set -e for exit on error in Bash Scripting - Mini Project: Build & Apply

Using set -e to Exit on Error in Bash Scripts

📖 Scenario: You are writing a simple bash script to run a few commands in order. You want the script to stop immediately if any command fails, so you don't continue with wrong results.

🎯 Goal: Learn how to use set -e in a bash script to make it exit on the first error.

📋 What You'll Learn

Create a bash script with three commands

Add set -e to make the script exit on error

Test the script to see it stops when a command fails

Print a message after the commands to confirm script behavior

💡 Why This Matters

🌍 Real World

In real scripts, <code>set -e</code> helps catch errors early and prevents running commands that depend on previous successful steps.

💼 Career

Many jobs require writing reliable bash scripts for automation. Knowing how to use <code>set -e</code> is essential for safe scripting.

1

Create a bash script with three commands

Create a bash script with these three commands exactly: echo "Start script", ls /nonexistentfolder, and echo "End script".

Bash Scripting

# Write the three commands below
# Your code here

Hint

Use echo to print messages and ls to list a folder that does not exist.

2

Add set -e to exit on error

Add the line set -e at the very top of the script to make it exit immediately if any command fails.

Bash Scripting

# Add set -e at the top

# Your code here

Hint

set -e must be the first line in the script.

3

Add a success message after commands

Add a new line at the end of the script that prints echo "Script completed successfully".

Bash Scripting

set -e
echo "Start script"
ls /nonexistentfolder
echo "End script"
# Add success message below
# Your code here

Hint

This message will only print if no command fails before it.

4

Run the script and observe output

Run the script and observe that it prints Start script, then stops with an error because ls /nonexistentfolder fails. It should NOT print End script or Script completed successfully. Use bash yourscript.sh to run.

Bash Scripting

# Run the script in your terminal
# Your code here

Hint

The script stops after the error. You will see the error message from ls.

Practice

1. What does the set -e command do in a bash script?

easy

A. It stops the script immediately if any command returns a non-zero exit status.

B. It prints all commands before executing them.

C. It ignores errors and continues running the script.

D. It runs the script in debug mode.

Solution

1. Step 1: Understand the purpose of set -e

   The set -e option tells bash to exit the script if any command fails (returns a non-zero status).

2. Step 2: Compare with other options

   Options like printing commands or ignoring errors do not match set -e's behavior.

3. Final Answer:

   It stops the script immediately if any command returns a non-zero exit status. -> Option A

4. Quick Check:

   set -e = exit on error [OK]

Hint: Remember: set -e means exit on any error [OK]

Common Mistakes:

• Confusing set -e with set -x (debug mode)
• Thinking it ignores errors instead of stopping
• Assuming it prints commands before running

2. Which of the following is the correct way to enable exit on error in a bash script?

easy

A. set -e

B. set +e

C. exit -e

D. enable -e

Solution

1. Step 1: Recall the syntax for enabling exit on error

   The correct syntax to enable exit on error is set -e.

2. Step 2: Eliminate incorrect options

   set +e disables exit on error, exit -e and enable -e are invalid commands.

3. Final Answer:

   set -e -> Option A

4. Quick Check:

   Enable exit on error = set -e [OK]

Hint: Use set -e to turn on exit on error [OK]

Common Mistakes:

• Using set +e which disables exit on error
• Confusing exit command with set
• Typing invalid commands like enable -e

3. Consider this script with set -e enabled:

#!/bin/bash
set -e
echo "Start"
false
echo "End"

What will be the output when this script runs?

medium

A. Start
End

B. No output

C. End

D. Start

Solution

1. Step 1: Analyze script commands with set -e

   The script prints "Start", then runs false which returns a non-zero exit status causing the script to exit immediately.

2. Step 2: Determine which commands run

   Because of set -e, the script stops after false, so "End" is never printed.

3. Final Answer:

   Start -> Option D

4. Quick Check:

   set -e stops script on error, so only "Start" prints [OK]

Hint: Script stops at first error, so later lines don't run [OK]

Common Mistakes:

• Expecting all echo statements to run
• Ignoring that false returns error
• Thinking set -e allows script to continue

4. You have this script:

#!/bin/bash
set -e
echo "Start"
ls /nonexistent_directory
echo "End"

The script exits immediately after the ls command. How can you fix it to continue even if ls fails?

medium

A. Remove set -e from the script.

B. Add || true after the ls command.

C. Change ls to ls -e.

D. Add set +e at the start of the script.

Solution

1. Step 1: Understand why script exits

   set -e causes the script to exit on any command failure, so ls /nonexistent_directory failing stops the script.

2. Step 2: Use || true to ignore error

   Appending || true makes the command always succeed, preventing exit.

3. Final Answer:

   Add || true after the ls command. -> Option B

4. Quick Check:

   Use command || true to bypass set -e exit [OK]

Hint: Add || true to ignore errors on specific commands [OK]

Common Mistakes:

• Removing set -e loses error protection
• Using invalid ls -e option
• Trying set +e disables exit globally, not selective

5. You want a bash script to stop on any error except for a specific command that might fail but should not stop the script. How do you write this script snippet correctly?

#!/bin/bash
set -e
# Your command here
other_command

Which snippet correctly allows your_command to fail without stopping the script?

hard

A. your_command ; true
other_command

B. set +e
your_command
other_command

C. your_command || true
other_command

D. trap '' ERR
your_command
other_command

Solution

1. Step 1: Understand selective error ignoring

   To allow one command to fail without stopping the script under set -e, append || true to that command.

2. Step 2: Compare other options

   your_command ; true other_command: your_command ; true fails because set -e exits after your_command before true. set +e your_command other_command disables set -e but doesn't re-enable it for other_command. trap '' ERR your_command other_command ignores ERR trap but doesn't restore it, affecting other_command.

3. Final Answer:

   your_command || true other_command -> Option C

4. Quick Check:

   Use command || true to ignore errors selectively [OK]

Hint: Add || true after command to ignore its errors [OK]

Common Mistakes:

• Disabling set -e globally instead of locally
• Using if without handling exit status properly
• Using traps unnecessarily complicates script