Are there any standard exit status codes in Linux?

Understanding Linux Exit Status Codes: A Comprehensive Guide

Explore the standard and conventional uses of exit status codes in Linux, crucial for robust scripting and error handling.

In the world of Linux and Unix-like operating systems, every command or script, upon completion, returns an integer value known as an 'exit status code' or 'return code'. This seemingly small detail is incredibly powerful, serving as a fundamental mechanism for inter-process communication and robust error handling in shell scripting. Understanding these codes is essential for writing reliable scripts that can react appropriately to the success or failure of commands they execute.

The Basics: Success and Failure

The most fundamental concept of exit status codes revolves around distinguishing between success and failure. By convention, an exit status of 0 signifies successful execution, while any non-zero value indicates some form of error or abnormal termination. This simple binary distinction forms the backbone of conditional execution in shell scripts, allowing you to chain commands or execute different branches of code based on the outcome of a previous operation.

ls /nonexistent_directory
echo $?

ls /etc
echo $?

Demonstrating exit codes for failed and successful commands.

Conventional Exit Status Codes

While 0 for success and non-zero for failure is universal, specific non-zero values often carry conventional meanings. These conventions are not strictly enforced by the kernel but are widely adopted by applications and utilities, making scripts more predictable and easier to debug. The range of possible exit codes is typically 0-255, as the shell often truncates values to the lower 8 bits.

Here are some commonly recognized conventional exit codes:

flowchart TD
    A[Command Execution] --> B{Exit Status?}
    B -- 0 --> C[Success]
    B -- 1 --> D["General Error (e.g., catchall)"]
    B -- 2 --> E["Misuse of Shell Builtins (e.g., invalid arguments)"]
    B -- 126 --> F["Command Invoked Cannot Execute (e.g., permission denied)"]
    B -- 127 --> G["Command Not Found (e.g., typo, not in PATH)"]
    B -- 128+N --> H["Fatal Error Signal N (e.g., 130 for Ctrl+C, SIGINT)"]
    B -- >128 --> I["Other Specific Errors (application-defined)"]

Common Linux Exit Status Code Conventions

It's important to note that specific applications or utilities might define their own set of non-zero exit codes for various error conditions. For instance, grep uses 0 for lines found, 1 for no lines found, and 2 for errors. Always consult the man page or documentation for a specific command to understand its unique exit code conventions.

Practical Application in Shell Scripting

Leveraging exit status codes is fundamental to writing robust shell scripts. They enable conditional execution, error logging, and graceful script termination. The $? special parameter holds the exit status of the most recently executed foreground command. You can use this in if statements, while loops, or with logical operators like && (AND) and || (OR).

# Example 1: Conditional execution
if grep -q "pattern" file.txt; then
    echo "Pattern found!"
else
    echo "Pattern not found or error occurred."
fi

# Example 2: Chaining commands
command_a && command_b && echo "Both succeeded" || echo "One or more failed"

# Example 3: Custom exit codes in your script
my_custom_function() {
    # ... do something ...
    if [ "$1" = "error" ]; then
        return 10 # Custom error code
    else
        return 0
    fi
}

my_custom_function "success"
echo "Success status: $?"

my_custom_function "error"
echo "Error status: $?"

Using exit codes for conditional logic and custom script returns.