SC2009

Consider using pgrep instead of grepping ps output.

Problematic Code:

ps ax | grep -v grep | grep "$service" > /dev/null

Correct Code:

pgrep -f "$service" > /dev/null

Rationale:

If you are just after a pid from a running program, then pgrep is a much safer alternative. Especially if you are also looking for a pid belonging to a certain user or group. All of the parameters are in one command and it can eliminate multiple greps, cuts, seds, awks, etc.

If you want a field that's not the pid, consider doing this through ps + pgrep instead of ps + grep:

for pid in $(pgrep '^python$')
do
  user=$(ps -o user= -p "$pid")
  echo "The process $pid is run by $user"
done

This is more robust than ps .. | grep python | cut .. because it does not try to match against unrelated fields, such as if the user's name was pythonguru.

Exceptions

pgrep is not POSIX. Please ignore this warning if you are targeting POSIX userlands.

You can ignore this error if you are trying to match against something that pgrep doesn't support:

# pgrep does not support filtering by 'nice' value
# shellcheck disable=SC2009
ps -axo nice=,pid= | grep -v '^  0'