Environment Variables

Environment variables configure how your shell and programs behave. PATH tells the shell where to find commands. HOME points to your home directory. Your apps read database URLs and API keys from them.

View Environment Variables

Terminal

$# All variables

$env

USER=john HOME=/home/john PATH=/usr/local/bin:/usr/bin:/bin SHELL=/bin/bash ...

$# Specific variable

$echo $PATH

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

$# Or use printenv

$printenv HOME

/home/john

Common Variables

Variable	Purpose
`PATH`	Directories to search for commands
`HOME`	User's home directory
`USER`	Current username
`SHELL`	Default shell
`EDITOR`	Default text editor
`LANG`	Language/locale setting
`PWD`	Current directory

Set Variables

Terminal

$# For current session only

$MY_VAR="hello"

$echo $MY_VAR

hello

$# Make available to child processes

$export MY_VAR="hello"

$bash -c 'echo $MY_VAR'

hello

Without export, variables are local to the current shell.

Persistent Variables

For Your User: ~/.bashrc

Terminal

$nano ~/.bashrc

Add:

hljs bash

export EDITOR=vim
export PATH="$HOME/bin:$PATH"
export API_KEY="your-key-here"

Then reload:

Terminal

$source ~/.bashrc

$# Or

$. ~/.bashrc

Shell Configuration Files

File	When Loaded
`~/.bashrc`	Interactive non-login shells
`~/.bash_profile`	Login shells
`~/.profile`	Login shells (if no .bash_profile)
`/etc/environment`	System-wide, all users
`/etc/profile`	System-wide, login shells

Which File to Use?

For most cases, use ~/.bashrc. For system-wide variables, use /etc/environment.

Modify PATH

Terminal

$# Add directory to beginning (searched first)

$export PATH="/opt/myapp/bin:$PATH"

$# Add to end

$export PATH="$PATH:/opt/myapp/bin"

Add to ~/.bashrc to make permanent:

hljs bash

# Add custom bin directories
export PATH="$HOME/bin:$HOME/.local/bin:$PATH"

Per-Command Variables

Terminal

$# Set just for one command

$DATABASE_URL=postgres://localhost/test ./migrate.sh

$# Multiple variables

$NODE_ENV=production PORT=3000 node server.js

Variable is only set for that command, doesn't affect your shell.

Using .env Files

Common pattern for applications:

hljs bash

# .env file
DATABASE_URL=postgres://localhost/mydb
API_KEY=secret123
DEBUG=true

Load it:

Terminal

$# In bash

$export $(cat .env | xargs)

$# Or source with set -a

$set -a && source .env && set +a

Don't Commit .env

Add .env to .gitignore. Never commit secrets to git.

Check If Variable Is Set

hljs bash

#!/bin/bash

# Check if set
if [[ -z "$API_KEY" ]]; then
    echo "API_KEY is not set"
    exit 1
fi

# Use default if not set
DB_HOST="${DB_HOST:-localhost}"

Unset Variables

Terminal

$export MY_VAR="something"

$echo $MY_VAR

something

$unset MY_VAR

$echo $MY_VAR

(empty)

Knowledge Check

What's the difference between 'VAR=x' and 'export VAR=x'?

Quick Reference

Command	Purpose
`echo $VAR`	View variable
`env`	List all variables
`export VAR=value`	Set and export
`unset VAR`	Remove variable
`source ~/.bashrc`	Reload config
`printenv VAR`	Print variable value

Key Takeaways

export makes variables available to child processes
Add to ~/.bashrc for persistence
PATH controls where commands are found
Use .env files for application config (don't commit them)
source reloads configuration files
Check required variables at script start

Next: creating shell aliases.

Environment Variables

View Environment Variables

Terminal

$# All variables

$env

USER=john HOME=/home/john PATH=/usr/local/bin:/usr/bin:/bin SHELL=/bin/bash ...

$# Specific variable

$echo $PATH

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

$# Or use printenv

$printenv HOME

/home/john

Common Variables

Variable	Purpose
`PATH`	Directories to search for commands
`HOME`	User's home directory
`USER`	Current username
`SHELL`	Default shell
`EDITOR`	Default text editor
`LANG`	Language/locale setting
`PWD`	Current directory

Set Variables

Terminal

$# For current session only

$MY_VAR="hello"

$echo $MY_VAR

hello

$# Make available to child processes

$export MY_VAR="hello"

$bash -c 'echo $MY_VAR'

hello

Without export, variables are local to the current shell.

Persistent Variables

For Your User: ~/.bashrc

Terminal

$nano ~/.bashrc

Add:

hljs bash

export EDITOR=vim
export PATH="$HOME/bin:$PATH"
export API_KEY="your-key-here"

Then reload:

Terminal

$source ~/.bashrc

$# Or

$. ~/.bashrc

Shell Configuration Files

File	When Loaded
`~/.bashrc`	Interactive non-login shells
`~/.bash_profile`	Login shells
`~/.profile`	Login shells (if no .bash_profile)
`/etc/environment`	System-wide, all users
`/etc/profile`	System-wide, login shells

Which File to Use?

For most cases, use ~/.bashrc. For system-wide variables, use /etc/environment.

Modify PATH

Terminal

$# Add directory to beginning (searched first)

$export PATH="/opt/myapp/bin:$PATH"

$# Add to end

$export PATH="$PATH:/opt/myapp/bin"

Add to ~/.bashrc to make permanent:

hljs bash

# Add custom bin directories
export PATH="$HOME/bin:$HOME/.local/bin:$PATH"

Per-Command Variables

Terminal

$# Set just for one command

$DATABASE_URL=postgres://localhost/test ./migrate.sh

$# Multiple variables

$NODE_ENV=production PORT=3000 node server.js

Variable is only set for that command, doesn't affect your shell.

Using .env Files

Common pattern for applications:

hljs bash

# .env file
DATABASE_URL=postgres://localhost/mydb
API_KEY=secret123
DEBUG=true

Load it:

Terminal

$# In bash

$export $(cat .env | xargs)

$# Or source with set -a

$set -a && source .env && set +a

Don't Commit .env

Add .env to .gitignore. Never commit secrets to git.

Check If Variable Is Set

hljs bash

#!/bin/bash

# Check if set
if [[ -z "$API_KEY" ]]; then
    echo "API_KEY is not set"
    exit 1
fi

# Use default if not set
DB_HOST="${DB_HOST:-localhost}"

Unset Variables

Terminal

$export MY_VAR="something"

$echo $MY_VAR

something

$unset MY_VAR

$echo $MY_VAR

(empty)

Knowledge Check

What's the difference between 'VAR=x' and 'export VAR=x'?

Quick Reference

Command	Purpose
`echo $VAR`	View variable
`env`	List all variables
`export VAR=value`	Set and export
`unset VAR`	Remove variable
`source ~/.bashrc`	Reload config
`printenv VAR`	Print variable value

Key Takeaways

export makes variables available to child processes
Add to ~/.bashrc for persistence
PATH controls where commands are found
Use .env files for application config (don't commit them)
source reloads configuration files
Check required variables at script start

Next: creating shell aliases.