command line tricks: ps, grep, awk, xargs, kill

I recently learned a little bit of awk; if it's not in your command line repertoire, it's worth looking into! awk lets you do things like this:

$ ps auxww | grep weka | grep -v grep | awk '{print $2}' | xargs kill -9

Let's unpack what's going on here. First, we list all processes, in wide format (that's the "auxww" options to ps), then we filter with grep to only include lines that include "weka". When I wrote this line, I was debugging a long-running machine learning task, so I would start it running, then if (when) I found a bug and wanted to restart, I used this command to kill it.

Now we have all the lines of output from ps that include "weka". Unfortunately, this includes the grep process that's searching for "weka"! No problem, just use "grep -v" to filter out the lines that include "grep".

This is where awk comes in. We want to get the process numbers out of the ps output. It seems like we could use cut to just get the second column, but we don't know how wide that column is going to be! Maybe there's a cut option for that, but I don't think there is. Instead, we just use a tiny awk script that prints the second whitespace-delimited thing on each line.

Finally, we use xargs to take the process numbers and make them be arguments to kill. xargs is great: it takes each line of its standard input and makes those lines argument to a program (ie, its first argument). Usually I use xargs in combination with find, "svn status", or "git status -s", to do the same thing to batches of files. Maybe delete them or add them to version control or whatever.

Thoughts? Better ways to do this sort of thing?