Why does "ps aux | grep x" give better results than "pgrep x"?

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?

0

5 Answers

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

or

ps aux | grep p[y]thon

6

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/i-am-a-perl-script.pl
• 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

7

diff <(ps aux|grep x) <(pgrep x) # :)

1

At this time, ps will give more complete output than pgep -f as pgrep is limited to the first 4,096 characters (often affecting Java users looking for the entry class of a Java program with a long classpath). The bug tracking this is:

1

It may not be better... when I tried to look for the tmux server process,

pgrep -l tmux

showed it, but

ps aux | grep tmux 

won't show it as the server but shows it as the command that triggered the start of the server. (tmux new -s foo)

Your Answer

Sign up or log in

Sign up using Google Sign up using Facebook Sign up using Email and Password Submit

Post as a guest

Post Your Answer Discard

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy