🚦 Day 6: Processes & Signals

Signals are just signals, not rocket science! 📡

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🎯 Today's Practice Task

Here you will learn killing a process, sending it to the background, and bringing it back to the foreground.

1️⃣ Creating a Dummy Process

1. Create a task using sleep:

   sleep 100

   • sleep in Kali is used to pause the terminal for a specific time (100 seconds here).
   • It is used here for Processes learning.

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💀 Killing the Process

1. Pause the Process: Press Ctrl + Z or open a split terminal/new tab.
2. Find the PID:
   ps # OR ps aux | grep sleep
   Pick the PID of the sleep 100 command.
3. Kill it:
   kill PID
   The process sleep 100 will be ended completely.
4. Force Kill (if needed):
   If kill PID failed, use:
   kill -9 PID
   The -9 flag sends a force kill signal.

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🔙 Background & Foreground

Sending to Background

1. Pause the process (as discussed above).
2. Find the Job Number:
   jobs
   Note the number in square brackets (e.g., [1]).
3. Send to Background:
   bg %1
   (Replace 1 with your job number. The % is mandatory).

Bringing to Foreground

1. Bring Back:
   fg %1

Remember:

• bg and fg always work in the same terminal where the process was initiated.
• Some processes (like network scanners) continue counting time when suspended, while others (like text editors) do not.

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📡 What are SIGNALS?

Imagine a worker and a boss. The boss might stop him from working, or say to continue his work. SIGNAL is the message, while the processes are the workers.

View all signals with:

kill -l

🔑 Important Signals

1. SIGINT (2): Ctrl + C

• Interrupt Signal.
• Usage: Stops a process manually.
• Catchable: Yes (Program can ask "Are you sure?").

2. SIGTERM (15): Request Stop

• Termination Signal.
• Usage: A polite request to stop. Default for kill PID.
• Catchable: Yes.

Difference between SIGINT and SIGTERM:

Feature	SIGINT	SIGTERM
Signal Number	2	15
Shortcut	Ctrl + C	None
Source	User (Keyboard)	System/Programs
Nature	Interrupt	Polite Stop

3. SIGKILL (9): Forced Kill

• Kill Signal.
• Usage: Forcefully kills the process. Use sparingly to avoid corruption.
• Catchable: NO.

4. SIGSTP (20): Pause (Soft)

• Stop Signal (Terminal).
• Usage: Ctrl + Z. Pauses/suspends the process.
• Catchable: Yes.

5. SIGSTOP (19): Pause (Hard)

• Stop Signal.
• Usage: Pauses process strictly.
• Catchable: NO.

Difference between SIGSTP and SIGSTOP:

Feature	SIGSTP	SIGSTOP
Signal Number	20	19
Shortcut	Ctrl + Z	None
Catchable	Yes	No

6. SIGCONT (18): Continue

• Continue Signal.
• Usage: Resumes a paused/suspended job.

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📝 Quick Summary of SIGNALS

Signal	Code	Shortcut	Catchable?	Purpose
SIGINT	2	Ctrl + C	Yes	Interrupt process (User).
SIGTERM	15	---	Yes	Polite termination request.
SIGSTOP	19	---	No	Strict suspension/freeze.
SIGSTP	20	Ctrl + Z	Yes	Suspend/freeze (Terminal).
SIGCONT	18	---	N/A	Resume a suspended process.
SIGKILL	9	---	No	Forcefully kill a process.

Catchable meaning: The program can intercept the signal (e.g., "Do you want to save your work?") before closing.

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🛠️ How to use SIGNALS

Format:

kill -SignalNumber PID
# OR
kill -SignalName PID

Example:

kill -9 1234

IMPORTANT:
If you start sleep 500, suspend it, and interpret it via SIGCONT in a different way, it might resume in the background.

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