How to try to start a service only if it isn't running? (Linux)

Question

I know I have to count how many instances are running:

ps x | grep apache2 | wc -l 

result if it's running: 2, or else: 1

I also know there is a command called test that I could use to perform the verification, but I don't know how to use test with wc

Answer 1

The startup scripts on most Linux distributions handle this, by saving the Process ID of the service (daemon) when they launch it.

These PIDs are typically stored in /var/run/. For example on my system Apache2 stores it original or parent process ID (useful for pre-forked processes) in /var/run/apache2.pid. So you can then send signals such as SIGHUP or SIGSTOP to the process to signal either a configuration reload or to stop the service (daemon).

One note to help with your ps ax style process searching is to use a little trick to escape the grep's regular expression pattern used for matching, such that it doesn't match itself. E.g. ps ax | grep [a]pache2 | wc -l

man test gives you the basics of the test command. The best is basic shell scripting, which is dependant on which shell you are using. Normally for basic scripting, writing scripts for the Borne Shell (/bin/sh) is recommended.

#!/bin/sh

RC= `ps ax | grep [a]pache2 | wc -l`

if [ $RC -gt 0 ]; then
  kill -SIGSTOP `cat /var/run/apache2.pid`
fi

Answer 2

If you are working with bash, you can do it with conditional checks:

#/bin/bash
num="1"
if [ $num -eq "1" ]; then
        echo "got 1"
fi

if [ $num -eq "2" ]; then
        echo "got 2"
fi

Answer 3

if (( $(ps x | grep [a]pache2 | wc -l) > 0 ))
then
    do_something
else
    do_something_else
fi

or

case $(ps x | grep [a]pache2 | wc -l) in
    0)
        do_zero_thing
        ;;
    1)
        do_one_thing
        ;;
    *)
        do_default_thing
        ;;
esac

The square brackets used in that way in the grep command suppresses the grep itself from being included in its own output. This will make the process count 0 if apache isn't running.