Linux command List

 Let's see the list of Linux commands which are commonly used. This list is helpful for beginners and professionals both.

Commands	Description
alias	Converts complex commands into simpler ones.
bzip2 / bunzip2	Compresses a file / Decompress a bzip2 file.
bzcat / bzmore	Displays files compressed with bzip2.
cal	Displays calendar.
cat	It is a multi-function command.
cd	Changes the current working directory.
chage	Sets an expiration date for a user account.
chgrp	Changes group.
chmod	Changes permission for a file or directory.
chown	Changes the owner of a file or directory.
chsh	Changes the shell.
clear	Clears the terminal.
comm	Compares two streams or files.
cp	Copies file content from one file to another file.
cut	Used to display the desired column from a file.
date	Displays the current date.
df	Checks the disk space in the system.
echo	Prints the typed word on the terminal.
exit	Exits from the current user group to the last group.
export	Exports shell variables to other shells.
file	Displays the type of file.
find	Finds files for a particular search.
gpasswd	Transfers group membership to another user.
grep	Filters lines of text containing a certain string.
groupadd	Creates a group.
groupdel	Permanently removes a group.
groupmod	Changes group name.
groups	Displays the group name to which the current user belongs to.
gzip / gunzip	Compresses a file / Decompress a gzip file.
head	Displays the first ten lines of a file.
history	Displays older commands from the shell command history.
HISTSIZE	Determines the number of commands to be stored in the current environment.
HISTFILE	Displays the file that contains the history.
HISTFILESIZE	Sets the number of commands kept in the history file.
id	Tells about the user's id in the system.
less	Displays file content according to the width of the terminal.
locate	Searches a file in the database.
ls	Lists all the files of a directory.
man	Displays the manual page for the specified command.
mkdir	Creates directory.
more	Displays one output screen at a time.
mv	Renames directories or files.
od	Displays a file content in octal format.
passwd	Set a password for a user group.
pwd	Display the current working directory location.
PS1	Change the prompt name in the terminal.
rename	Renames more than one file at once.
rm	Removes a file.
rmdir	Removes a directory.
set -o noclobber	Prevents file from getting overwritten.
set +o noclobber	Allows overwriting in the existing file.
set -u	Displays undefined variables as an error.
set +u	Displays nothing for an undefined variable.
set -x	Displays shell expansion.
set +x	Disables shell expansion.
sed	Performs editing in streams.
sleep	Waits for the specified number of seconds.
sort	sorts the content in alphabetical order.
su	It allows a user to run a shell as another user.
sudo	It allows a user to start a program with the credentials of another user.
tac	Displays file content in the opposite order.
tail	Displays the last ten lines of a file.
tar	Compresses a directory.
tee	Puts stdin on stdout and then into a file.
time	Displays time taken to execute a command.
touch	It creates an empty file.
tr	Translates characters.
type	Displays information about command type.
uniq	Sorts and display multi times repeating lines only once.
unset	Removes a variable from a shell.
useradd	Adds users.
userdel	Deletes users.
usermod	Modifies the properties of a user.
vi	Opens vi editor to write a program.
w	Displays who is logged on and what are they doing.
wc	Counts words, lines, and characters.
who	Tells who is logged on the system.
whoami	Tells the name of the user.
who am i	Displays the line pointing to your current session.
zcat / zmore	Views the files compressed with gzip.

Linux export Command

 The export command is a built-in utility of Linux Bash shell. It is used to ensure the environment variables and functions to be passed to child processes. It does not affect the existing environment variable.

Environment variables are set when we open a new shell session. At any time, if we change any variable value, the shell has no way to select that change. The export command allows us to update the current session about the changes that have been made to the exported variable. We do not need to wait to start a new shell session.

Syntax:

1. export [-f] [-n] [name[=value] ...] or export -p  

Let's have a look at various examples of the export command:

Example1: The export command without any argument

The basic export command will display all the exported environment variables of your system. It is executed as follows:

1. export  

Consider the below snap of output:

Example2: Display all exported variable on current shell

To display all the exported environment variable of the current shell, execute the command with -p option as follows:

1. export -p  

Consider the below snap of output:

Example3: Using export with functions

To use a function with the export command, use the -f option. If we do not use this option, it will be considered as a variable, not function.

Syntax:

1. export -f function_name  

We are exporting a function 'name' as follows:

1. name() { echo "Javatpoint";}  

To export the above function, execute the command as follows:

1. export -f name  

Now, invoke the bash shell to execute the function:

1. bash  

To call the function, enter the function name:

1. name  

Consider the below output:

let's create another function 'hello,' execute the command as follows:

1. function hello  
2. > {  
3. > echo hello, welcome to javatpoint  
4. > }  

To export the above function, execute the command as follows:

1. export -f hello  

Consider the below output:

Example4: Assign a value before exporting a function or variable:

The export command allows us to assign a value before exporting a function. Consider the below command:

1. export name[=value]  

For example, assign a value to a variable as follows:

1. a=5  

now export it as:

1. export a  

we can verify the assignment by using the printenv command as follows:

1. printenv a  

Consider the below output:

Example5: Set vim as default editor:

The vim editor is the most widely used text editor for the Linux systems. We can set the vim as default text editor by using the export command.

To set the vim as a default text editor, execute the following command:

1. export EDITOR=/usr/bin/vim  
2. export | grep EDITOR  

The above commands will not show any confirmation. Consider the below output:

Example6: Set an environment variable

To create a new variable, use the export command followed by a variable name and its value.

Syntax:

1. export NAME=VALUE  

To create a new variable, 'sys,' execute the command as follows:

1. export  sys=50  

The echo command is used to display the variable:

1. echo sys  

To display the value of the variable, use the $ symbol before the variable name

1. echo $sys  

Consider the below output:

Linux Set Command

 Linux set command is used to set and unset certain flags or settings within the shell environment. These flags and settings determine the behavior of a defined script and help in executing the tasks without facing any issue. The values of shell attributes and parameters can be changed or displayed by using the set command.

Syntax:

1. set [options]  

To know about how to set the environment variable in Linux, visit Here.

Options: The supported options by the set command are as following:

• -a: It is used to mark variables that are modified or created for export.
• -b: It is used to notify of job termination immediately.
• -e: It is used to exit immediately if a command exits with a non-zero status.
• -f: It is used to disable the file name generation (globbing).
• -h: It is used to save the location of commands where they looked up.
• -k: It is used to place all assignment arguments in the environment variable of a command, except those that precede the command name.
• -m: It is used to enable Job control.
• -n: It is used to read commands.
• -o: It is used for option-name.
• -p: It is used to disable the processing of the '$ENV' file and import shell functions. It is turned on whenever the real and effective user ids do not match. Turning off this option may cause the working uid and gid to be set as the authorized uid and gid.
• -t: It is used to exit from the command after executing one command.
• -u: It is used to treat unset variables as an error when substituting.
• -v: It is used to print shell input lines.
• -x: It is used to print commands and their arguments in a sequential way (as they are executed).
• -B: It is used to perform brace expansion by the Shell.
• -C: It is used to disallow existing regular files to be overwritten by redirection of output.
• -E: It is used if the ERR trap is inherited by the shell functions.
• -H: It is used to enable style history substitution. By default, it is on when the shell is interactive.
• -P: It is used if we do not want to follow symbolic links when executing commands.
• -T: If this flag is set, the DEBUG trap is inherited by the shell functions.

Note: To turn off the above flags, use the '+' rather than '-' symbol.

To better understand the set command, let's see a brief introduction of the shell variable:

Shell Variable

A variable is a character string that can hold a value. The assigned value could be anything such as number, filename, text, or any other data type. It is just like a pointer to the actual data. The shell allows us to create, delete, and assign variables.

The variable name can have any letters, numbers, or underscore (_) character. We cannot have the characters such as "! *, or -", because these special characters have other meanings for the shell. According to Unix naming convention, the Unix shell variable must have their names in UPPERCASE.

Examples of the set command

Turn debugging information on or off.

The '-x' option is used with the set command to show command and their arguments. It is useful for debugging the shell script.

Execute the following command:

To turn on the debugging information:

1. set -x  

To turn off the debugging information:

1. set +x  

Disable Bash's default behavior.

To disable the default behavior of Bash, execute the command as follows:

1. set -C  

Stop a script immediately.

To stop a script immediately, execute the following command:

1. set -e  

Getting Help

If you get stuck during the use of the set command, you can access the help documentation from your terminal. To access the help manual, execute the below command:

1. set --help  

Linux Set Environment Variable

 The environment variables are dynamic values that are stored within a system and used by applications launched in shells or sub-shells. These variables have a name and their respected value. The environment variable customizes the system performance and the behavior of an application.

The environment is the track for a computer application to interact with the system. The environment variable can have information about the default applications of the system, the system locale, the path of the executable file and the keyboard layout setting, and more. The environment variable makes an app available as per the system.

Common Environment Variables

Some standard environment variables are as follows:

• PATH

This variable contains a list of directories in which our system looks for files. It separates directories by a (:) colon.

• USER

This variable holds the username.

• HOME

This variable holds the default path to the user's home directory.

• EDITOR

This variable contains the path to the specified editor.

• UID

This variable contains the path to the user's unique id.

• TERM

This variable contains the path to the default terminal emulator.

• SHELL

This variable contains the path to the default shell that is being used by the user.

• ENV

This variable displays all the environment variable.

How to set Environment Variable in Linux?

There are multiple commands in Linux that allow us to set and create the environment variable.

Use the export command to set a new environment variable.

To create a new variable, use the export command followed by a variable name and its value.

Syntax:

1. export NAME=VALUE  

To create a new variable say new_variable, execute the command as follows:

1. export new_variable=10  

The echo command is used to display the variable:

1. echo new_variable  

To display the value of the variable, use the $ symbol before the variable name:

1. echo $new_variable  

Consider the below output:

To set Java Environment Variable, execute the command as follows:

1. export PATH=$PATH:/home/jdk1.8/bin/  

We can also create a user to define a variable by directly declaring it on the terminal.

If we want to create a variable new_variable2, we can create it as follows:

1. new_variable2=100  

Consider the below output:

Accessing the value of Environment Variable

To access the value of a variable, execute the echo command as follows:

1. echo $variable name  

Note: The variables are case sensitive; we cannot use any variable name 'new_variable' as 'New_variable.'

The env command

The env command is used to display all the available variables in the system.

1. env   

Output:

SHELL=/bin/bash
SESSION_MANAGER=local/javatpoint-GB-BXBT-2807:@/tmp/.ICE-unix/1458,unix/javatpoint-GB-BXBT-2807:/tmp/.ICE-unix/1458
COLORTERM=truecolor
XDG_CONFIG_DIRS=/etc/xdg/xdg-ubuntu:/etc/xdg
XDG_MENU_PREFIX=gnome-
GNOME_DESKTOP_SESSION_ID=this-is-deprecated
GTK_IM_MODULE=ibus
LANGUAGE=en_IN:en
QT4_IM_MODULE=ibus
MANDATORY_PATH=/usr/share/gconf/ubuntu.mandatory.path
GNOME_SHELL_SESSION_MODE=ubuntu
SSH_AUTH_SOCK=/run/user/1000/keyring/ssh
XMODIFIERS=@im=ibus
DESKTOP_SESSION=ubuntu
SSH_AGENT_PID=1362
GTK_MODULES=gail:atk-bridge
PWD=/home/javatpoint
LOGNAME=javatpoint
XDG_SESSION_DESKTOP=ubuntu
XDG_SESSION_TYPE=x11
GPG_AGENT_INFO=/run/user/1000/gnupg/S.gpg-agent:0:1
XAUTHORITY=/run/user/1000/gdm/Xauthority
GJS_DEBUG_TOPICS=JS ERROR;JS LOG
WINDOWPATH=2
HOME=/home/javatpoint
USERNAME=javatpoint
IM_CONFIG_PHASE=1
LANG=en_IN
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.m4a=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.oga=00;36:*.opus=00;36:*.spx=00;36:*.xspf=00;36:
XDG_CURRENT_DESKTOP=ubuntu:GNOME
VTE_VERSION=5802
GNOME_TERMINAL_SCREEN=/org/gnome/Terminal/screen/dc6e48ed_7868_43d5_a086_fd6d6a90a74a
INVOCATION_ID=fadf556d1cf5422ea55b247bf57c3c99
MANAGERPID=1252
CLUTTER_IM_MODULE=ibus
GJS_DEBUG_OUTPUT=stderr
LESSCLOSE=/usr/bin/lesspipe %s %s
XDG_SESSION_CLASS=user
TERM=xterm-256color
DEFAULTS_PATH=/usr/share/gconf/ubuntu.default.path
LESSOPEN=| /usr/bin/lesspipe %s
USER=javatpoint
GNOME_TERMINAL_SERVICE=:1.216
DISPLAY=:0
SHLVL=1
QT_IM_MODULE=ibus
XDG_RUNTIME_DIR=/run/user/1000
JOURNAL_STREAM=9:35179
XDG_DATA_DIRS=/usr/share/ubuntu:/usr/local/share/:/usr/share/:/var/lib/snapd/desktop
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
GDMSESSION=ubuntu
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
_=/usr/bin/env
javatpoint@javatpoint-GB-BXBT-2807:~$ 

Removing an Environment Variable

By removing an environment variable we can remove all existing component of particular variable.

To remove an environment variable, execute the unset command followed by variable name:

1. unset variable name  

The above command will delete the specified variable and its components from the system.

To remove a variable new_variable from the system, execute the command as follows:

1. unset new_variable  

Consider the below output:

Linux Licensing

 Linus Torvalds has given linux kernel license to GNU General Public License (GPL) version 2. GNU make sure that any software source code licensed under it have to make originating source code open and freely availble to all its users. Here, freely doesn't mean by cost but it means that it is freely available to users to distribute and modify the code.

There is the third version of GNU, GNU Lesser General Public License (LGPL) version 3. But it imposes some more permissions on the license. Torvalds doesn't like some provisions in version 3 and have announced that linux kernel will not come under version 3.