trap for cleanup on exit in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the trap command do in bash scripting?

The trap command lets you run a specific command or script when the shell receives certain signals or when the script exits. It is often used to clean up temporary files or reset settings.

beginner

How do you use trap to run a cleanup function when a script exits?

You write a cleanup function and then use trap cleanup EXIT. This means the cleanup function runs automatically when the script finishes or is interrupted.

intermediate

Why is it useful to trap the EXIT signal in a script?

Trapping EXIT ensures that cleanup code runs no matter how the script ends, whether normally or due to an error or interruption. This helps keep your system clean and avoids leftover temporary files.

intermediate

What happens if you trap signals like INT or TERM in a script?

Trapping INT (interrupt) or TERM (terminate) lets your script handle those signals gracefully, such as cleaning up before exiting instead of stopping immediately.

beginner

Show a simple example of a bash script that creates a temp file and deletes it on exit using trap.

Example:

#!/bin/bash

tempfile=$(mktemp)

echo "Temp file created: $tempfile"

cleanup() {
  echo "Removing temp file"
  rm -f "$tempfile"
}

trap cleanup EXIT

# Script work here
sleep 5

What signal does trap cleanup EXIT catch in a bash script?

A Keyboard interrupt only

B System shutdown

C Script exit (normal or interrupted)

D File deletion

The EXIT signal triggers when the script ends for any reason, normal or interrupted.

Which command creates a temporary file in bash?

A tempfile

B mktemp

C touch temp

D createfile

mktemp safely creates a unique temporary file.

Why use trap to catch signals like INT or TERM?

A To run cleanup code before the script stops

B To speed up the script

C To ignore errors

D To create new files

Trapping these signals lets the script clean up before exiting.

What happens if you don’t use trap for cleanup in a script that creates temp files?

A Files are deleted automatically

B Script runs faster

C Script won’t run

D Temp files may remain and clutter the system

Without cleanup, temporary files can stay and waste space.

Which of these is a correct way to set a trap for cleanup on exit?

A trap cleanup EXIT

B trap EXIT cleanup

C trap cleanup SIGINT

D trap EXIT

The syntax is trap <command> <signal>, so trap cleanup EXIT is correct.

Explain how the trap command helps with cleanup in bash scripts.

Think about what happens when a script ends or is interrupted.

Describe a simple bash script scenario where trapping EXIT is important and what it does.

Imagine you create a file that should be deleted no matter how the script ends.

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