What is the difference between kill , pkill and killall?

I am familiar with kill command , and most of the time we just use kill -9 to kill a process forcefully, there are many other signals that can be used with kill. But I wonder what are the use cases of pkill and killall, if there is already a kill command.

Do pkill and killall use the kill command in their implementation? I mean they are just wrappers over kill or they have their own implementation?

I would also like to know how pgrep command gets the process id from the process name.

Do all these commands use the same underlying system calls? Is there any difference from a performance point of view, which one is faster?

Asked By: Ijaz Ahmad

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kill and killall are tools which provide a way to kill a process. The first by its PID, the second by its name.
pgrep (list) and pkill (kill by default) are tools which provide a way to send message to a process by its name or other attributes
see: http://linux.die.net/man/1/pkill
For more info about signals: http://linux.die.net/man/7/signal

Answered By: dervishe

The kill command is a very simple wrapper to the kill system call, which knows only about process IDs (PIDs). pkill and killall are also wrappers to the kill system call, (actually, to the libc library which directly invokes the system call), but can determine the PIDs for you, based on things like, process name, owner of the process, session id, etc.

How pkill and killall work can be seen using ltrace or strace on them. On Linux, they both read through the /proc filesystem, and for each pid (directory) found, traverses the path in a way to identify a process by its name or other attributes. How this is done is technically speaking, kernel and system specific. In general, they read from /proc/<PID>/stat which contains the command name as the 2nd field. For pkill -f and pgrep examine the /cmdline entry for each PID’s proc entry.

pkill and pgrep use the readproc system call, whereas killall does not. I couldn’t say if there’s a performance difference: you’ll have to benchmark that on your own.

Answered By: Otheus

A more practical answer:

  • kill will send a SIGTERM (terminate a process) signal to the OS for a given process id (e.g. 10341), by default this is SIGTERM. You can actually specify other signals too (e.g. -9 to force terminate)

  • killall will send SIGTERM to all processes matching a certain name exactly. It also comes wit other goodies such as subsetting by user.

  • pkill will send SIGTERM to all processes that match a given pattern. It uses pgrep under the hood.

Answered By: fny
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