Exit Codes

Exit codes (also called return codes or exit status) indicate whether a command succeeded or failed. Understanding exit codes is fundamental to error handling.

What Are Exit Codes?

Every command returns a number when it finishes:

• 0 = Success
• 1-255 = Failure (various error types)

#!/bin/bash
ls /home
echo "Exit code: $?"    # 0 (success)

ls /nonexistent
echo "Exit code: $?"    # 2 (no such file)

The $? Variable

$? holds the exit code of the last command:

#!/bin/bash
grep "pattern" file.txt
RESULT=$?

if [ $RESULT -eq 0 ]; then
    echo "Pattern found"
elif [ $RESULT -eq 1 ]; then
    echo "Pattern not found"
else
    echo "Error occurred"
fi

Common Exit Codes

Code	Meaning
0	Success
1	General error
2	Misuse of command
126	Command not executable
127	Command not found
128	Invalid exit argument
128+N	Fatal signal N (e.g., 130 = Ctrl+C)
255	Exit status out of range

Exercise: Check Exit Code

Use exit code to check success:

Setting Exit Codes

Use exit to set your script's exit code:

#!/bin/bash

# Exit with success
exit 0

# Exit with error
exit 1

# Exit with specific code
if [ ! -f "$1" ]; then
    echo "File not found" >&2
    exit 2
fi

The exit Command

#!/bin/bash

check_file() {
    if [ ! -f "$1" ]; then
        echo "Error: File not found" >&2
        exit 1    # Exits the ENTIRE script
    fi
}

check_file "missing.txt"
echo "This won't print"

Return vs Exit

• exit terminates the script
• return exits only the function

#!/bin/bash

validate() {
    if [ -z "$1" ]; then
        return 1    # Function returns, script continues
    fi
    return 0
}

if validate ""; then
    echo "Valid"
else
    echo "Invalid"    # This prints
fi

echo "Script continues"    # This also prints

Exit on Error

Using set -e:

#!/bin/bash
set -e    # Exit on first error

echo "Step 1"
false     # This will cause exit
echo "Step 2"    # Never reached

Better: control where to exit:

#!/bin/bash
set -e

# Critical section
do_important_thing

set +e    # Disable for non-critical
optional_command    # Won't exit on failure
set -e

# Back to strict mode
do_another_thing

Exercise: Exit with Code

Set script exit code:

Checking Commands in Conditions

#!/bin/bash

# Direct check in if
if grep -q "pattern" file.txt; then
    echo "Found"
fi

# Command success/failure
if command; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

# Negation
if ! command; then
    echo "Command failed"
fi

Conditional Execution

#!/bin/bash

# && = run if previous succeeded
mkdir project && cd project && echo "Created and entered"

# || = run if previous failed
rm file.txt || echo "Could not remove file"

# Combined
command && echo "Success" || echo "Failure"

Exit Code Best Practices

#!/bin/bash

# Define exit codes as constants
readonly EXIT_SUCCESS=0
readonly EXIT_ERROR=1
readonly EXIT_USAGE=2
readonly EXIT_NO_FILE=3

# Main script
main() {
    if [ $# -lt 1 ]; then
        echo "Usage: $0 <file>" >&2
        exit $EXIT_USAGE
    fi

    if [ ! -f "$1" ]; then
        echo "Error: File not found: $1" >&2
        exit $EXIT_NO_FILE
    fi

    # Process file
    process "$1" || exit $EXIT_ERROR

    exit $EXIT_SUCCESS
}

main "$@"

Key Takeaways

• Exit code 0 means success, non-zero means failure
• $? contains the last command's exit code
• Use exit N to set script's exit code
• Use return N in functions (doesn't exit script)
• set -e exits on any command failure
• && runs next command only on success
• || runs next command only on failure
• Define meaningful exit codes for your scripts