A command which returns either a process ID, if running, or some other output if not?

Question

I'm looking for a command that will give me the PID, and it's not running it should give me some output other than blank.

In my program, sometimes the library I'm using gives blank output even if that process is running and I'm not able to detect whether the blank output is due to the libray or due to the process having stopped!

Changing the library call will take time, so I'm looking for suggestions of such a shell command as workaround?

Solution 1

Finding PID's

There are other tools rather than ps for looking up processes based on their names.

$ pgrep name

Example

$ sleep 100 &
[1] 2556

$ pgrep sleep
2556

Above we can see the process named sleep is started and then backgrounded. If we use pgrep to look for processes with that name we get back its PID.

Checking status

As to your question about the output being blank. When commands return they'll often times return a status code that gets stored in a special variable, $? in Bourne shells, such as Bash. So you simply need to interrogate this variable's contents to find out if the process name you were looking for was present or not.

Example

Similar example using sleep again.

$ sleep 100 &
[1] 3228

Look for the PID of a process named sleep.

$ pgrep sleep
3228

Check its status variable, $?, a zero means it ran successfully, and found what we were looking for.

$ echo $?
0

Now if we look for a bogus process named sleepy.

$ pgrep sleepy

Not only do we get no output, but the status variable has a one in it, denoting that the command we ran, was unsuccessful in finding what we were looking for.

$ echo $?
1

Solution 2

You can combine commands as follows:

ps <pid> || echo "message"

If the pid exist you will get pid else you will get message printed.

Comments

[MongoLearner]

i got the idea.. but i dont know pid, i want to get it ..i know process name

[MongoLearner]

hi again, i have tried with pgrep <process> || echo "message" and its working ..simple .thanks a lot