kill command

kill - send a signal to a process. The default signal for kill is TERM.

The kill command in Linux is a utility used to terminate processes by sending signals to them. It’s essential for stopping misbehaving programs, freeing resources, or managing tasks identified with tools like ps.

Usage: kill [options] [pid] [...]

• PID: The process ID(s) to target (find with ps).
• options: Flags to specify signals (e.g., -9 for SIGKILL).

Examples

• Basic Usage

  Send the default signal (SIGTERM) to a process to request termination.

  kill 1234
  • Sends SIGTERM to process with PID 1234.
  • SIGTERM politely asks the process to stop, allowing it to clean up.

  Find PID First:

  ps aux | grep firefox
  • Output (example): alice 5678 1.2 3.4 123456 78901 ? S 10:00 0:01 firefox
  • Then:

    kill 5678

• Forcing Termination

  Use -9 (or -SIGKILL) to forcefully stop a process.

  kill -9 5678
  • Sends SIGKILL, immediately terminating PID 5678 without cleanup.
  • Use this as a last resort—processes can’t ignore it.

• Specifying Signals

  kill supports various signals. Use -SIGNALNAME or -NUMBER.

  Common Signals:

  Signal	Number	Description
  SIGTERM	15	Terminate gracefully (default)
  SIGKILL	9	Force immediate termination
  SIGHUP	1	Hang up (reload config for some daemons)
  SIGINT	2	Interrupt (like Ctrl+C)

  Example with Signal Name:

  kill -SIGHUP 1234
  • Sends SIGHUP to reload configuration (e.g., for some servers).

  Example with Number:

  kill -1 1234
  • Same as above.

• Listing Signals

  See all available signals with -l (list).

  kill -l
  • Output (partial): 1) SIGHUP 2) SIGINT 3) SIGQUIT 9) SIGKILL 15) SIGTERM
  • Numbers match signal names.

• Killing Multiple Processes

  Pass multiple PIDs to terminate several processes at once.

  kill 1234 5678 9012
  • Stops all listed PIDs with SIGTERM.

• Using with Process Names (via pkill)

  While kill uses PIDs, pkill (a related tool) targets processes by name.

  pkill firefox
  • Sends SIGTERM to all Firefox processes.
  • Use pkill -9 firefox for SIGKILL.

To get help related to the kill command use --help option

$ kill --help
kill: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
    Send a signal to a job.

    Send the processes identified by PID or JOBSPEC the signal named by
    SIGSPEC or SIGNUM.  If neither SIGSPEC nor SIGNUM is present, then
    SIGTERM is assumed.

    Options:
      -s sig    SIG is a signal name
      -n sig    SIG is a signal number
      -l        list the signal names; if arguments follow `-l' they are
                assumed to be signal numbers for which names should be listed
      -L        synonym for -l

    Kill is a shell builtin for two reasons: it allows job IDs to be used
    instead of process IDs, and allows processes to be killed if the limit
    on processes that you can create is reached.

    Exit Status:
    Returns success unless an invalid option is given or an error occurs.

For more details, check the manual with man kill