kill and signal types in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the kill command do in Linux?

The kill command sends a signal to a process, usually to stop or control it. It can ask a process to stop gently or force it to quit.

beginner

What is the difference between SIGTERM and SIGKILL signals?

SIGTERM politely asks a process to stop, allowing it to clean up. SIGKILL forces the process to stop immediately without cleanup.

beginner

How do you send a SIGINT signal using the kill command?

Use kill -2 <pid> or kill -SIGINT <pid>. SIGINT is like pressing Ctrl+C to interrupt a process.

beginner

What signal number is usually used to force kill a process?

Signal number 9 (SIGKILL) is used to force kill a process immediately.

beginner

Why should you prefer SIGTERM over SIGKILL when stopping a process?

SIGTERM lets the process close files and save data safely. SIGKILL stops it instantly, which might cause data loss.

Which signal number corresponds to SIGKILL?

A 9

B 15

C 2

D 1

SIGKILL is signal number 9, used to forcefully stop a process.

What does the kill -15 <pid> command do?

A Sends SIGHUP to reload the process

B Sends SIGKILL to force stop the process

C Sends SIGINT to interrupt the process

D Sends SIGTERM to politely stop the process

kill -15 sends SIGTERM, which politely asks the process to stop.

Which signal is sent when you press Ctrl+C in the terminal?

A SIGTERM

B SIGINT

C SIGKILL

D SIGSTOP

Ctrl+C sends SIGINT to interrupt the running process.

If a process ignores SIGTERM, which signal should you use next?

A SIGKILL

B SIGINT

C SIGHUP

D SIGSTOP

If SIGTERM is ignored, SIGKILL (signal 9) forces the process to stop.

What is the purpose of the SIGHUP signal?

A Terminate the process immediately

B Pause the process

C Reload configuration or restart the process

D Ignore the process

SIGHUP usually tells a process to reload its configuration or restart.

Explain how the kill command works and name three common signals it can send.

Think about how you stop or control a process using signals.

Describe the difference between SIGTERM and SIGKILL and why you might choose one over the other.

Consider what happens to a process when it receives each signal.

Practice

1. What is the default signal sent by the kill command if no signal is specified?

easy

A. SIGINT (interrupt the process)

B. SIGTERM (terminate gracefully)

C. SIGSTOP (pause the process)

D. SIGKILL (force kill immediately)

Solution

1. Step 1: Understand the default behavior of kill

   The kill command sends signals to processes. If no signal is specified, it sends the default signal.

2. Step 2: Identify the default signal

   The default signal is SIGTERM, which asks the process to terminate gracefully, allowing cleanup.

3. Final Answer:

   SIGTERM (terminate gracefully) -> Option B

4. Quick Check:

   Default kill signal = SIGTERM [OK]

Hint: Remember: kill defaults to SIGTERM for polite stop [OK]

Common Mistakes:

• Confusing SIGKILL as default signal
• Thinking SIGSTOP pauses instead of terminates
• Assuming SIGINT is default for kill

2. Which of the following is the correct syntax to send a SIGKILL signal to a process with PID 1234?

easy

A. kill 1234 -9

B. kill 1234 -s SIGKILL

C. kill 1234 -SIGKILL

D. kill -9 1234

Solution

1. Step 1: Recall the syntax for sending signals with kill

   The kill command can use -s SIGNAL or numeric signal with a dash before the number.

2. Step 2: Identify the correct numeric signal and position

   -9 is the numeric code for SIGKILL. The correct syntax is kill -9 PID. All other options incorrectly place the signal specification after the PID, which is invalid syntax.

3. Final Answer:

   kill -9 1234 -> Option D

4. Quick Check:

   kill -9 PID sends SIGKILL [OK]

Hint: Use kill -9 PID to force kill quickly [OK]

Common Mistakes:

• Placing signal after PID
• Using -SIGKILL instead of -s SIGKILL
• Confusing signal names and numbers

3. What will be the output of the following command sequence?

sleep 100 &
kill -15 $!

Assuming $! is the PID of the sleep process.

medium

A. The sleep process stops gracefully after receiving SIGTERM

B. The sleep process is immediately killed without cleanup

C. The sleep process ignores the signal and continues running

D. Syntax error due to incorrect kill command

Solution

1. Step 1: Understand the commands

   sleep 100 & runs sleep in background. $! stores its PID. kill -15 $! sends SIGTERM (signal 15) to that PID.

2. Step 2: Effect of SIGTERM on sleep

   SIGTERM asks the process to stop gracefully. Sleep will stop waiting and exit cleanly.

3. Final Answer:

   The sleep process stops gracefully after receiving SIGTERM -> Option A

4. Quick Check:

   kill -15 sends SIGTERM = graceful stop [OK]

Hint: Signal 15 (SIGTERM) politely stops processes [OK]

Common Mistakes:

• Confusing SIGTERM with SIGKILL immediate stop
• Assuming sleep ignores SIGTERM
• Thinking kill command syntax is wrong

4. You try to stop a stubborn process with kill -15 5678 but it keeps running. What is the best fix?

medium

A. Use kill -1 5678 to send SIGHUP

B. Restart the system to clear the process

C. Use kill -9 5678 to force kill immediately

D. Use kill 5678 without signal

Solution

1. Step 1: Understand why SIGTERM might fail

   SIGTERM (signal 15) asks process to stop nicely, but some processes ignore or handle it differently.

2. Step 2: Use SIGKILL to force stop

   SIGKILL (signal 9) forces immediate termination and cannot be ignored. Using kill -9 is the best fix for stubborn processes.

3. Final Answer:

   Use kill -9 5678 to force kill immediately -> Option C

4. Quick Check:

   kill -9 forces stop when SIGTERM fails [OK]

Hint: If kill -15 fails, try kill -9 to force stop [OK]

Common Mistakes:

• Using kill without signal defaults to SIGTERM
• Using SIGHUP does not force stop
• Restarting system is last resort, not first

5. You want to pause a process with PID 4321 temporarily and then resume it later. Which signals should you send in order?

hard

A. Send SIGSTOP to pause, then SIGCONT to resume

B. Send SIGTERM to pause, then SIGKILL to resume

C. Send SIGKILL to pause, then SIGSTOP to resume

D. Send SIGINT to pause, then SIGTERM to resume

Solution

1. Step 1: Identify signals to pause and resume

   SIGSTOP pauses a process immediately without termination. SIGCONT resumes a paused process.

2. Step 2: Understand incorrect options

   SIGTERM and SIGKILL terminate processes, not pause or resume. SIGINT interrupts but does not pause/resume.

3. Final Answer:

   Send SIGSTOP to pause, then SIGCONT to resume -> Option A

4. Quick Check:

   Pause = SIGSTOP, Resume = SIGCONT [OK]

Hint: Pause with SIGSTOP, resume with SIGCONT signals [OK]

Common Mistakes:

• Using SIGTERM or SIGKILL to pause processes
• Confusing SIGINT with pause/resume signals
• Thinking kill without signal pauses process