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I've just tried the following command on my Ubuntu, it doesn't show a thing:

pgrep php5

shouldn't it return the process id of php5(which the following command just does)?:

ps aux | grep php5

So, what's the difference between these two commands?

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up vote 38 down vote accepted

ps aux includes the full command line (path and parameters), while pgrep only looks at the first 15 characters of the executable's names

ps aux returns the full command line of each process, while pgrep only looks at the names of the executables.

That means that grepping ps aux output will match anything that occurs in the path or the parameters of a process' binary: e.g. `

  • ps aux | grep php5 will match /usr/share/php5/
  • but pgrep php5 won't

Take an example from my system -- only we'll use python instead of php5:

  • ps aux | grep python gives us:
izx       2348  0.0  0.7 514928 15644 ?        Sl   Jun24   0:00 /usr/bin/python /usr/lib/unity-lens-video/unity-lens-video
izx       2444  0.0  0.9 547392 18864 ?        Sl   Jun24   0:01 /usr/bin/python /usr/lib/unity-scope-video-remote/unity-scope-video-remote
root      2805  0.0  0.5  95436 12204 ?        S    Jun24   0:00 /usr/bin/python /usr/lib/system-service/system-service-d
izx       6272  0.0  2.9 664400 60320 ?        SNl  Jun24   1:16 /usr/bin/python /usr/bin/update-manager --no-focus-on-map
root     11729  0.0  0.9 180508 19516 ?        S    Jun25   0:00 python /usr/lib/software-properties/software-properties-dbus
  • But pgrep python returns only 11729, which you'll see from the above list is:
root     11729  0.0  0.9 180508 19516 ?        S    Jun25   0:00 python /usr/lib/software-properties/software-properties-dbus
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"pgrep -l" also truncates the process to 15 characters, which is probably a bug but has been like this for ages – Thorsen Jun 28 '12 at 10:23
Well it's in launchpad as an upstream bug as of 2008 :) "top and ps truncate command after 15 chars" – Thorsen Jun 28 '12 at 10:42
Hah, this explains it, from comment #3: This is due to the fact that some procedures of procs get the command name from /proc/<pid>/stat but not from /proc/<pid>/cmdline. OK, @Thorsen, you win the bug spray, it's a bug :P – izx Jun 28 '12 at 10:57
Thank you so much. According to your answer, pgrep is quite an unreasonable command, because it could well ignore all the running process in some circumstances(e.g.,my php5). Is there any other convincible reason that this command should exist? – xczzhh Jun 28 '12 at 13:22
@xczzhh pgrep is not an unreasonable command. It works well and as designed. The issue is simply you were missing an option when you run it, you can't blame pgrep for that. Using ps aux | grep xxx is unreliable so need hacks to filter out grep itself from the output and might give false positives like with ps aux | grep root. – jlliagre Aug 1 '13 at 22:23

The ps aux | grep x command gives "better" results than pgrep x essentially because you are missing an option with the latter.

Simply use the -f option for pgrep to search the full command line and not only the process name which is its default behavior, eg:

pgrep -f php5

Unlike the ps | grep construction with which you need to filter out the grep line or use pattern tricks, pgrep just won't pick itself by design.

Moreover, should your pattern appear in ps USER column, you'll get unwanted processes in the output, pgrep doesn't suffer from this flaw.

If you want full details instead of just the pids, you can use:

ps wup $(pgrep -f python)

which is simpler and more reliable than

ps aux | grep python | grep -v grep


ps aux | grep p[y]thon
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Also add the -a (--list-full) option if you want to see the full command line and not just the pid. (Older pgrep had no -a, did this on -fl.) – Beni Cherniavsky-Paskin Aug 12 '14 at 8:51
The original question was asking about the difference but this actually gives those of us trying to get pgrep to play nice the solution. +1 – 2rs2ts Jun 25 '15 at 21:32
@2rs2ts Thanks, I was indeed missing to answer to the question asked. Fixed now. – jlliagre Jun 25 '15 at 22:20
diff <(ps aux|grep x) <(pgrep x) # :)
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This may answer the answer the question - but perhaps you can expand your answer to explain what this one line command does. – fossfreedom Jun 28 '12 at 18:46

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