Signal handling with trap in Bash Scripting - Step-by-Step Execution

Concept Flow - Signal handling with trap

Start script

↓

Set trap for signal

↓

Run main loop or commands

↓

Signal received?

No→Continue running

Yes↓

Execute trap command

↓

Continue or exit script

The script sets a trap to catch signals, runs commands, and when a signal arrives, it runs the trap command before continuing or exiting.

Execution Sample

Bash Scripting

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

This script prints 'Running...' every second until you press Ctrl+C, which triggers the trap to print a message and exit.

Execution Table

Step	Action	Signal Received	Trap Command Executed	Output	Script State
1	Start script and set trap	No	No		Running
2	Print 'Running...'	No	No	Running...	Running
3	Sleep 1 second	No	No		Running
4	Print 'Running...'	No	No	Running...	Running
5	Sleep 1 second	No	No		Running
6	User presses Ctrl+C (SIGINT)	Yes	Yes	Signal received, exiting	Exiting
7	Exit script	No	No		Exited

💡 User pressed Ctrl+C, SIGINT caught, trap executed, script exited

Variable Tracker

Variable	Start	After 1	After 2	After SIGINT	Final
Script State	Not started	Running	Running	Exiting	Exited

Key Moments - 3 Insights

Why does the script not exit immediately when started?

Because the trap only runs when the specified signal (SIGINT) is received. Until then, the script keeps running the loop as shown in rows 2-5 of the execution_table.

What happens when Ctrl+C is pressed during the sleep?

The signal SIGINT is sent, the trap command runs (row 6), printing the message and exiting the script immediately.

Can the trap command run multiple times if multiple signals are sent?

Yes, each time the signal is received, the trap command runs. But in this example, the trap exits the script, so it runs only once.

Visual Quiz - 3 Questions

Look at the execution_table, what is the script state after step 4?

A Exiting

B Exited

C Running

D Not started

💡 Hint

Check the 'Script State' column at step 4 in the execution_table

At which step does the trap command execute?

A Step 6

B Step 4

C Step 2

D Step 7

💡 Hint

Look for 'Trap Command Executed' marked 'Yes' in the execution_table

If the trap command did not call exit, what would happen after step 6?

A Script would exit immediately

B Script would continue running the loop

C Script would pause indefinitely

D Script would restart

💡 Hint

Refer to the key_moments explanation about trap behavior without exit

Concept Snapshot

trap "command" SIGNAL
- Sets a command to run when SIGNAL is received
- Commonly used to handle Ctrl+C (SIGINT)
- Allows cleanup or graceful exit
- Script continues unless trap command exits
- Example: trap "echo Exiting; exit" SIGINT

Full Transcript

This visual trace shows how a bash script uses trap to handle signals. The script sets a trap for SIGINT (Ctrl+C). It runs a loop printing 'Running...' every second. When the user presses Ctrl+C, the signal is caught, the trap command runs printing 'Signal received, exiting', then the script exits. Variables like script state change from running to exiting to exited. Key moments include understanding that the trap only runs on signal, and that exit in the trap stops the script. The quiz tests understanding of script state and trap execution steps.

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