What is the difference between kill , pkill and killall?

Question

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?

Solution 1

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.

Solution 2

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

Comments

[Jpark822]

Two things: Why use kill -9 by default? -15 (please stop) and -1 (modem has hung up, please CLEANLY closed yourself) are much more polite. Secondly. Beware of using killall on non-linux boxes. It might behave differently. (E.g. on solaris it kills all. NOT FILTERED on process names).

[Byte Commander]

Related: unix.stackexchange.com/questions/91527/…

[Ijaz Ahmad]

Do you mean killall is used to kill a process by its name? and it uses pgrep for this purpose? and killall also kills all the child processes ,? what signal killall uses by default?

[Jenny D]

...unless you're on Solaris, in which case killall will kill all processes you have the right to kill, so that if you're root, you're effectively rebooting the server.

[dervishe]

yep: killall chromium will kill the chromium process, pgrep chromium will give you the PID list, pkill chromium will kill chromium. killall will send by default SIGTERM signal (as pkill)