Testing if a process is running via if and -z

Question

I'm have a process on my system which is notorious for crashing, and is kind of mission critical, my network manager. In any case, I need to write a loop which tests if it's running and starts it if it isn't. This is what I've come up with thus far:

#!/bin/bash

while [ true ]; do 
    if [ -z $(ps aux | grep "[n]m-applet") ]; then
        echo "Bugger died, resurrecting..."
        nm-applet >/dev/null 2>/dev/null &
        disown $!
    fi

    sleep 3
done 

Unfortunately, this isn't quite doing the trick, as it seems to be starting the process even when not necessary, and after the first run, I get the following error output:

line 4: [: too many arguments

What am I doing wrong here?

Answer 1

Try wrapping the $(...) in double quotes:

if [ -z "$(ps aux | grep '[n]m-applet')" ]; then

But you might want to try using pgrep or ps axo cmd | grep '[n]m-applet' instead.

Answer 2

That error comes from giving multiple arguments to -z. It's a unary operator, and if the $() expands to something with the $IFS in it, it will see multiple arguments. To fix that, you can put quotes around it like so: [ -z "$(ps...)" ].

In this case, you actually don't need the test ([]) because grep will return non-zero if it doesn't find anything. You can do:

if ps aux | grep '[n]m-applet' > /dev/null; then

And you might want to see if the systems you plan on deploying this to have pgrep. It's is made for looking for processes that match a pattern.

Also, you might want the script to not background the nm-applet. This way, the script will block until nm-applet dies.

Finally, you might want to look at Monit, which is made for doing what your script does.

Answer 3

I'm unsure about your reasons for while [ true ]; do and I second Arcege's suggestion to use pgrep.

#!/bin/bash
while true; do 
    if ! pgrep nmapplet &>/dev/null; then
       echo "Bugger died, resurrecting..."
       nm-applet &>/dev/null
       disown $!
    fi
    sleep 3
done