What is the difference between kill , pkill and killall?

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.

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