Ubuntu – What’s the difference between ‘killall’ and ‘pkill’

command lineprocess

After using just plain kill <some_pid> on Unix systems for many years, I learned pkill from a younger Linux-savvy ~~co-worker~~ colleague¹.

I soon accepted the Linux-way,pgrep-ing and pkill-ing through many days and nights, through slow-downs and race conditions. This was all well and good.

But now I see nothing but killall. How-to's seem to only mention killall, and I'm not sure if this is some kind of parallel development, or if killall is a successor to pkill, or something else.

It seems to function as a more targeted pkill, but I'm sure I'm missing something.

Can an Ubuntu/Debian-savvy² person explain when (or why) killall should be used, especially if it should be used in preference to pkill (when pkill often seems easier, because I can be sloppier with name matching, at least by default).

When speaking of killall, I'm not thinking of the command that on some Unix systems (Solaris, AIX, ?) would kill all user processes. Here's a description of that version, from a manpage for IBM's AIX:

The killall command cancels all
processes that you started, except
those producing the killall process.
This command provides a convenient
means of canceling all processes
created by the shell that you control.
When started by a root user, the
killall command cancels all
cancellable processes except those
processes that started it. If several
Signals are specified, only the last
one is effective.

¹ 'colleague' is free upgrade from 'co-worker', so might as well.
² Originally I thought this was a Linux or Debian thing, but some sources are saying that the Linux killall is derived from BSD-flavored Unix.

Best Answer

I think you see killall in how-to's because by default it requires the precise process name, whereas pkill does basic pattern matching. Thus, killall is safer for users to blindly copy and paste.

Pkill and killall both have distinguishing options. Killall has a flag to match by process age, pkill has a flag to only kill processes on a given tty. Etcetera ad nauseum. Neither is better, they just have different specialties.

I see from their man pages that killall comes from the psmisc package, which has several process managing utilities, but notably doesn't contain ps. It's the procps package that has ps, top, kill, and pkill (among others). I'd wager procps didn't originally have pkill, so psmisc scratched an itch and came up with killall.

The pkill/pgrep man page says they were introduced in Solaris 7. As you mention, jgbelacqua, Solaris's killall was not the utility psmisc provides, thus Solaris probably only had the procps package. Someone wanted a pattern matching process tool, thus pkill and pgrep. Whether it was developed by the procps dev or added in afterwards, I don't know. Regardless, it made it in and became part of *nixes everywhere.

More sources:

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