Managing Environment Variables

1. Viewing All Variables

Command	Description	Notes
env	Display all environment variables	Simple and widely supported
printenv	Print all or specified environment variables	More portable than `env`
set	Display all shell variables (including environment)	Includes shell-specific variables

Example: Using `env` to view variables

env
# Output includes variables like:
# PATH=/usr/local/bin:/usr/bin:/bin
# HOME=/home/user
# USER=user

Example: Using `printenv` to view specific variable

printenv HOME
# Output:
# /home/user

2. Displaying Specific Variable

Command	Description	Example
echo $VARIABLE_NAME	Displays the value of a specific environment variable	`echo $HOME`
printenv VARIABLE_NAME	Prints the value of a specific environment variable	`printenv PATH`
env \| grep VARIABLE_NAME	Filters environment variables to show only the specified one	`env \| grep USER`

3. Setting Temporary Variables

Command	Description	Example
export	Sets a temporary environment variable for the current shell session	`export MY_VAR="value"`
unset	Removes a temporary environment variable from the current shell session	`unset MY_VAR`

Example: Setting and Using a Temporary Variable

export PATH="$PATH:/new/path"
echo $PATH

Note: These variables are only available in the current shell session and will not persist after closing the terminal.

4. Unsetting Variables

Command	Description	Notes
unset	`unset VAR_NAME`	Removes the variable from the environment
unset -f	`unset -f function_name`	Unsets a function definition
unset -v	`unset -v VAR_NAME`	Unsets a variable (explicit)

Example: Unsetting a Variable

export MY_VAR="test"
echo $MY_VAR
# Output: test

unset MY_VAR
echo $MY_VAR
# Output: (empty)

Note: Using unset only affects the current shell session and does not persist across new sessions.

5. Setting Permanent Variables

File	Content	Description
~/.bashrc	`export MY_VAR="value"`	Applies to bash shell sessions
~/.profile	`export MY_VAR="value"`	Applies to login shells and GUI sessions
/etc/environment	`MY_VAR=value`	System-wide environment variables (requires PAM)
/etc/profile.d/custom.sh	`export MY_VAR="value"`	System-wide script for custom variables

Example: Adding a variable to ~/.bashrc

echo 'export PATH="$PATH:/usr/local/bin"' >> ~/.bashrc

Note: Changes to ~/.bashrc take effect in new terminal sessions or after running source ~/.bashrc.

Warning: Modifying /etc/environment may require root privileges and can affect all users.

6. Setting System-Wide Variables

File	Content	Description
/etc/environment	`PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"`	System-wide environment variables for login shells
/etc/profile	`export NODE_ENV=production`	Global shell initialization script for login shells
/etc/bash.bashrc	`alias ll='ls -la'`	System-wide bash configuration for interactive shells

Example: Adding a system-wide variable

# Edit /etc/environment
echo "JAVA_HOME=/usr/lib/jvm/java-17-openjdk" | sudo tee -a /etc/environment

Note: Changes to /etc/environment require a new login or reboot to take effect.

Warning: Modifying system files directly can affect all users and should be done carefully.

7. Managing PATH Variable

Operation	Description	Command	Notes
View PATH	Display current PATH variable	`echo $PATH`	Shows all directories where commands are searched
Add to PATH	Add a new directory to PATH permanently	`export PATH=$PATH:/new/directory`	Appends to current session; use `~/.bashrc` or `~/.zshrc` for persistence
Remove from PATH	Remove a directory from PATH	`export PATH=$(echo $PATH \| sed 's/\/old\/directory://g')`	Uses `sed` to remove specific path from list
Reset PATH	Restore default PATH	`export PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin`	Typical default PATH on most Linux systems

Example: Adding a Custom Binary Directory to PATH

mkdir -p ~/bin
echo 'export PATH=$PATH:~/bin' >> ~/.bashrc
source ~/.bashrc

Note: Changes to PATH in current session only last until shell is closed. To make them permanent, update shell config files like ~/.bashrc, ~/.zshrc, or /etc/environment.

8. Using Variable Substitution

Feature	Description	Example
Variable Expansion	Expands variables using `${VAR}` syntax	`echo "Hello ${USER}"`
Default Value	Provides default if variable is unset or empty	`echo "${HOME:-/tmp}"`
Alternate Value	Uses alternate value if variable is unset or empty	`echo "${USER:-$(whoami)}"`
Variable Length	Expands to the length of a variable's value	`echo "${#USER}"`
Substring Extraction	Extracts substring using `${VAR:start:length}`	`echo "${USER:0:3}"`
Variable Replacement	Replaces all occurrences of a substring	`echo "${USER//u/U}"`

Example: Variable Substitution in Shell Script

#!/bin/bash
name="John"
echo "Hello ${name}, welcome to ${HOME}"
echo "Your home directory is ${HOME:-/tmp}"
echo "First letter of name: ${name:0:1}"

9. Setting Variables in Scripts

Method	Description	Example
export	Sets and exports a variable to child processes	`export MY_VAR="value"`
local	Declares a variable local to a function (bash/zsh)	`function myfunc() { local localVar="localValue" }`
declare	Declares a variable with optional attributes (bash)	`declare -r readonlyVar="value"`

Example: Setting and Using Variables in a Script

#!/bin/bash
export DATABASE_URL="postgresql://localhost:5432/mydb"
export API_KEY="abc123"

echo "Database URL: $DATABASE_URL"
echo "API Key: $API_KEY"

Note: Variables set without export are only available in the current shell session and not inherited by child processes.

10. Exporting Variables to Child Processes

Command	Description	Example
export	Exports a variable to child processes	`export MY_VAR="value"`
env	Runs a command with environment variables set	`env VAR=value command`

Example: Setting and Exporting a Variable

export PATH="$PATH:/usr/local/bin"
echo $PATH

Example: Running Command with Temporary Variable

env DEBUG=true npm start