UNIX for Dummies Questions & Answers

If I run a script called 'abc.sh' and then execute the following :

ps -ef | grep 'abc.sh'

I always get two rows of output, one for the executing script, and the other for the grep command that I have triggered after the pipe.

Questions: Why does the second row turn up in the results. My (flawed) understanding of how the above should work is :
1. ps -ef returns a list of all processes running on the machine
2. grep works on that list to fetch processes with abc.sh in them

So when 'ps -ef' is triggered in step 1, 'grep' isn't running. Then why does it turn up in the results. Can someone explain what happens in the background when two commands are piped?

Thanks for the reply. Is there any way to mark this thread as closed?

There is no option to close this thread. (Only admin and moderate people can do it)

It will be opened and others also express their views. ( May be you will some other answers - which is not in the above link )

If you think about your question, it will logically make sense. You are "grepping" for a name or word and at the time the ps and grep run, there are two process that contain the target word, the process itself and the grep command, hence they are both returned by the grep command.

If it really bothers you, you can add a second pipe to ignore the grep:

Code:

ps -ef| grep 'abc.sh'| grep -v grep

(author's note: boy, talk about a run on sentence. )

A better way is to change the pattern so it doesn't match itself.

Code:

ps aux | grep '[m]yprocessname'

The [m] matches 'myprocessname' but doesn't match '[m]yprocessname'.

Thanks for your suggestions. I actually know how to avoid the grep process in the output but was curious about the reason it turned up in the output at all.

While this thread is open, I have a lingering question. This code actually runs in one my scripts in a never ending loop to check whether the script is still running. To compensate for the extra grep command in the output, I actually made the code such that it checked for 2 lines in the output of the aforesaid combination of commands, i.e.

Code:

if [ `ps -ef | grep 'abc.sh'` -le 1 ]; then
 echo "Script ends"
fi

This code runs in a never ending while loop. Problem is, the o/p is not consistent. Sometimes, after ten-fifteen minutes of expected results, the loop terminates and echoes the message indicating the script has ended, while in the background the script still runs. I know that this way to monitor script's running status is probably not the best way to do it. But still, why would the output be inconsistent? Does this have to do with multiprocessor setup? Are there instances where ps -ef command has been triggered, and the grep command has still not been triggered? As per the linked article above, the processes might "wait" for their turn, but are fired together.