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Get the grep Out!

Oct 24, 2007  By admin

While using grep to search for a running process from a ps ax output, it also includes the grep command, but in some cases (for example in a script) this is undesirable. To have a 'clean' output you can use brackets in this way for the grep command:

$ ps ax | grep s[s]h
4920 ?    Ss 0:00   /usr/sbin/sshd
$

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pgrep

oneguycoding's picture
Submitted by oneguycoding on Tue, 12/04/2007 - 15:00.

I find pgrep to be really useful for identifying processes,

pgrep ssh
3161
3815
3957
4098
4168
4320
7089
9647
19727

and then you can do fun stuff like,

for p in $(pgrep ssh) ; do
ps -p $p -f --no-headers
done

and this eliminates the need to do any tweeks to avoid grepping grep.

:wq

How does this work? I type

Gordon J Milne's picture
Submitted by Gordon J Milne (not verified) on Sun, 12/02/2007 - 16:51.

How does this work?

I type "ps -ef | grep g[e]tty" and get this:

root      1878     1  0 Nov23 tty1     00:00:00 /sbin/getty 38400 tty1
root      1879     1  0 Nov23 tty2     00:00:00 /sbin/getty 38400 tty2
root      1880     1  0 Nov23 tty3     00:00:00 /sbin/getty 38400 tty3
root      1881     1  0 Nov23 tty4     00:00:00 /sbin/getty 38400 tty4
root      1882     1  0 Nov23 tty5     00:00:00 /sbin/getty 38400 tty5
root      1883     1  0 Nov23 tty6     00:00:00 /sbin/getty 38400 tty6

So it seems to work.

Now, if I type "ps -ef | grep getty >xxx" and then "cat xxx" I get this:

root      1878     1  0 Nov23 tty1     00:00:00 /sbin/getty 38400 tty1
root      1879     1  0 Nov23 tty2     00:00:00 /sbin/getty 38400 tty2
root      1880     1  0 Nov23 tty3     00:00:00 /sbin/getty 38400 tty3
root      1881     1  0 Nov23 tty4     00:00:00 /sbin/getty 38400 tty4
root      1882     1  0 Nov23 tty5     00:00:00 /sbin/getty 38400 tty5
root      1883     1  0 Nov23 tty6     00:00:00 /sbin/getty 38400 tty6
root     11504 11455  0 10:44 pts/0    00:00:00 grep getty

As you would expect, xxx contains the grep command. Now, I type "cat xxx | grep g[e]tty" and find that it gives me:

root      1878     1  0 Nov23 tty1     00:00:00 /sbin/getty 38400 tty1
root      1879     1  0 Nov23 tty2     00:00:00 /sbin/getty 38400 tty2
root      1880     1  0 Nov23 tty3     00:00:00 /sbin/getty 38400 tty3
root      1881     1  0 Nov23 tty4     00:00:00 /sbin/getty 38400 tty4
root      1882     1  0 Nov23 tty5     00:00:00 /sbin/getty 38400 tty5
root      1883     1  0 Nov23 tty6     00:00:00 /sbin/getty 38400 tty6
root     11504 11455  0 10:44 pts/0    00:00:00 grep getty

Why does the same trick not work?

Frankly, I cannot see how the square brackets trick works. Can you enlighten me please.

Dumb Me

gjmilne's picture
Submitted by gjmilne on Sun, 12/02/2007 - 18:04.

Sorry, I am dumb. I now see how the grep 'trick' works.