Running Commands & PATH Resolution

In Plain English

When you type a command in Bash, the shell executes a strict search hierarchy: Aliases -> Reserved Keywords -> Functions -> Built-ins -> $PATH disk executable binaries. Understanding PATH resolution prevents common "command not found" errors.

The Mental Model

"The $PATH variable is like a prioritized delivery route list: when you ask for a package (command), Bash checks each warehouse address in order from left to right until it finds the binary."

Deep Dive: How It Works

Lookup Hierarchy: Bash checks alias -> function -> builtin -> hash table -> $PATH entries.

$PATH Variable: A colon-separated list of directories (/usr/local/bin:/usr/bin:/bin) searched sequentially.

type and which: Use `type -a <cmd>` to inspect whether a command is an alias, builtin, or disk binary.

Syntax Blueprint

type command_name
which command_name
export PATH="/custom/bin:$PATH"

type displays command type classification; which prints the executable binary disk location.

Core Rules to Remember

Search Order: First match in $PATH wins. Prepending directories allows overriding system defaults.

Built-in Commands: Commands like cd, pwd, and exit run inside the shell process without spawning child processes.

Absolute vs Relative: You can execute local scripts via ./script.sh or absolute paths like /opt/app/bin/start.

Common Beginner Traps & How to Fix Them

Mistake:Executing a script in the current directory with just "script.sh" without "./".

Why it happens: For security reasons, the current directory (.) is not included in $PATH by default.

How to fix: Always prefix local executables with ./ (e.g. ./build.sh).

Pro Tip: Use `type -t <name>` in scripts to programmatically check if a command is a builtin, file, or alias.

Live Interactive Example

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Command Type Inspection

Git 2.45 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: cd is an internal shell builtin because changing directories must affect the parent shell process.

Your Turn: Micro Challenge

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Micro Exercise

Check Command Classification

Use `type cd` to check the classification of the cd command.

Sandbox Output

Click "Run & Check" to test your solution.

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