2024-04-01
unset
The unset
command is used to remove variables from the current shell’s environment. Once unset, the variable is no longer accessible within the current shell session. It’s important to note that unset
only affects the current shell; variables in subshells or other sessions remain unaffected.
Syntax:
unset [option] variable...
While options are rarely used, let’s briefly discuss the only commonly encountered option:
-f
or --function
: This option specifies that the arguments are function names, rather than variable names.Environment variables are inherited by child processes. unset
allows you to remove them, preventing their propagation.
Example:
Let’s say you have an environment variable MY_VAR
set to “hello”:
export MY_VAR="hello"
echo $MY_VAR # Output: hello
To remove MY_VAR
:
unset MY_VAR
echo $MY_VAR # Output: (nothing, the variable is unset)
Shell variables, unlike environment variables, are local to the current shell. unset
works identically for these.
Example:
MY_SHELL_VAR="world"
echo $MY_SHELL_VAR # Output: world
unset MY_SHELL_VAR
echo $MY_SHELL_VAR # Output: (nothing)
unset
with the -f
option allows you to remove defined shell functions.
Example:
my_function() {
echo "This is my function!"
}
my_function # Output: This is my function!
unset -f my_function
my_function # Output: (command not found)
unset
can remove multiple variables simultaneously.
Example:
VAR1="one"
VAR2="two"
VAR3="three"
unset VAR1 VAR2 VAR3
echo $VAR1 $VAR2 $VAR3 # Output: (nothing)
If you attempt to unset
a variable that doesn’t exist, unset
will typically produce no error message. This behavior might differ slightly depending on your shell.
unset
proves useful in shell scripts for managing temporary variables or cleaning up after a specific process. This allows for better resource management and avoids potential conflicts. For instance, you might unset
variables after they are no longer needed within a loop or function.
Remember that unset
only removes variables from the current shell instance. It does not affect variables in subshells or other terminal sessions. Also, attempting to unset
a read-only variable will result in an error in most shells.