Signal handling with trap in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output when SIGINT is sent?

Consider this bash script that traps SIGINT (Ctrl+C) and prints a message before exiting. What will be printed if the user presses Ctrl+C while the script is running?

Bash Scripting

trap 'echo "Caught SIGINT, exiting..."; exit 1' SIGINT
while true; do
  echo "Running..."
  sleep 1
done

A Running...\nRunning...\nCaught SIGINT, exiting...

B Running...\nRunning...\nTerminated

C Caught SIGINT, exiting...

D Running...\nRunning...\nRunning... (script never stops)

💡 Hint

Think about what the trap command does when SIGINT is received.

The trap command catches the SIGINT signal and runs the echo and exit commands. So the script prints 'Running...' repeatedly until Ctrl+C is pressed, then prints the message and exits.

💻 Command Output

intermediate

What happens if trap is not set for SIGTERM?

Given this script, what will happen if it receives a SIGTERM signal?

Bash Scripting

while true; do
  echo "Waiting..."
  sleep 2
done

A The script terminates immediately without any message.

B The script restarts automatically.

C The script prints 'Terminated' and exits.

D The script ignores SIGTERM and continues running.

💡 Hint

What is the default behavior of SIGTERM if not trapped?

By default, SIGTERM causes the process to terminate immediately without running any custom code.

📝 Syntax

advanced

Identify the syntax error in this trap command

Which option contains the correct syntax to trap SIGUSR1 and run a function named handle_usr1?

Bash Scripting

function handle_usr1() {
  echo "SIGUSR1 received"
}

# trap command here

A trap 'handle_usr1' SIGUSR1

B trap handle_usr1 SIGUSR1

C trap "handle_usr1" SIGUSR1

D trap handle_usr1() SIGUSR1

💡 Hint

trap expects a command string or function name without parentheses.

Option B correctly passes the function name without quotes or parentheses. Options B and C pass a string that calls the function but as a string literal, which works but is less direct. Option B is invalid syntax.

🔧 Debug

advanced

Why does this trap not work as expected?

This script is supposed to print 'Exiting...' when it receives SIGINT, but it doesn't. Why?

Bash Scripting

trap 'echo "Exiting..."' SIGINT

while true; do
  sleep 1
done

A The trap command syntax is wrong; single quotes prevent execution.

B SIGINT cannot be trapped in an infinite loop.

C The trap command is missing a command to exit, so the script continues running after printing.

D The trap command is correct; the script ignores SIGINT because sleep is blocking.

💡 Hint

What happens after the trap command runs?

The trap prints the message but does not exit, so the script continues running. Adding 'exit' after echo fixes this.

🚀 Application

expert

How to safely clean up temporary files on script termination?

You want a bash script to create a temporary file and delete it when the script ends, whether normally or due to signals like SIGINT or SIGTERM. Which trap setup ensures cleanup in all cases?

Bash Scripting

tmpfile=$(mktemp)
echo "Temporary file created: $tmpfile"

# trap commands here

# Simulate work
sleep 30

# End of script

A trap 'rm -f "$tmpfile"; exit' EXIT SIGINT SIGTERM

B trap 'rm -f "$tmpfile"; exit' SIGINT SIGTERM

C trap 'rm -f "$tmpfile"' EXIT SIGINT SIGTERM

D trap 'rm -f "$tmpfile"' EXIT

💡 Hint

EXIT trap runs on script exit regardless of reason.

The EXIT trap runs when the script exits for any reason, including signals that cause termination. Using EXIT alone ensures cleanup. Adding exit in the trap for signals can cause double exit calls or missed cleanup.

Practice

1. What does the trap command do in a bash script?

easy

A. It creates a loop that repeats commands indefinitely.

B. It catches signals like Ctrl+C and runs specified commands.

C. It defines a function to be used later in the script.

D. It pauses the script until a key is pressed.

Solution

1. Step 1: Understand the purpose of trap

   The trap command is used to catch signals sent to the script, such as Ctrl+C (SIGINT).

2. Step 2: What trap does with signals

   When a signal is caught, trap runs the commands specified by the user to handle that signal.

3. Final Answer:

   It catches signals like Ctrl+C and runs specified commands. -> Option B

4. Quick Check:

   trap catches signals = B [OK]

Hint: Trap runs commands on signals like Ctrl+C [OK]

Common Mistakes:

• Thinking trap creates loops
• Confusing trap with function definition
• Assuming trap pauses script

2. Which of the following is the correct syntax to run echo "Bye" when the script receives a SIGINT signal?

easy

A. trap 'echo "Bye"' SIGINT

B. trap echo "Bye" SIGINT

C. trap (echo "Bye") SIGINT

D. trap {echo "Bye"} SIGINT

Solution

1. Step 1: Check trap syntax for commands

   The commands to run must be quoted as a single string, usually with single quotes to preserve special characters.

2. Step 2: Identify correct quoting style

   trap 'echo "Bye"' SIGINT uses single quotes around the entire command string, which is the correct syntax.

3. Final Answer:

   trap 'echo "Bye"' SIGINT -> Option A

4. Quick Check:

   trap 'commands' SIGNAL = A [OK]

Hint: Use single quotes around commands in trap [OK]

Common Mistakes:

• Not quoting commands properly
• Using parentheses or braces incorrectly
• Omitting quotes causing syntax errors

3. What will be the output of this script when you press Ctrl+C during its execution?

#!/bin/bash
trap 'echo "Interrupted! Cleaning up..."; exit' SIGINT
while true; do
  echo "Running..."
  sleep 2
done

medium

A. The script ignores Ctrl+C and keeps running.

B. It crashes with an error message.

C. It prints "Running..." once and then exits.

D. It prints "Interrupted! Cleaning up..." and exits.

Solution

1. Step 1: Understand trap on SIGINT

   The trap command catches the SIGINT signal (Ctrl+C) and runs the echo and exit commands.

2. Step 2: Effect of trap during infinite loop

   When Ctrl+C is pressed, the script prints the message and exits immediately, stopping the loop.

3. Final Answer:

   It prints "Interrupted! Cleaning up..." and exits. -> Option D

4. Quick Check:

   trap on SIGINT prints message and exits = D [OK]

Hint: trap runs commands on Ctrl+C, then script exits [OK]

Common Mistakes:

• Thinking script ignores Ctrl+C
• Assuming loop stops without message
• Expecting error instead of clean exit

4. Identify the error in this script snippet:

trap echo "Exiting" SIGTERM
sleep 10

medium

A. sleep command cannot be used with trap.

B. SIGTERM is not a valid signal name.

C. Missing quotes around the command to run in trap.

D. trap must be placed after sleep command.

Solution

1. Step 1: Check trap command syntax

   The command to run on signal must be quoted as a single string to avoid syntax errors.

2. Step 2: Identify missing quotes

   Here, echo "Exiting" is not quoted, so the shell misinterprets the command.

3. Final Answer:

   Missing quotes around the command to run in trap. -> Option C

4. Quick Check:

   trap commands need quotes = C [OK]

Hint: Always quote trap commands to avoid syntax errors [OK]

Common Mistakes:

• Not quoting trap commands
• Using invalid signal names
• Placing trap after commands needing it

5. You want a bash script to delete a temporary file /tmp/mytempfile when it exits, whether normally or due to Ctrl+C. Which trap command correctly achieves this?

hard

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

B. trap 'rm -f /tmp/mytempfile' SIGINT

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

D. trap 'rm -f /tmp/mytempfile' SIGINT SIGTERM

Solution

1. Step 1: Understand EXIT signal

   EXIT is a special trap signal that runs commands when the script ends for any reason, including normal exit or Ctrl+C.

2. Step 2: Why EXIT is better than SIGINT or SIGTERM alone

   Using EXIT ensures cleanup runs on all script exits, not just specific signals.

3. Final Answer:

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

4. Quick Check:

   Use EXIT to clean up on all exits = A [OK]

Hint: Use trap on EXIT to clean up on any script exit [OK]

Common Mistakes:

• Using only SIGINT or SIGTERM misses normal exit
• Not quoting the command
• Expecting multiple signals needed for all exits