Conditionally run process in background with bash

Question

I would like to run a program conditionally in the background, if the condition is met, we run it as a daemon, otherwise we wait for the result:

if [ -z "${CONDITION}" ]; then
    echo "background" && npm install -g yarn &
else
    echo "foreground" && npm install -g yarn
fi

is there a shorthand way to do this? (I assume in the else block, that given the syntax that this process will not run in the background). I guess we could also conditionally add "wait" instead of an "&" character.

Answer 1

I think your wait idea is good. I can't think of a better way.

npm install -g yarn &
[ -n "$CONDITION" ] && { echo "waiting for install"; wait; }

Not sure that you want the echo stuff or not, but if it's time consuming, you might want an indication.

HTH

Update: Simple script to show how it works

C=""
date
sleep 5s &
[ -n "$C" ] && { echo "waiting"; wait; }
date

If C="", you get an output like so:

Thu Dec  8 11:42:54 CET 2016
Thu Dec  8 11:42:54 CET 2016

(and of course, sleep 5s is still running while your script is finished)

If you set C="something", you get:

Thu Dec  8 11:42:42 CET 2016
waiting
Thu Dec  8 11:42:47 CET 2016