Custom exit codes (exit N) in Bash Scripting - Mini Project: Build & Apply

Custom Exit Codes in Bash Scripts

📖 Scenario: You are writing a small bash script to check if a user is allowed to enter a club based on their age. The script will exit with different codes to show if the user is too young, just right, or too old.

🎯 Goal: Build a bash script that uses custom exit codes with exit N to indicate different age categories.

📋 What You'll Learn

Create a variable age with a specific value.

Create a variable min_age to set the minimum allowed age.

Use if statements to check the age and exit with custom codes.

Print a message showing the exit code after running the script.

💡 Why This Matters

🌍 Real World

Scripts often need to tell other programs or users what happened by using exit codes. Custom exit codes help identify specific situations like errors or special cases.

💼 Career

Understanding exit codes is important for automation, debugging, and writing scripts that work well with other tools in system administration and DevOps.

1

Set the user's age

Create a variable called age and set it to 20.

Bash Scripting

# Create a variable called age and set it to 20
# Your code here

Hint

Use age=20 to assign the value.

2

Set the minimum allowed age

Create a variable called min_age and set it to 18.

Bash Scripting

age=20
# Create a variable called min_age and set it to 18
# Your code here

Hint

Use min_age=18 to assign the value.

3

Use if statements to set custom exit codes

Write an if statement that checks if age is less than min_age. If yes, exit with code 1. Otherwise, if age is between min_age and 30 inclusive, exit with code 0. Else, exit with code 2.

Bash Scripting

age=20
min_age=18
# Write if statements to exit with codes 1, 0, or 2 based on age
# Your code here

Hint

Use if, elif, and else with exit N commands.

4

Print the exit code after running the script

Run the script and print the exit code using echo and $?.

Bash Scripting

age=20
min_age=18
if [ "$age" -lt "$min_age" ]; then
    exit 1
elif [ "$age" -le 30 ]; then
    exit 0
else
    exit 2
fi

# Print the exit code below
# Your code here

Hint

After running the script, use echo $? in the terminal to see the exit code.

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