Custom exit codes (exit N) in Bash Scripting

Introduction

Custom exit codes help your script tell other programs or users what happened when it finished.

You want to show if a script worked or failed in a specific way.

You need to stop a script early and explain why.

You want other scripts to react differently based on your script's result.

Syntax

Bash Scripting

exit N

Replace N with a number from 0 to 255.

0 means success; any other number means an error or special condition.

Examples

Ends the script and says everything worked fine.

Bash Scripting

exit 0

Ends the script and signals a general error.

Bash Scripting

exit 1

Ends the script with a custom code 42 to show a specific error.

Bash Scripting

exit 42

Sample Program

This script checks if a file given as the first argument exists. It exits with 0 if yes, or 2 if no.

Bash Scripting

#!/bin/bash

# Check if a file exists
if [ -f "$1" ]; then
  echo "File exists."
  exit 0
else
  echo "File does not exist."
  exit 2
fi

Important Notes

Exit codes are numbers between 0 and 255 because the system uses 8 bits to store them.

Use different exit codes to help other scripts understand what happened.

Always use exit 0 for success to follow common practice.

Summary

Use 'exit N' to stop a script and send a number as the result.

0 means success; other numbers mean different errors or states.

Custom exit codes help scripts communicate clearly with each other.

Practice

1. What does the command exit 0 mean in a bash script?

easy

A. The script will continue running after this command.

B. The script encountered an error and stopped.

C. The script finished successfully without errors.

D. The script restarts from the beginning.

Solution

1. Step 1: Understand exit codes in bash

   In bash, exit 0 means the script ended without any errors.

2. Step 2: Interpret the meaning of 0

   Exit code 0 always signals success to the system or calling program.

3. Final Answer:

   The script finished successfully without errors. -> Option C

4. Quick Check:

   exit 0 = success [OK]

Hint: Remember: 0 means success, anything else means error [OK]

Common Mistakes:

• Thinking exit 0 means error
• Assuming script continues after exit
• Confusing exit 0 with restart

2. Which of the following is the correct syntax to exit a bash script with code 5?

easy

A. exit 5

B. exit code 5

C. exit(5)

D. exit -5

Solution

1. Step 1: Recall bash exit syntax

   The correct way to exit with a code is exit N, where N is a number.

2. Step 2: Check each option

   exit 5 uses exit 5, which is valid syntax. Others are invalid in bash.

3. Final Answer:

   exit 5 -> Option A

4. Quick Check:

   exit N syntax = exit 5 [OK]

Hint: Use just 'exit' followed by a number, no extra words [OK]

Common Mistakes:

• Adding 'code' after exit
• Using parentheses like a function
• Using negative numbers for exit

3. What will be the exit code of this script?

#!/bin/bash
if [ -f "/tmp/file.txt" ]; then
  exit 3
else
  exit 0
fi

medium

A. 0 if /tmp/file.txt exists, 3 if not

B. 3 if /tmp/file.txt exists, 0 if not

C. Always 0

D. Always 3

Solution

1. Step 1: Understand the if condition

   The script checks if the file /tmp/file.txt exists using -f.

2. Step 2: Analyze exit codes for each case

   If the file exists, it exits with code 3; otherwise, it exits with 0.

3. Final Answer:

   3 if /tmp/file.txt exists, 0 if not -> Option B

4. Quick Check:

   File exists -> exit 3; else -> exit 0 [OK]

Hint: Match exit code to condition true/false carefully [OK]

Common Mistakes:

• Mixing up the if and else branches
• Assuming exit 0 always means file exists
• Ignoring the file check condition

4. Identify the error in this script snippet:

#!/bin/bash
exit 256

medium

A. Exit code 256 is invalid; exit codes must be 0-255.

B. Missing quotes around 256.

C. exit command should be lowercase.

D. exit requires a string, not a number.

Solution

1. Step 1: Check valid exit code range

   Bash exit codes must be between 0 and 255 inclusive.

2. Step 2: Analyze the given exit code

   256 is outside this range, so it is invalid and will wrap around or cause unexpected behavior.

3. Final Answer:

   Exit code 256 is invalid; exit codes must be 0-255. -> Option A

4. Quick Check:

   Exit codes range 0-255 [OK]

Hint: Exit codes must be 0-255, no higher numbers [OK]

Common Mistakes:

• Using numbers above 255
• Thinking quotes are needed
• Confusing case sensitivity of exit

5. You want a script to exit with code 10 if a user is not root, and 0 if root. Which snippet correctly implements this?

hard

A. if [ "$USER" != "root" ]; then exit 10; else exit 0; fi

B. if [ "$UID" -eq 0 ]; then exit 10; else exit 0; fi

C. if [ "$USER" == "root" ]; then exit 10; else exit 0; fi

D. if [ "$UID" -ne 0 ]; then exit 10; else exit 0; fi

Solution

1. Step 1: Identify how to check root user

   Root user has UID 0, so checking $UID -ne 0 means user is not root.

2. Step 2: Match exit codes to condition

   If user is not root, exit 10; else exit 0 for root user.

3. Final Answer:

   if [ "$UID" -ne 0 ]; then exit 10; else exit 0; fi -> Option D

4. Quick Check:

   UID 0 means root; non-zero means not root [OK]

Hint: Use UID 0 to check root, not USER string [OK]

Common Mistakes:

• Comparing USER string instead of UID
• Reversing the exit codes
• Using == instead of -eq for numbers