Environment Variables in Linux

Variable	Description
$HOME	Home directory of the current user
$PWD	Present working directory
$USER	Current user
$SHELL	Current shell
$PATH	List of directories to search for executable files
$EDITOR	Default text editor
$LANG	Default language
$HOSTNAME	Hostname of the computer

Setting Environment Variables

Command	Description
export VAR=value	Set an environment variable
export VAR=value1:value2	Set an environment variable with multiple values
export VAR=$VAR:value	Append a value to an environment variable
unset VAR	Remove an environment variable
env	Display all environment variables
printenv	Display all environment variables
echo $VAR	Display the value of an environment variable

Parameter Expansion

Command	Description
${VAR}	Value of the variable VAR
${VAR:-value}	Value of VAR if set, otherwise value
${VAR:=value}	Value of VAR if set, otherwise value and set VAR to value
${VAR:?message}	Value of VAR if set, otherwise print message and exit
${VAR:+value}	Value of value if VAR is set, otherwise nothing