Kill

The kill command in Unix-like systems is used to send signals to processes. It can be used to terminate processes, stop them, or even send specific signals to change their behavior. Below is a breakdown of the usage and examples:

Syntax:

kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid
kill -l [exit_status]
kill -l [sigspec]

Key Options:

• -l: List all signal names.
• -s: Send a specific signal (can be either signal name or number).
• -n: Send a signal by its number.

If no signal is specified, SIGTERM (signal 15) is used by default to terminate the process.

Signals:

• SIGTERM (15): Default signal used by kill, requests a graceful shutdown.
• SIGKILL (9): Forcefully terminates the process (cannot be caught or ignored).
• SIGINT (2): Interrupts a process, typically triggered by pressing Ctrl + C.
• SIGHUP (1): Hangup signal, often used to tell a daemon to reload its configuration.
• SIGSTOP (17,19,23): Stops a process temporarily (can be resumed later).
• SIGABRT (6): Used by programs to abort execution.

Common Examples:

1. Kill a Process: First, identify the process using ps or any other method to get its PID (Process ID).

   ps
   PID TTY      TIME CMD
   1293 pts/5    00:00:00 MyProgram

   Then, use the kill command to terminate it:

   kill 1293

   If successful, the output will be:

   [2]+ Terminated MyProgram

2. Force Kill a Process (SIGKILL): If the process does not respond to SIGTERM, you can forcefully kill it with SIGKILL (signal number 9):

   kill -9 1293

3. Send a Custom Signal: You can send a custom signal to a process. For example, to send SIGSTOP (to pause the process):

   kill -s SIGSTOP 1293

4. List All Signals: To list all available signal names:

   kill -l

5. Kill a Process After a Delay: Run a command in the background, then kill it after a delay of 5 seconds:

   my_command & sleep 5
   kill -0 $! && kill $!

   The $! variable holds the PID of the last background process.