Command line interface

The command line interface (CLI) or shell interface is an old fashioned, non-graphical interface in a shell, which is still convenient for serious Linux users. It may seem old fashioned, but seasoned Linux afficionados find it to be much faster, more convenient, more effective, and more powerful for many tasks than using a GUI.

1 Command line interface: General commands

Command	Function
↑	Move up in command history
↓	Move down in command history
!!	Repeat last command
sudo !!	Repeat last command with sudo privileges
com1 && com2	Run command #2 if command #1 is successful (command1 && command2)
com1 ; com2	Run multiple commands, e.g., command1 ; command 2 ; command3 (execution proceeds whether previous commands were successfully executed or not)
com1 \|\| com2	Run multiple commands: Run command2 if command1 is unsuccessful (command1 \|\| command2 runs command2 if command1 does not execute successfully)
!cmd#	Re-execute a command by its history number
cmd –help	Brief help information on a command
cmd &	Start a process and send it to the background
cmd < file	Read input of command from a file
cmd 2> file	Error output (stderr) of cmd to file
& cmd2	stderr of cmd1 to cmd2
cmd1; cmd2	Execute separate commands in succession
#	Shell prompt symbol used for superuser
>	Redirect: Send output of a process to a file (create a new file or overwrite an existing file)
>& OR &>	Send output to a location
>>	Append: Append output of a process to the end of an existing file
\|	command2)
$	Shell prompt symbol for normal user
1>	Send stdoutput (standard output, e.g., to the screen) to a location
1>2&	Redirect stdout (standard output) to stderr (error file)
2>	Send stderror (error messages) to a location
2> /dev/null	Redirect stderr to bitbucket
apropos keyword	Display command names based on keyword search
bc	Launch a high-precision calculator shell
cal	Show a calendar for the current month
cal -3	Shows calendar for three months: previous, current, and following month
cal -n no.	Shows calendar for specified number of months around the current date
cal month	Shows calendar for specified month
cal month# year	Shows calendar for specified month (by number) and year (e.g., cal 1 1812 = January 1812)
cal year	Shows calendar for a specified year (from 01 to 9999 CE)
cd -	Move to previous directory
chsh	Change shell
clear	Clear screen
CTL-ALT [F1, F2, F3...]	Switch to alternate system terminal (tty)
date -d	Print formatted date.
dmesg -k	Display system messages
echo text	Echo a message to the standard output (i.e., the screen)
echo text > file	Redirect text to a new file (or overwrite an existing file)
at time	at 12.00)
echo “ “Hello, world.” ”	Display a message containing quotation marks or other escape characters, e.g., “ ‘ [ ] \| / \ ( ) { } * ?
exit	Exit the current shell
export	A command for assigning or removing an environment variable.
expr	Evaluate an expression.
history	Displays history of previously entered commands in the shell
grep keyword	Search history for a particular command
localectl	Show locale related environmental settings
ls -S	List and sort files by file size
man cmd	Full manual entry for command
screenfetch	Display system, OS and DM information
script	Record terminal session activity and create output for the session in a file
source script . script	Load a configuration file into current script or shell session
stderr	Standard error, i.e., error messages
stdin	Standard input (input to a command)
stdout	Standard output, e.g., of a command to the screen
stty	Set key commands or shortcuts
su -	Log in as superuser
sudo	Root or superuser privilege to execute commands
sudo -ll	Show commands that the user is allowed to execute
sudo !!	Rerun last command with sudo privileges
tee	Read stdin and send output to stdout and file

1.1 Sudo behavior

If you find the timeout for sudo authentication to be too short, this can be modified using the program visudo (which should come pre-installed). Visudo allows for modifying the sudo configuration file and locking it at the same time so that no other programs modify it.

 sudo visudo

At the end of the file, append the following line to change the timeout, for example, to 30 minutes.

Defaults:USER timestamp_timeout=30

2 CLI environment variables

Command	Function
$HOME	Home directory
$HOME/.bashrc	Location of user bash configuration file
$PATH	Executable search path, i.e., paths for commands and executable files
$SHELL	Current shell
alias	Show defined aliases
alias [alias]=”[command]”	Define a new alias in the bash environment file
cat etcvconsole.conf	See keyboard settings for terminal sessions
echo "STRING"	Output string
echo $NAME	Output value of variable NAME
echo $Variable_name	Output the value of a defined variable on the terminal.
env	Display environment variables.
export NAME = value	set "value to NAME
export PATH="$PATH:/some/addition"	Export new variable to bash configuration
export VAR	Export a variable to the current shell and child processes
export Variable = value	Assign an environment variable a new value.
localectl	Show locale related environmental settings
printenv	Print defined environmental variables
source .bashrc	Reload bash shell settings after editing configuration file
Unset	Remove or delete a variable.
variable_name = variable_value	Assign a variable value to variable name
vi /etc/locale.gen	Edit localization file for environmental variables

3 CLI keyboard shortcuts

Command	Function
!!	Execute a previously executed command.
CTL + C	Kill a running process.
CTL + G	Exit command history.
CTL + K	Cut a line fragment succeeding the cursor.
CTL + O	Execute the previously recalled command.
CTL + R	Recall the last used command.
CTL + U	Cut a line fragment preceding the cursor.
CTL + W	Cut a single word preceding the cursor.
CTL + Y	Paste from clipboard.
CTL + Z	Stop or suspend a currently active process.
exit	Sign off from an active session.