Overview
The unset command removes one or more environment variables from the current Nash shell session. Once unset, the variable is no longer available and will not appear in env output.
Syntax
Description
The unset command removes environment variables by name. Multiple variables can be removed in a single command. After unsetting, attempting to reference the variable will result in an empty expansion.
Options
The unset command does not accept any flags or options. It only accepts variable names as arguments.
Examples
Unset a Single Variable
export API_KEY=secret123
echo $API_KEY
# Output: secret123
unset API_KEY
echo $API_KEY
# Output: (empty)
Unset Multiple Variables
export DB_HOST=localhost DB_PORT=5432 DB_NAME=myapp
env | grep DB
# Output:
# DB_HOST=localhost
# DB_PORT=5432
# DB_NAME=myapp
unset DB_HOST DB_PORT DB_NAME
env | grep DB
# Output: (empty)
Verify Variable is Unset
export TEMP_VAR=temporary
env | grep TEMP_VAR
# Output: TEMP_VAR=temporary
unset TEMP_VAR
env | grep TEMP_VAR
# Output: (empty - no match)
Unset Non-Existent Variable
unset DOES_NOT_EXIST
# No error - command succeeds silently
Practical Use Cases
Clean Up Temporary Variables
# Set temporary variables for a task
export BUILD_ID=12345
export BUILD_TEMP=/tmp/build-12345
# Use variables...
echo "Building $BUILD_ID in $BUILD_TEMP"
# Clean up when done
unset BUILD_ID BUILD_TEMP
Reset Configuration
# Remove all API-related variables
unset API_KEY API_URL API_TIMEOUT
# Verify cleanup
env | grep API
# Output: (empty)
Security: Clear Sensitive Data
# Use password temporarily
export DB_PASSWORD=secret123
# ... perform database operations ...
# Immediately unset sensitive data
unset DB_PASSWORD
Revert to Defaults
# Override default
export PORT=3000
# Later, remove override to use system default
unset PORT
Clean Environment Before Tests
# Clear all test-related variables
unset TEST_ENV TEST_MODE TEST_DB TEST_API
# Set fresh test environment
export TEST_ENV=local
export TEST_MODE=debug
Conditional Unset
# Unset if variable matches condition
test "$DEBUG" = "false" && unset DEBUG
Variable Name vs. Value
Provide the variable name without the $ prefix:# Correct
unset MY_VAR
# Incorrect - this tries to unset the value, not the variable
unset $MY_VAR
Comparison with Export
Sets or modifies variables:export KEY=value
echo $KEY
# Output: value
Removes variables completely:unset KEY
echo $KEY
# Output: (empty)
System Variables
Be careful when unsetting system variables:# Unsetting PATH makes commands harder to locate
unset PATH
# Unsetting HOME can break scripts that depend on it
unset HOME
# Unsetting PWD may cause confusion
unset PWD
export to override rather than unset:export PATH=/custom/path:$PATH
Subshell Behavior
Changes in subshells don’t affect the parent:
export OUTER=value
# Unset in subshell
(unset OUTER; echo $OUTER)
# Output: (empty in subshell)
echo $OUTER
# Output: value (still set in parent)
Checking if Variable is Set
# Set a variable
export MY_VAR=value
# Check if set using test
test -n "$MY_VAR" && echo "Set" || echo "Not set"
# Output: Set
# Unset it
unset MY_VAR
# Check again
test -n "$MY_VAR" && echo "Set" || echo "Not set"
# Output: Not set
Multiple Unsets
# Set several variables
export VAR1=a VAR2=b VAR3=c
# Unset all at once
unset VAR1 VAR2 VAR3
# Or unset one by one
unset VAR1
unset VAR2
unset VAR3
Common Patterns
Cleanup Script Section
# At end of script
echo "Cleaning up..."
unset TEMP_DIR TEMP_FILE BUILD_ID
echo "Cleanup complete"
Environment Reset
# Clear all custom variables
unset API_KEY API_URL
unset DB_HOST DB_PORT DB_NAME
unset DEBUG LOG_LEVEL
# Verify clean state
env | grep -E "API|DB|DEBUG|LOG"
# Output: (empty)
Conditional Cleanup
# Only unset if variable exists
test -n "$TEMP_VAR" && unset TEMP_VAR
Notes
- Unsetting a non-existent variable does not produce an error
- Once unset, the variable returns to an undefined state
- Unsetting does not affect other shell sessions or subshells
- Variable names are case-sensitive
- You cannot unset read-only variables (if Nash supports them)
Exit Status
The unset command always returns exit code 0 (success), even when unsetting non-existent variables.
- export - Set or modify environment variables
- env - Display all environment variables
- test - Test if variables are set
- echo - Display variable values
