trap for cleanup on exit in Bash Scripting - Mini Project: Build & Apply

Using trap for Cleanup on Exit in Bash

📖 Scenario: You are writing a bash script that creates a temporary file to store some data. To keep your system clean, you want to make sure this temporary file is deleted automatically when the script finishes or if it is interrupted.

🎯 Goal: Build a bash script that creates a temporary file, sets up a trap to delete this file when the script exits, and then prints a message confirming the cleanup.

📋 What You'll Learn

Create a variable tempfile with the value /tmp/mytempfile.txt

Set up a trap command to delete $tempfile on script exit

Create the temporary file using touch

Print Cleanup done! after the file is deleted

💡 Why This Matters

🌍 Real World

Scripts often create temporary files or resources that must be cleaned up to avoid clutter or security risks. Using trap ensures cleanup happens even if the script is interrupted.

💼 Career

Knowing how to use trap for cleanup is important for system administrators, DevOps engineers, and anyone writing reliable shell scripts that manage temporary resources.

1

Create the temporary file variable

Create a variable called tempfile and set it to the string /tmp/mytempfile.txt.

Bash Scripting

# Create a variable called tempfile with the value /tmp/mytempfile.txt
# Your code here

Hint

Use tempfile=/tmp/mytempfile.txt to assign the path.

2

Set up the trap for cleanup

Add a trap command that runs rm -f "$tempfile" and then echo "Cleanup done!" when the script exits.

Bash Scripting

tempfile=/tmp/mytempfile.txt
# Set up trap to remove tempfile and print message on exit
# Your code here

Hint

Use trap 'rm -f "$tempfile"; echo "Cleanup done!"' EXIT to run commands on exit.

3

Create the temporary file

Use the touch command to create the file at the path stored in tempfile.

Bash Scripting

tempfile=/tmp/mytempfile.txt
trap 'rm -f "$tempfile"; echo "Cleanup done!"' EXIT
# Create the tempfile using touch
# Your code here

Hint

Use touch "$tempfile" to create the file.

4

Test the cleanup on exit

Run the script and observe that when it finishes, it prints Cleanup done! and the temporary file is deleted.

Bash Scripting

tempfile=/tmp/mytempfile.txt
trap 'rm -f "$tempfile"; echo "Cleanup done!"' EXIT
touch "$tempfile"
# Run the script and check output
# Your code here

Hint

Run the script normally. The message Cleanup done! should appear when it exits.

Practice

1. What is the main purpose of using trap in a bash script?

easy

A. To define functions inside the script

B. To create loops that repeat commands

C. To run specific commands when the script exits or receives signals

D. To declare variables with default values

Solution

1. Step 1: Understand the role of trap

   The trap command is designed to catch signals or script exit events and run commands in response.

2. Step 2: Compare options with trap's purpose

   Options B, C, and D describe other bash features unrelated to trap. Only To run specific commands when the script exits or receives signals correctly describes its purpose.

3. Final Answer:

   To run specific commands when the script exits or receives signals -> Option C

4. Quick Check:

   trap runs commands on exit/signals = A [OK]

Hint: Trap runs cleanup commands on exit or signals [OK]

Common Mistakes:

• Confusing trap with loops or functions
• Thinking trap declares variables
• Assuming trap runs commands continuously

2. Which of the following is the correct syntax to run cleanup function on script exit using trap?

easy

A. trap EXIT cleanup

B. trap 'cleanup' on EXIT

C. trap cleanup on EXIT

D. trap 'cleanup' EXIT

Solution

1. Step 1: Recall trap syntax for commands

   The command(s) to run must be quoted as a string, followed by the signal or event name.

2. Step 2: Check each option's syntax

   trap 'cleanup' EXIT correctly quotes the command and uses the signal EXIT. trap EXIT cleanup places the signal before the command (wrong order). Options C and D use incorrect keywords like 'on'.

3. Final Answer:

   trap 'cleanup' EXIT -> Option D

4. Quick Check:

   trap 'command' SIGNAL = B [OK]

Hint: Always quote commands in trap before the signal [OK]

Common Mistakes:

• Omitting quotes around commands
• Using 'on' keyword incorrectly
• Placing signal before command

3. What will be the output of this script?

#!/bin/bash
cleanup() {
  echo "Cleaning up temp files"
}
trap 'cleanup' EXIT

echo "Script is running"

medium

A. Script is running

B. Script is running\nCleaning up temp files

C. No output

D. Cleaning up temp files

Solution

1. Step 1: Understand script flow

   The script prints "Script is running" first, then exits.

2. Step 2: Effect of trap on EXIT

   On exit, the cleanup function runs, printing "Cleaning up temp files" after the first echo.

3. Final Answer:

   Script is running\nCleaning up temp files -> Option B

4. Quick Check:

   trap runs cleanup on exit, so both lines print [OK]

Hint: trap runs cleanup after script finishes [OK]

Common Mistakes:

• Thinking trap runs before script output
• Expecting only cleanup message
• Ignoring trap on EXIT

4. Identify the error in this script snippet that uses trap for cleanup:

#!/bin/bash
trap cleanup EXIT

cleanup() {
  echo "Cleaning up"
}

medium

A. No error, script is correct

B. EXIT signal is invalid

C. trap command missing quotes around cleanup

D. cleanup function is defined after trap command

Solution

1. Step 1: Check trap command syntax

   For simple function names like cleanup, quotes are not required around the command in trap.

2. Step 2: Analyze script order and signals

   Defining cleanup after trap is allowed because bash processes function definitions before executing traps. EXIT is valid. No syntax errors present.

3. Final Answer:

   No error, script is correct -> Option A

4. Quick Check:

   unquoted function name OK, post-definition OK [OK]

Hint: trap accepts unquoted simple function names [OK]

Common Mistakes:

• Thinking functions must be defined before trap
• Believing EXIT is an invalid signal
• Assuming quotes are required around simple commands

5. You want to ensure a temporary file /tmp/mytempfile is deleted when your bash script exits, even if interrupted. Which trap command correctly achieves this?

hard

A. trap 'rm -f /tmp/mytempfile' EXIT INT TERM

B. trap rm -f /tmp/mytempfile EXIT INT TERM

C. trap 'rm -f /tmp/mytempfile' ONEXIT

D. trap 'rm -f /tmp/mytempfile' EXIT SIGNAL

Solution

1. Step 1: Understand trap for multiple signals

   To catch script exit and interruptions, use signals EXIT, INT, and TERM together.

2. Step 2: Check syntax correctness

   Commands must be quoted. trap 'rm -f /tmp/mytempfile' EXIT INT TERM quotes the command and lists valid signals. Options B misses quotes, C uses invalid signal ONEXIT, D uses invalid SIGNAL keyword.

3. Final Answer:

   trap 'rm -f /tmp/mytempfile' EXIT INT TERM -> Option A

4. Quick Check:

   trap 'command' SIGNALS with quotes = A [OK]

Hint: Quote commands and list all signals to catch [OK]

Common Mistakes:

• Missing quotes around commands
• Using invalid signal names
• Not including INT and TERM for interrupts