Waiting for things

From FBSD_tips

Waiting for something to happen or, if you can grep(1) for it you can wait for it.

Sometimes you have to wait for something to happen in the system before you can proceed to another task. This is farily easy to spot manually, but what if you want to automate things? Well, grep will exit with '0' if it gets what you are are grepping for. Suppose I wanted to wait until a mirror was rebuilt before I went ahead with some task.

Gmirror status will tell us what we want to know.

# gmirror status
      Name    Status  Components
mirror/gm0  COMPLETE  ad0
                                     ad1

Grep will find this for us too.

# gmirror status | grep COMPLETE
mirror/gm0  COMPLETE  ad0

And even simpler, the exit code can be read.

# gmirror status | grep COMPLETE > /dev/null
# echo $?
0

So we can loop on the the grep in an until structure, when it becomes true it will go on. The 'sleep 1' is just to keep from hammering the system too much, adjust according to the situation.

until sleep 1; gmirror status | grep COMPLETE > /dev/null
do 
 echo "done"
done

Gongo 19:44, 16 October 2007 (UTC)