TIL: Why cd in a Script Does Nothing

I wanted a shortcut to jump to a scratch directory:

mkdir -p /tmp/scratch && cd /tmp/scratch

My first thought was to put it in a script. That does not work.

The problem

A script runs in a child process — a separate shell spawned to execute the file. The cd happens inside that child. When the script finishes, the child exits, and you are right back where you started. Your shell never moved.

# scratch.sh
#!/bin/bash
mkdir -p /tmp/scratch && cd /tmp/scratch

$ pwd
/Users/todd
$ bash scratch.sh
$ pwd
/Users/todd   # nothing changed

The same applies to environment variables. A script can set variables all day — your shell will never see them. The child process is disposable.

The fix: shell functions

A function runs in your current shell, not a child process. It can change your directory, set variables, and modify your environment.

Add this to ~/.zshrc:

scratch() { mkdir -p /tmp/scratch && cd /tmp/scratch; }

Now scratch drops you straight into /tmp/scratch. It works because the function executes in your shell, not a throwaway subprocess.

When to use what

Need	Use	Why
Change directory	Function in .zshrc	Must run in current shell
Set environment variables	Function or source	Must affect current shell
Standalone task (build, deploy, lint)	Script in ~/bin/ or ~/.local/bin/	Does not need to change shell state
Simple command shortcut	Alias in .zshrc	Shorter syntax for common commands

The rule of thumb: if it needs to change the state of your shell, it cannot be a script. It has to be a function or alias loaded into the configuration of your shell.

Where these live

• ~/.zshrc — loads for every new interactive shell. Functions, aliases, and shell options go here.
• ~/.zshenv — loads for every shell, including non-interactive ones (scripts, cron). Environment variables that everything needs go here.
• ~/bin/ or ~/.local/bin/ — directories on your PATH for standalone scripts. These run as child processes.

Another piece of Linux I was using without understanding. Now it makes sense why source .env works but bash .env does not change anything — same concept.