Neat bash tricks

One idiom I like is:

ssh $host "commands;" 2>&1 | while read line ; do

    # react to any error messages or messages from commands in $line

done

For instance, say you were running x11vnc on a remote host. x11vnc has the annoying habit of using a port other then the one you specify, if the one you want is already taken. Very annoying. So:

ssh $host "x11vnc ...." 2>&1 | while read line ; do

    if [[ $line =~ 'PORT=([[:digit:]]+)' ]] ; then

        port=${BASH_REMATCH[1]}

        # now set up some sort of port forwarding so that $port is a sane, known port

        ssh -L '*:9600:localhost'$port $host

    fi

done

This has some problems, in that the second ssh can survive x11vnc exiting. I thought "hey, how about $?" but that has other problems; say the second ssh exits before its time. The $? you saved could be reused. The kill you'd want to do would provoke hilarity. While complaining about this on IRC, a wise soul suggested that I open a lock file, and then any process with that file still open must be killed. I don't need locking, and didn't want to learn about flock in bash right away, so what I roughly did was:

child_kill () {

    if [[ ! $LOCKFILE ]] ; then

        return 0

    fi

    lsof -F '' $LOCKFILE | while read ppid ; do

            if [[ $ppid =~ '^p([[:digit:]]+)$' ]] ; then

                pid=${BASH_REMATCH[1]}

                if [[ $pid != $$ ]] ; then

                    kill -HUP $pid

                fi

            fi

        done

    rm -f $LOCKFILE

}



local LOCKFILE=$(mktemp -p /tmp)

trap "child_kill" EXIT

ssh ... | while read line ; do 

    ....

    ( exec 123>$LOCKFILE

      ssh -L ..... $host &

    )

done

child_kill

I wish there was a better way to deal with lsof's output, but this works so why complain?

What's more, I wish I could use SSH's ControlMaster to make the second connection that much faster. But the quick testing I did with 4.9p1 failed. Bugger