My Shell, My Bell: While/Xargs by Example

Earlier today I logged into a jumpbox session and was greeted with some lovely error messages:

$ channel 12: open failed: administratively prohibited: open failed
channel 13: open failed: administratively prohibited: open failed
channel 19: open failed: administratively prohibited: open failed
channel 20: open failed: administratively prohibited: open failed
channel 21: open failed: administratively prohibited: open failed
channel 22: open failed: administratively prohibited: open failed
channel 23: open failed: administratively prohibited: open failed
channel 24: open failed: administratively prohibited: open failed
channel 25: open failed: administratively prohibited: open failed

Was a little odd, considering that I hadn’t done anything yet. So I looked through a list of my running processes and noticed that I had a ton of ssh-agent and ssh -R sessions still running. That’s what I get for scripting and not cleaning up, but that’s an issue for another day.

My main issue was figuring out how to get all those PIDs and delete them in a loop, because why would I go through probably a hundred processes one by one? (I may be exaggerating a little… but not by much.)

To create the PID lists, I used:

ps -o pid,cmd -u quinn | grep "ssh-agen[t]  " | awk '{print $1}'
ps -o pid,cmd -u quinn | grep "ssh -R 2022" | awk '{print $1}'

(I actually have ps -o pid,cmd -u ${USERNAME} aliased to myps to make my life even easier.) To delete I could use either a WHILE loop or XARGS. Since the list of processes was relatively small, either would work in this case. For WHILE:

ps -eo pid,cmd -u quinn | grep "ssh-agen[t]  " | awk '{print $1}' | while read PID; do kill ${PID}; done

And for XARGS:

ps -o pid,cmd -u quinn | grep "ssh -R 2022" | awk '{print $1}' | xargs -n1 kill -TERM

A Quick XARGS Note

A quick overview of the syntax for the xargs command as I used its: the -n1 tells xargs to "group" the arguments individually, so it’s actually running kill -TERM ${PID} for each PID. For situations like rm where you could want multiple arguments at once, e.g. find /home -name '*.bak' | xargs rm to recursively remove all the *.bak files out of /home, then larger groups could be used. When a grouping size isn’t specified, like in the rm example, then xargs will simply use the largest group size that particular command can handle.