The PATH Variable

The PATH variable is one of the most important environment variables. It tells the shell where to find executable programs.

What is PATH?

PATH is a colon-separated list of directories where the shell looks for commands:

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

How PATH Works

When you type a command:

1. Shell checks if it's a built-in
2. Shell searches PATH directories (left to right)
3. First match is executed
4. If not found: "command not found"

Viewing PATH

echo $PATH

Make it readable:

echo $PATH | tr ':' '\n'

Common PATH Directories

Directory	Contains
/usr/local/bin	Locally installed software
/usr/bin	Standard system commands
/bin	Essential system commands
/sbin	System administration commands
~/bin or ~/.local/bin	User's personal scripts

Exercise: View Your PATH

Display the PATH variable:

Adding to PATH

Temporarily (Current Session)

export PATH=$PATH:/new/directory
export PATH=/new/directory:$PATH    # Add to front

Permanently

Add to ~/.bashrc:

echo 'export PATH=$PATH:$HOME/bin' >> ~/.bashrc
source ~/.bashrc    # Reload

Order Matters

Directories are searched left to right. First match wins:

# If /usr/local/bin is first, its python runs
PATH=/usr/local/bin:/usr/bin

# If /usr/bin is first, system python runs
PATH=/usr/bin:/usr/local/bin

Finding Commands in PATH

which python        # Shows which python runs
type python         # Shows command type and location

Common PATH Modifications

Add Local Scripts

export PATH="$HOME/bin:$PATH"
export PATH="$HOME/.local/bin:$PATH"

Add Programming Tools

# Node.js
export PATH="$HOME/.npm-global/bin:$PATH"

# Go
export PATH="$PATH:/usr/local/go/bin"

# Rust
export PATH="$HOME/.cargo/bin:$PATH"

Troubleshooting PATH Issues

Command Not Found

# Check if command exists
which mycommand
# If not found, check where it is
find /usr -name mycommand
# Add its directory to PATH

Wrong Version Running

# See which one runs
which python
# Check all locations
type -a python
# Reorder PATH if needed

Exercise: Modify PATH

Add a directory to your PATH:

Security Note

Never put . (current directory) at the start of PATH!

# DANGEROUS - allows hijacking
PATH=.:$PATH

# An attacker could put a malicious 'ls' in any directory

Key Takeaways

• PATH tells shell where to find commands
• Directories separated by colons
• Search happens left to right
• Add directories with export PATH=$PATH:/new/dir
• Make permanent by adding to ~/.bashrc