Bash by Example: Exit Codes

Bash 5.0+

Understanding exit statuses through the $? special variable that holds the return code of the last executed command, controlling script success and failure with the exit built-in command, using exit codes for error handling and conditional logic, following conventions where 0 means success and non-zero indicates errors, and implementing proper exit code propagation in functions and scripts.

Code

#!/bin/bash

# Run a command that succeeds
ls / > /dev/null
echo "ls / exit code: $?"

# Run a command that fails
ls /non_existent_folder 2> /dev/null
echo "ls fail exit code: $?"

# Custom function with exit codes
check_number() {
    if [ "$1" -gt 10 ]; then
        return 0  # Success
    else
        return 1  # Failure
    fi
}

check_number 15
if [ $? -eq 0 ]; then
    echo "Check 15: Success"
fi

check_number 5
# Directly using the exit code in 'if'
if ! check_number 5; then
    echo "Check 5: Failed (as expected)"
fi

# Exiting the script with a specific code
# exit 127

Explanation

In the Unix world, every command returns a numeric Exit Code (or return status) when it finishes. This code tells the system whether the command succeeded or failed. The convention is strict: 0 means Success, and any non-zero value (1-255) means Failure. This is the opposite of many programming languages where "true" is often 1.

The special variable $? holds the exit code of the most recently executed command. You must check $? immediately after the command runs, because running another command (even a simple echo) will overwrite it with its own exit code.

You can control your own script's exit code using the exit N command. If your script encounters a critical error (like a missing dependency or invalid input), you should exit 1 (or another non-zero number) to signal to any parent process that the script failed. This allows your script to be part of larger pipelines and automation workflows.

Code Breakdown

5

$? captures the result of ls /. Since the root directory exists, ls returns 0.

14

return 0 in a function sets $? to 0 for the caller. This mimics the behavior of standard commands.

27

if ! command; then. The if statement implicitly checks exit codes. ! inverts it, so "if exit code is NOT 0 (failure), then execute block".

Samplebadu

Bash by Example: Exit Codes

Code

Explanation

Code Breakdown