Day 2 – The PATH and Making Your Own Commands

Today's Focus

Understand what happens when you type a command, how the shell finds it, and how to create your own commands that work from anywhere on the system.

What is PATH?

When you type a command like ls or git, the shell doesn't search your entire filesystem — it only looks in a specific list of directories called the PATH. The PATH is an environment variable containing a colon-separated list of directories:

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

The shell searches these directories from left to right and runs the first match it finds. If no match is found, you see command not found.

Run this to see your current PATH:

echo $PATH

Run this to see exactly which version of a command the shell will use:

which git
which python3
which ls

Key Concepts

ConceptExplanation
$PATHEnvironment variable listing directories the shell searches for commands.
Left-to-right orderThe shell uses the first match found — earlier directories take priority.
which <cmd>Shows the full path of the executable that would run for a given command.
exportMakes an environment variable available to the current shell session and any child processes.
~/.zshrc / ~/.bashrcShell configuration files that run every time a new interactive shell session starts.
~/.zprofile / ~/.profileLogin shell configuration files that run once at login — used for PATH changes that should apply system-wide.
sourceReload a config file in the current session without opening a new terminal: source ~/.zshrc.

Tasks

  • Print your PATH and identify each directory in it. Run ls on two or three of those directories to see what commands live there.

  • Use which to locate ls, git, python3, and echo. Open one of those directories in your terminal and confirm the binary is there.

  • Understand order: create two scripts with the same name in two different directories, put both on your PATH in different positions, and observe which one runs. Then swap the order and see the result change.

  • Create a personal bin directory and add a custom command to it:

    mkdir -p ~/bin

    Create a script ~/bin/hello with the following content:

    #!/bin/sh
echo "Hello from my own command!"

    Make it executable:

    chmod +x ~/bin/hello

  • Try running hello — it will fail with command not found because ~/bin is not on your PATH yet.

  • Add ~/bin to your PATH for the current session only (disappears when the terminal closes):

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

    Run hello again — it should work now. Run which hello to confirm the shell found it in ~/bin.

  • Make the change permanent by adding the export to your rc file. Open ~/.zshrc (or ~/.bashrc if you use bash) and add the line at the bottom:

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

    Reload the file without closing the terminal:

    source ~/.zshrc

    Open a new terminal tab and confirm hello still works.

  • Explore the difference between ~/.zshrc (runs for every interactive shell) and ~/.zprofile (runs once at login). For PATH changes, ~/.zprofile is the more appropriate place on macOS — move your export there and test it by logging out and back in, or by running zsh --login -c 'echo $PATH'.

Reading / Reference

  • man zshrc / man bash — search for the "STARTUP FILES" section to understand the order rc files are loaded.
  • echo $SHELL — tells you which shell you are running so you know which rc file to edit.
  • The Linux Command Line (William Shotts) — Chapter 11 covers the environment and startup files.