Process and job control


Typing kill -l shows you the instructions that kill passes to a process. The following are the most relevant ones for your daily work:

  • SIGHUP: This tells a process to restart immediately after terminating and is often used to tell servers to parse modified configuration files.
  • SIGTERM: This request to terminate allows the process to clean up.
  • SIGKILL: This signal forces a process to terminate come what may. But in some cases, it takes more to get rid of the process. After waiting in vain for a timeout, you have no alternative but to reboot.
  • SIGSTOP: Interrupts the process until you enter SIGCONT to continue.

To send a signal to a process, you can enter either the signal name or number followed by the process ID – for example, kill -19 9201. Also, you can specify multiple process IDs. If you call kill without any parameters but with the PID, it will send the SIGTERM signal to the process.

Seek and Ye Shall Find

To find the right process ID, you can run ps as described previously. The shell command can be combined with other tools, such as grep, in the normal way. For example, you could do this (Listing 3) to find processes with ssh in their names.

Listing 3

grep ssh


Besides the SSH server (sshd), the list includes all of your SSH connections. To send the same signal to all of these processes, you would normally list the PIDs in the kill command line, which can be tricky if the list is too long.

The killall gives you a workaround – the tool understands all of the kill signals but expects process names instead of IDs.

The killall -19 ssh command sends all your SSH connections to sleep (SIGSTOP). If you do not specify the signal, killall assumes you mean SIGTERM, just like kill.

Because killall really does remove the processes in one fell swoop, it is a good idea to switch to interactive mode (-i option). For each process, the tool prompts you to decide whether to terminate.

More Detective Work

If you are looking for process IDs, a combination of ps and grep is a good idea, but you can save some typing by running pgrep instead.

To find all processes with ssh in their names, do the following:

$ pgrep ssh

If you need more context, add the -l parameter and pgrep will reveal the names. To discover the full command line, including all arguments, combine -l and -f:

$ pgrep -lf ssh
2816 /usr/sbin/sshd
3992 ssh -X chicken@asteroid
4249 ssh chicken@nugget

Buy this article as PDF

Express-Checkout as PDF
Price $2.95
(incl. VAT)

Buy Linux Magazine

Get it on Google Play

US / Canada

Get it on Google Play

UK / Australia

Related content

  • Command Line – Pgrep

    Pgrep is a valuable tool for tracking down processes.

  • Command Line: Processes

    Innumerable processes may be running on your Linux system. We’ll show you how to halt, continue, or kill tasks, and we’ll examine how to send the remnants of crashed programs to the happy hunting grounds.

  • Shell Scripts in Waiting: the Waitmax Tool

    Linux consultant Matthias Kettner has just released Version 1.0 of his Waitmax Software. It gives programs a specified time to complete after terminating them.

  • Command Line – Killing Processes

    Linux offers a variety of tools for shutting down systems and processes. We describe some important commands.

  • Pipe Cleaner

    Detecting programs where the standard output has frozen can require a deep dive into terminal emulation basics. Go plumber Mike Schilli builds a plunger to free up the pipe works.

comments powered by Disqus
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters

Support Our Work

Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.

Learn More