ps -e | grep python until some python process runs in the system

Question

I want to see if some python process is running. I've set up a python script that updates noip, and a cron runs it every 1 minute, but it takes only 2 seconds to run and finish, so I'd like to know if there's a way to do ps -e | grep python and wait until some python process appear in the screen

Answer 1

If your goal is simply to observe is process appears, you can just do this:

watch -n1  "ps -ef | grep command | grep -v grep"

Personally , I frequently use polling technique with pgrep and while loop like so:

while ! pgrep -f command ; do true; done && echo "Process started"

This is a good approach if you want to run additional command if a process appears or disappears.

Test run:

The way this works is very simple:

• loop runs pgrep -f command on each iteration and negates its exit status. Thus if process not found the exit status of 1 negates to 0 , thus keeping the loop running.
• Once the process appears, exit status becomes 0 , it's negated to 1, causing for loop to terminate and display message with echo

This is a pretty universal approach, works with any shell and scripting language. For instance, I used it in a python script that detects ip change and script to run commands when process appears