🌿 Environment Variables

What are Environment Variables?

Environment variables are key-value pairs that affect process behavior:

• Tell programs where to find things
• Configure program behavior
• Pass information between processes

---

Viewing Variables

printenv                    # All environment variables
printenv PATH               # Specific variable
echo $PATH                  # Also works
echo $HOME
echo $USER
env                         # Same as printenv

Important Variables

| Variable | Purpose | Example | |----------|---------|---------| | PATH | Where to find commands | /usr/bin:/bin | | HOME | Your home directory | /home/alice | | USER | Your username | alice | | SHELL | Your shell | /bin/bash | | EDITOR | Default text editor | vim or nano | | LANG | Language/locale | en_US.UTF-8 | | PWD | Current directory | /home/alice/project | | TERM | Terminal type | xterm-256color |

---

Setting Variables

For Current Session

MY_VAR="hello"              # Set (not exported)
export MY_VAR="hello"       # Set and export to children
export PATH="$PATH:/new/path"   # Add to existing PATH

Exported vs Non-Exported

PRIVATE="secret"            # Only this shell sees it
export PUBLIC="visible"     # Child processes see it too

# Test:
echo $PRIVATE               # Works in current shell
bash -c 'echo $PRIVATE'     # Empty (not exported)
bash -c 'echo $PUBLIC'      # "visible" (exported)

---

Making Variables Permanent

~/.bashrc — Interactive Non-Login Shells

# Add to ~/.bashrc
export MY_VAR="permanent"
export PATH="$PATH:$HOME/bin"
alias ll='ls -la'

~/.profile or ~/.bash_profile — Login Shells

# Add to ~/.profile
export EDITOR=vim

Reload Configuration

source ~/.bashrc            # Reload bashrc
. ~/.profile                # Shorthand for source

---

The PATH Variable

PATH tells the shell where to find commands:

echo $PATH
# /usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin

which python                # Shows /usr/bin/python
which ls                    # Shows /usr/bin/ls

Adding to PATH

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

# Permanently (add to ~/.bashrc)
export PATH="$HOME/.local/bin:$PATH"   # Prepend (higher priority)
export PATH="$PATH:/opt/tools"          # Append

Custom Scripts

mkdir -p ~/bin
# Put scripts there, add to PATH:
export PATH="$HOME/bin:$PATH"

---

Unsetting Variables

unset MY_VAR                # Remove variable

---

Startup File Order

When you log in:

1. /etc/profile (system-wide)
2. ~/.bash_profile OR ~/.bash_login OR ~/.profile

When you open a terminal:

1. /etc/bash.bashrc (system-wide)
2. ~/.bashrc

Best Practice

Put everything in ~/.bashrc, and have ~/.profile source it:

# In ~/.profile
if [ -f ~/.bashrc ]; then
    . ~/.bashrc
fi

---

Common Customizations

Add to ~/.bashrc:

# Aliases
alias ll='ls -la'
alias la='ls -A'
alias ..='cd ..'
alias ...='cd ../..'
alias grep='grep --color=auto'

# Custom prompt
export PS1='\u@\h:\w\$ '

# Editor
export EDITOR=vim
export VISUAL=vim

# History settings
export HISTSIZE=10000
export HISTFILESIZE=20000
export HISTCONTROL=ignoredups:erasedups

---

Try It!

Practice environment variables in the terminal:

Exercises:

1. View your PATH: echo $PATH
2. Create a variable: export MY_NAME="Ubuntu User"
3. Use it: echo "Hello, $MY_NAME"
4. View all variables: printenv | head -20

Try it in the terminal

A sandboxed shell with this lesson’s commands. Type help to see what’s available.