Environment Variables¶

Environment variables store configuration that programs can access. Understanding how they work is essential for proper shell configuration.

What Are Environment Variables?¶

Environment variables are:

Key-value pairs available to all processes
Inherited by child processes
Used to configure program behavior

# View an environment variable
echo $HOME

/Users/username

Shell vs Environment Variables¶

There's an important distinction:

Shell variables - Local to current shell:

myvar="hello"           # Not exported
echo $myvar             # Works in this shell
bash -c 'echo $myvar'   # Empty - not inherited

Environment variables - Inherited by child processes:

export myvar="hello"    # Exported
bash -c 'echo $myvar'   # Works - inherited

Setting Variables¶

Basic Assignment¶

NAME="value"            # No spaces around =
NAME=value              # Quotes optional if no spaces
NAME="value with spaces"
NAME='literal $value'   # Single quotes: no expansion

Export¶

Make variable available to child processes:

export PATH="/new/path:$PATH"
export EDITOR=vim

# Or combine
export NAME="value"

One-Time Environment¶

Set for single command:

DEBUG=1 ./script.sh     # DEBUG only for this command

Unset¶

Remove a variable:

unset VARNAME

Common Environment Variables¶

System Variables¶

Variable	Purpose
`HOME`	User's home directory
`USER`	Current username
`SHELL`	User's default shell
`PWD`	Current working directory
`OLDPWD`	Previous directory
`HOSTNAME`	Machine hostname
`TERM`	Terminal type
`LANG`	Language/locale
`TZ`	Timezone

Shell Configuration¶

Variable	Purpose
`PATH`	Executable search path
`PS1`	Primary prompt
`PS2`	Continuation prompt
`HISTSIZE`	History length
`HISTFILE`	History file location
`HISTCONTROL`	History behavior

Program Configuration¶

Variable	Purpose
`EDITOR`	Default text editor
`VISUAL`	Visual editor
`PAGER`	Default pager (less, more)
`BROWSER`	Default web browser
`MANPATH`	Manual page search path

The PATH Variable¶

PATH tells the shell where to find executables:

echo $PATH

/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

Directories are separated by colons, searched left to right.

Adding to PATH¶

# Prepend (searched first)
export PATH="$HOME/.local/bin:$PATH"

# Append (searched last)
export PATH="$PATH:/opt/myapp/bin"

Best Practice¶

Check before adding to avoid duplicates:

# Add only if not already present
add_to_path() {
    [[ ":$PATH:" != *":$1:"* ]] && export PATH="$1:$PATH"
}

add_to_path "$HOME/.local/bin"

Finding Commands¶

which python          # First match in PATH
type python           # What python resolves to
command -v python     # POSIX way to check

Viewing Variables¶

All Environment Variables¶

env                   # Environment variables only
printenv              # Same as env
export                # Exported variables with values

All Variables (including shell)¶

set                   # All variables and functions
declare -p            # Variables with types

Specific Variable¶

echo $VARNAME
printenv VARNAME

Variable Scope¶

Global (Exported)¶

Available everywhere after export:

export GLOBAL="accessible"

Local to Script¶

Not exported, only in current shell:

LOCAL="only here"

Local to Function¶

Using local keyword:

myfunc() {
    local myvar="function scope"
    echo $myvar
}
myfunc           # "function scope"
echo $myvar      # Empty

Default Values¶

Use Default if Unset¶

echo ${NAME:-default}      # Use 'default' if NAME unset
echo ${NAME:=default}      # Set and use 'default' if unset
echo ${NAME:+alternate}    # Use 'alternate' if NAME IS set
echo ${NAME:?error msg}    # Error if NAME unset

Examples:

unset NAME
echo ${NAME:-John}         # John

NAME="Alice"
echo ${NAME:-John}         # Alice

echo ${UNDEFINED:?Must be set}  # Error: Must be set

Persistent Configuration¶

For Interactive Shells¶

Add to ~/.bashrc:

export EDITOR=vim
export PAGER=less

Add to ~/.bash_profile (sources .bashrc):

# ~/.bash_profile
source ~/.bashrc

System-Wide¶

Add to /etc/environment or /etc/profile.d/*.sh:

# /etc/profile.d/custom.sh
export COMPANY_ENV="production"

Common Patterns¶

XDG Base Directories¶

Standard locations for application data:

export XDG_CONFIG_HOME="$HOME/.config"
export XDG_DATA_HOME="$HOME/.local/share"
export XDG_CACHE_HOME="$HOME/.cache"

Language-Specific¶

# Python
export PYTHONPATH="$HOME/lib/python"
export VIRTUAL_ENV="$HOME/.venv"

# Node.js
export NODE_PATH="$HOME/.node_modules"
export NPM_CONFIG_PREFIX="$HOME/.npm-global"

# Go
export GOPATH="$HOME/go"
export GOBIN="$GOPATH/bin"

# Rust
export CARGO_HOME="$HOME/.cargo"
export RUSTUP_HOME="$HOME/.rustup"

Development¶

export DEBUG=1
export LOG_LEVEL=debug
export API_URL="https://api.example.com"

Special Variables¶

Variables set by bash:

Variable	Meaning
`$?`	Last command exit status
`$$`	Current shell PID
`$!`	Last background job PID
`$0`	Script name
`$#`	Number of arguments
`$@`	All arguments (individually quoted)
`$*`	All arguments (as single word)
`$_`	Last argument of previous command

See Special Variables for complete reference.

Security Considerations¶

Never Export Secrets¶

Don't put sensitive data in shell config:

# Bad - visible in ps, env dumps
export API_KEY="secret123"

# Better - use files with restricted permissions
chmod 600 ~/.secrets
source ~/.secrets

Sanitize User Input¶

When using variables in commands:

# Quote to prevent word splitting and globbing
rm "$file"              # Not: rm $file

# Validate before use
[[ "$input" =~ ^[a-z]+$ ]] || exit 1

Debugging¶

Trace Variable Expansion¶

set -x                  # Show commands as executed
echo $PATH
set +x

Check If Set¶

[[ -v VARNAME ]] && echo "Set"
[[ -z "$VARNAME" ]] && echo "Empty or unset"
[[ -n "$VARNAME" ]] && echo "Not empty"

Try It¶

Explore current environment:

env | sort | head -20
echo $PATH | tr ':' '\n'

Practice variable scope:

myvar="local"
echo $myvar
bash -c 'echo $myvar'   # Empty

export myvar
bash -c 'echo $myvar'   # Works now

Use defaults:

unset NAME
echo "Hello, ${NAME:-World}"

NAME="Alice"
echo "Hello, ${NAME:-World}"

Modify PATH:

echo $PATH
export PATH="$HOME/test-bin:$PATH"
echo $PATH

Summary¶

Action	Syntax
Set shell variable	`VAR=value`
Export to environment	`export VAR=value`
View variable	`echo $VAR`
View all environment	`env` or `printenv`
View all variables	`set`
Unset variable	`unset VAR`
Default if unset	`${VAR:-default}`
Set default if unset	`${VAR:=default}`
Prepend to PATH	`export PATH="$NEW:$PATH"`