. Once you ‘rm -rf‘ a directory all the files and the directory itself is lost forever, all of a sudden. Use it with caution.

32. Command: echo

echo as the name suggest echoes a text on the standard output. It has nothing to do with shell, nor does shell reads the output of echo command. However in an interactive script, echo passes the message to the user through terminal. It is one of the command that is commonly used in scripting, interactive scripting.

root@tecmint:~# echo "Tecmint.com is a very good website" 

Tecmint.com is a very good website

creating a small interactive script

1. create a file, named ‘interactive_shell.sh‘ on desktop. (Remember ‘.sh‘ extension is must).
2. copy and paste the below script, exactly same, as below.

#!/bin/bash 
echo "Please enter your name:" 
   read name 
   echo "Welcome to Linux $name"

Next, set execute permission and run the script.

root@tecmint:~# chmod 777 interactive_shell.sh

root@tecmint:~# ./interactive_shell.sh

Please enter your name:
Ravi Saive
Welcome to Linux Ravi Saive

Note: ‘#!/bin/bash‘ tells the shell that it is an script an it is always a good idea to include it at the top of script. ‘read‘ reads the given input.

33. Command: passwd

This is an important command that is useful for changing own password in terminal. Obviously you need to know your current passowrd for Security reason.

root@tecmint:~# passwd 

Changing password for tecmint. 
(current) UNIX password: ******** 
Enter new UNIX password: ********
Retype new UNIX password: ********
Password unchanged   [Here was passowrd remians unchanged, i.e., new password=old password]
Enter new UNIX password: #####
Retype new UNIX password:#####

34. Command: lpr

This command print files named on command line, to named printer.

root@tecmint:~# lpr -P deskjet-4620-series 1-final.pdf

Note: The ‘lpq‘ command lets you view the status of a printer (whether it’s up or not), and the jobs (files) waiting to be printed.

35. Command: cmp

compare two files of any type and writes the results to the standard output. By default, ‘cmp‘ Returns 0 if the files are the same; if they differ, the byte and line number at which the first difference occurred is reported.

To provide examples for this command, lets consider two files:

file1.txt

root@tecmint:~# cat file1.txt

Hi My name is Tecmint

file2.txt

root@tecmint:~# cat file2.txt

Hi My name is tecmint [dot] com

Now, let’s compare two files and see output of the command.

root@tecmint:~# cmp file1.txt file2.txt 

file1.txt file2.txt differ: byte 15, line 1

36. Command: wget

Wget is a free utility for non-interactive (i.e., can work in background) download of files from the Web. It supportsHTTP, HTTPS, FTP protocols and HTTP proxies.

Download ffmpeg using wget

root@tecmint:~# wget http://downloads.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2

--2013-05-22 18:54:52--  http://downloads.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2
Resolving downloads.sourceforge.net (downloads.sourceforge.net)... 216.34.181.59
Connecting to downloads.sourceforge.net (downloads.sourceforge.net)|216.34.181.59|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: http://kaz.dl.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2 [following]
--2013-05-22 18:54:54--  http://kaz.dl.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2
Resolving kaz.dl.sourceforge.net (kaz.dl.sourceforge.net)... 92.46.53.163
Connecting to kaz.dl.sourceforge.net (kaz.dl.sourceforge.net)|92.46.53.163|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 275557 (269K) [application/octet-stream]
Saving to: ‘ffmpeg-php-0.6.0.tbz2’

100%[===========================================================================>] 2,75,557    67.8KB/s   in 4.0s   

2013-05-22 18:55:00 (67.8 KB/s) - ‘ffmpeg-php-0.6.0.tbz2’ saved [275557/275557]

37. Command: mount

Mount is an important command which is used to mount a filesystem that don’t mount itself. You need root permission to mount a device.

First run ‘lsblk‘ after plugging-in your filesystem and identify your device and note down you device assigned name.

root@tecmint:~# lsblk 

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT 
sda      8:0    0 931.5G  0 disk 
├─sda1   8:1    0 923.6G  0 part / 
├─sda2   8:2    0     1K  0 part 
└─sda5   8:5    0   7.9G  0 part [SWAP] 
sr0     11:0    1  1024M  0 rom  
sdb      8:16   1   3.7G  0 disk 
└─sdb1   8:17   1   3.7G  0 part

From this screen it was clear that I plugged in a 4 GB pendrive thus ‘sdb1‘ is my filesystem to be mounted. Become a root to perform this operation and change to /dev directory where all the file system is mounted.

root@tecmint:~# su
Password:

root@tecmint:~# cd /dev

Create a directory named anything but should be relevent for reference.

root@tecmint:~# mkdir usb

Now mount filesystem ‘sdb1‘ to directory ‘usb‘.

root@tecmint:~# mount /dev/sdb1 /dev/usb

Now you can navigate to /dev/usb from terminal or X-windows system and acess file from the mounted directory.

Time for Code Developer to know how rich Linux environment is

38. Command: gcc

gcc is the in-built compiler for ‘c‘ language in Linux Environment. A simple c program, save it on ur desktop asHello.c (remember ‘.c‘ extension is must).

#include <stdio.h>
int main()
{
  printf("Hello world\n");
  return 0;
}

Compile it

root@tecmint:~# gcc Hello.c

Run it

root@tecmint:~# ./a.out 

Hello world

Note: On compiling a c program the output is automatically generated to a new file “a.out” and everytime you compile a cprogram same file “a.out” gets modified. Hence it is a good advice to define a output file during compile and thus there is no risk of overwrite to output file.

Compile it this way

root@tecmint:~# gcc -o Hello Hello.c

Here ‘-o‘ sends the output to ‘Hello‘ file and not ‘a.out‘. Run it again.

root@tecmint:~# ./Hello 

Hello world

39. Command: g++

g++ is the in-built compiler for ‘C++‘ , the first object oriented programming language. A simple c++ program, save it on ur desktop as Add.cpp (remember ‘.cpp‘ extension is must).

#include <iostream>

using namespace std;

int main() 
    {
          int a;
          int b;
          cout<<"Enter first number:\n";
          cin >> a;
          cout <<"Enter the second number:\n";
          cin>> b;
          cin.ignore();
          int result = a + b;
          cout<<"Result is"<<"  "<<result<<endl;
          cin.get();
          return 0;
     }

Compile it

root@tecmint:~# g++ Add.cpp

Run it

root@tecmint:~# ./a.out

Enter first number: 
...
...

Note: On compiling a c++ program the output is automatically generated to a new file “a.out” and everytime you compile ac++ program same file “a.out” gets modified. Hence it is a good advice to define a output file during compile and thus there is no risk of overwrite to output file.

Compile it this way

root@tecmint:~# g++ -o Add Add.cpp

Run it

root@tecmint:~# ./Add 

Enter first number: 
...
...

40. Command: java

Java is one of the world’s highly used programming language and is considered fast, secure, and reliable. Most of the the web based service of today runs on java.

Create a simple java program by pasting the below test to a file, named tecmint.java (remember ‘.java‘ extension is must).

class tecmint {
  public static void main(String[] arguments) {
    System.out.println("Tecmint ");
  }
}

compile it using javac

root@tecmint:~# javac tecmint.java

Run it

root@tecmint:~# java tecmint

Note: Almost every distribution comes packed with gcc compiler, major number of distros have inbuilt g++ and java compiler, while some may not have. You can apt or yum the required package.

Don’t forget to mention your valueable comment and the type of article you want to see here. I will soon be back with an interesting topic about the lesser known facts about Linux.