How to test if a process exists in Bash?

Question

I want to make sure node is running when logging in. So in my .bashrc file I have:

pkill node
sleep 1
node server.js &

Of course, that doesn't check if node is running... it simply kills it and starts it up again. Instead I'd like something like this:

node_process = $(pidof node)
if [not_exists node_process]; then
  node server.js &
fi

The problem is the not_exists method doesn't seem to exist :). How do I test the existence of a number or string in Bash and is this the best way to ensure node is up and running upon login?

Possible duplicate of How to check if a process id (PID) exists — Trevor Boyd Smith
– Trevor Boyd Smith, Commented Jun 7, 2016 at 15:44

chepner · Accepted Answer · 2013-10-10 12:21:30Z

7

You can check if a string is empty using -z:

node_process_id=$(pidof node)
if [[ -z $node_process_id ]]; then
    node server.js &
fi

pidof returns nothing if no matching processes are found, so node_process_id would be set to an empty string.

answered Oct 10, 2013 at 12:21

chepner

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3 Comments

at. Over a year ago

This seems like exactly what I'm looking for. Can I just move the $(pidof node) inside the if condition? Curious why I need 2 square brackets around the if condition?

chepner Over a year ago

The double brackets are just a more friendly bash version of the single brackets. And yes, I think if ! pidof node; then node server.js & fi should work fine.

Dario Corno Over a year ago

Thanks a lot, was banging my head to find a clean solution. Works like a charm.

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