LearnExit Codes ($?), Error Handling & Custom Exits

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Lesson 57 of 69

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Exit Codes ($?), Error Handling & Custom Exits

In Plain English

Every command, binary, and script in Unix returns an integer Exit Status (Exit Code) between 0 and 255 upon termination. By universal standard, `0` signifies success, while any non-zero value (`1–255`) indicates an error or failure condition. The special parameter `$?` captures the exit code of the most recently executed command.

The Mental Model

"An exit code is like a doctor's report at the end of an examination: `0` is a clean bill of health; any number > 0 specifies the exact medical code for what went wrong."

Deep Dive: How It Works

Special Variable `$?`: Immediately stores the exit status of the previous foreground command.

Standard Conventions: `0` (Success), `1` (General error), `2` (Misuse of shell builtins), `126` (Command invoked cannot execute / permission denied), `127` (Command not found), `130` (Terminated by Ctrl+C / SIGINT 128+2).

The `exit` Command: `exit 0` ends the script with success; `exit 1` ends with error.

Syntax Blueprint

command
if [[ $? -ne 0 ]]; then
  echo "Command failed with code $?"
  exit 1
fi

# Or idiomatic short-circuit:
command || { echo "Failed"; exit 1; }

Inspect $? immediately after running a command or use short-circuit logic ||.

Core Rules to Remember

0 Means Success: Unlike boolean true/false in other languages, in Unix 0 is truthy (success).

$? is Overwritten Instantly: Every single command run rewrites $?. Save it into a local variable (`EXIT_CODE=$?`) if needed.

Custom Exit Codes: Define clear exit codes in your scripts (e.g. 2 for invalid args, 3 for network timeout).

Common Beginner Traps & How to Fix Them

Mistake:Running `echo $?` followed by `if [ $? -eq 0 ]`.

Why it happens: The `echo` command succeeds, resetting `$?` to 0 before the `if` statement can test it.

How to fix: Save `$?` to a variable immediately: `STATUS=$?; echo "Status: $STATUS"; if [ $STATUS -eq 0 ]...`.

Pro Tip: In Bash functions, return non-zero exit codes with `return 1` to indicate validation failure without killing the whole parent script.

Live Interactive Example

Hit Run Code to see it live

Capturing Exit Codes and Handling Failures

Git 2.45 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: ls returned 0 (success) and grep returned 1 (no match found).

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Capture and Test an Exit Code

Run `true` (a command that always succeeds with 0).
Capture `$?` into `CODE=$?`.
Echo "Exit Code: $CODE".

Sandbox Output

Click "Run & Check" to test your solution.

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