Exit codes (also called return codes or status codes) are numeric values that programs return to the shell when they finish executing. They indicate whether a command succeeded (0) or failed (non-zero), enabling scripts and system processes to make decisions based on command outcomes.

Key Concepts

• Success Code: Always 0, indicates successful execution
• Error Codes: 1-255, indicate various types of failures
• $? Variable: Shell variable containing the last command’s exit code
• Signal Termination: Codes 128+ often indicate process killed by signal
• Command Chaining: Exit codes enable conditional execution

Command Syntax

Exit codes are automatically set by commands, but you can:

• Check with: echo $?
• Set manually with: exit [code]
• Use in conditionals: command && success_action || fail_action

Common Exit Codes

0 - Success/True 1 - General error 2 - Misuse of shell command 126 - Command not executable 127 - Command not found 128+n - Fatal error signal “n” 130 - Script terminated by Ctrl+C

Practical Examples

Example 1: Checking exit codes

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ls /existing/directory
echo $?
# Output: 0

ls /nonexistent/directory
echo $?
# Output: 2

Demonstrates success vs failure exit codes

Example 2: Using in conditionals

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if grep "pattern" file.txt; then
    echo "Pattern found"
else
    echo "Pattern not found"
fi

Conditional execution based on grep’s exit code

Example 3: Command chaining

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mkdir backup && cp file.txt backup/ && echo "Success"
# Only proceeds if each command succeeds

command1 || echo "Command1 failed"
# Executes echo only if command1 fails

Logical operators using exit codes for flow control

Example 4: Setting custom exit codes

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#!/bin/bash
if [ $# -eq 0 ]; then
    echo "No arguments provided"
    exit 1
fi
echo "Processing: $1"
exit 0

Script that sets appropriate exit codes

Use Cases

• Script Error Handling: Check if operations succeeded
• Automated Systems: Monitor service health
• CI/CD Pipelines: Determine build success/failure
• System Monitoring: Alert on command failures
• Conditional Execution: Run commands based on previous results

Related Commands

echo $? - Display last exit code exit [n] - Set exit code and terminate test or [ - Return exit codes for conditions true - Always returns 0 false - Always returns 1

Tips & Troubleshooting

• Always check $? immediately after the command
• Exit codes are overwritten by each new command
• Use set -e in scripts to exit on any non-zero code
• Document custom exit codes in your scripts
• Remember: 0 = success, everything else = failure
• Some commands use specific codes for different errors
• Pipeline exit codes reflect the last command unless using set -o pipefail