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?
Best Answer
The
killcommand is a very simple wrapper to thekillsystem call, which knows only about process IDs (PIDs).pkillandkillallare also wrappers to thekillsystem 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
pkillandkillallwork can be seen usingltraceorstraceon them. On Linux, they both read through the/procfilesystem, 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>/statwhich contains the command name as the 2nd field. Forpkill -fandpgrepexamine the/cmdlineentry for each PID's proc entry.pkillandpgrepuse thereadprocsystem call, whereaskillalldoes not. I couldn't say if there's a performance difference: you'll have to benchmark that on your own.