Linux/Unix command to determine if process is running?

While pidof and pgrep are great tools for determining what's running, they are both, unfortunately, unavailable on some operating systems. A definite fail safe would be to use the following: ps cax | grep command

The output on Gentoo Linux:

14484 ?        S      0:00 apache214667 ?        S      0:00 apache219620 ?        Sl     0:00 apache221132 ?        Ss     0:04 apache2

The output on OS X:

42582   ??  Z      0:00.00 (smbclient)46529   ??  Z      0:00.00 (smbclient)46539   ??  Z      0:00.00 (smbclient)46547   ??  Z      0:00.00 (smbclient)46586   ??  Z      0:00.00 (smbclient)46594   ??  Z      0:00.00 (smbclient)

On both Linux and OS X, grep returns an exit code so it's easy to check if the process was found or not:

#!/bin/bashps cax | grep httpd > /dev/nullif [ $? -eq 0 ]; then  echo "Process is running."else  echo "Process is not running."fi

Furthermore, if you would like the list of PIDs, you could easily grep for those as well:

ps cax | grep httpd | grep -o '^[ ]*[0-9]*'

Whose output is the same on Linux and OS X:

3519 3521 3523 3524

The output of the following is an empty string, making this approach safe for processes that are not running:

echo ps cax | grep aasdfasdf | grep -o '^[ ]*[0-9]*'

This approach is suitable for writing a simple empty string test, then even iterating through the discovered PIDs.

#!/bin/bashPROCESS=$1PIDS=`ps cax | grep $PROCESS | grep -o '^[ ]*[0-9]*'`if [ -z "$PIDS" ]; then  echo "Process not running." 1>&2  exit 1else  for PID in $PIDS; do    echo $PID  donefi

You can test it by saving it to a file (named "running") with execute permissions (chmod +x running) and executing it with a parameter: ./running "httpd"

#!/bin/bashps cax | grep httpdif [ $? -eq 0 ]; then  echo "Process is running."else  echo "Process is not running."fi

WARNING!!!

Please keep in mind that you're simply parsing the output of ps ax which means that, as seen in the Linux output, it is not simply matching on processes, but also the arguments passed to that program. I highly recommend being as specific as possible when using this method (e.g. ./running "mysql" will also match 'mysqld' processes). I highly recommend using which to check against a full path where possible.

References:

http://linux.about.com/od/commands/l/blcmdl1_ps.htm

http://linux.about.com/od/commands/l/blcmdl1_grep.htm

You SHOULD know the PID !

Finding a process by trying to do some kind of pattern recognition on the process arguments (like pgrep "mysqld") is a strategy that is doomed to fail sooner or later. What if you have two mysqld running? Forget that approach. You MAY get it right temporarily and it MAY work for a year or two but then something happens that you haven't thought about.

Only the process id (pid) is truly unique.

Always store the pid when you launch something in the background. In Bash this can be done with the $! Bash variable. You will save yourself SO much trouble by doing so.

How to determine if process is running (by pid)

So now the question becomes how to know if a pid is running.

Simply do:

ps -o pid= -p <pid>

This is POSIX and hence portable. It will return the pid itself if the process is running or return nothing if the process is not running. Strictly speaking the command will return a single column, the pid, but since we've given that an empty title header (the stuff immediately preceding the equals sign) and this is the only column requested then the ps command will not use header at all. Which is what we want because it makes parsing easier.

This will work on Linux, BSD, Solaris, etc.

Another strategy would be to test on the exit value from the above ps command. It should be zero if the process is running and non-zero if it isn't. The POSIX spec says that ps must exit >0 if an error has occurred but it is unclear to me what constitutes 'an error'. Therefore I'm not personally using that strategy although I'm pretty sure it will work as well on all Unix/Linux platforms.

On most Linux distributions, you can use pidof(8).

It will print the process ids of all running instances of specified processes, or nothing if there are no instances running.

For instance, on my system (I have four instances of bashand one instance of remmina running):

$ pidof bash remmina6148 6147 6144 5603 21598

On other Unices, pgrep or a combination of ps and grep will achieve the same thing, as others have rightfully pointed out.