Custom exit codes (exit N) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does exit N do in a bash script?

exit N stops the script and sends the number N as the exit code to the system. This code tells if the script ended successfully or with an error.

beginner

Why use custom exit codes instead of just exit 0 or exit 1?

<p>Custom exit codes let you show different reasons for failure or success. For example, <code>exit 2</code> might mean "file not found," while <code>exit 3</code> means "permission denied." This helps other scripts or users understand what happened.</p>

beginner

What is the valid range for exit codes in bash scripts?

Exit codes must be between 0 and 255. 0 means success, and any number from 1 to 255 means different errors or statuses.

beginner

How can you check the exit code of the last command in bash?

Use the special variable $?. It holds the exit code of the last command or script run.

beginner

Example: What exit code will this script return?

#!/bin/bash
if [ ! -f myfile.txt ]; then
  exit 2
else
  exit 0
fi

If myfile.txt does not exist, the script exits with code 2. Otherwise, it exits with 0 meaning success.

What does exit 0 usually mean in a bash script?

A The script ran successfully

B The script failed

C The script is still running

D The script has a syntax error

exit 0 means the script finished without errors.

Which exit code range is valid in bash scripts?

A Any positive number

B 1 to 100

C 0 to 1000

D 0 to 255

Bash exit codes must be between 0 and 255.

How do you get the exit code of the last command in bash?

A $!

B $?

C $#

D $$

$? holds the exit code of the last command.

If a script ends with exit 3, what does it mean?

A The script succeeded

B The script is running in background

C The script failed with a specific error

D The script is paused

Exit codes other than 0 usually mean different errors or statuses.

What happens if you use exit 300 in a bash script?

A It exits with code 44 (300 modulo 256)

B It exits with code 300

C It causes an error and script stops immediately

D It exits with code 0

Exit codes wrap around modulo 256, so 300 becomes 44.

Explain how and why you would use custom exit codes in a bash script.

Think about how scripts tell the system if they succeeded or failed.

Describe how to check the exit code of the last command and why it is useful.

This is how you know if a command worked before moving on.

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