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man pgrep says the following about -f option:

   -f     The  pattern  is normally only matched against the process name.
              When -f is set, the full command line is used.

What does it mean to say the full command line is used?

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up vote 8 down vote accepted

It means that without -f, pgrep only searches for the provided regexp in the command names, while with -f it searches it in their arguments (where the first argument (argv[0]) is not necessarily the same as the command name).

$ sleep 2000 &
[1] 15768
$ pgrep 2000
$ pgrep -f 2000

And if we cause argv[0] to be different from the command name (on Linux as seen in /proc/$pid/stat) as when using zsh's ARGV0:

$ ARGV0=foo sleep 2000 &
[2] 15881
$ ps -fp $!
chazelas 15881 13126  0 19:55 pts/1    00:00:00 foo 2000
$ ps -o comm= $!
$ awk '{print $2}' /proc/$!/stat
$ pgrep foo
$ pgrep sleep
$ pgrep -f sleep
$ pgrep -f foo

There's a lot of potential confusion regarding process name on Unix and Linux. There are 3 attributes of a process that could claim being the process name:

  • The base name of the executable that was passed to the last execve(2) system call that the process made. On Linux, that is used to initialise the process name, as reported by ps or found in /proc/$pid/stat. However, it should be noted that on Linux, it can be changed using prctl(PR_SET_NAME...).
  • any path to the executable that is currently mmaped for execution in the process and preferably the path as it was invoked (for scripts, that would be the path provided in the shebang line for instance). On Linux, you can get it with a readlink or /proc/$pid/exe. That one, you can't change without calling another execve (though you could in theory load a new executable in memory and execute its code without calling execve, see for instance some attempts at a user land execve).
  • The first argument passed along the execve system call. By convention, that argument is meant to tell the application its name to let it know how to behave accordingly so is often what is meant by the process name. On Linux, it can be found in /proc/$pid/cmdline, but a process can also change it by modifying the memory pointed to by argv[0].

It should also be noted that there's a lot of variation among the difference Unices.

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you wrote "(where the first argument (argv[0]) is not necessarily the same as the command name)." First argument is ways different than the command name right ? For example when you did sleep 2000 & , "2000" is the first argument to the command "sleep ". Am I missing something fundamental here ? – Geek Jan 24 '13 at 4:03
what does the " $!" part in ps -fp $! indicate ? – Geek Jan 24 '13 at 4:04
@Geek, depends how you want to count. In "sleep 2000", you call "/usr/bin/sleep" with two arguments: "sleep" and "2000". Now, you may want to call them the 1st and 2nd arguments or the 0th and 1st arguments since the 0th one is treated specially by most applications (and not as much as an argument they should act upon, but one that tells them their name). $! is the PID of the last command run in background. – Stéphane Chazelas Jan 24 '13 at 7:26

when you ps -f you will see more details about the processes including the associated commands and process information. When you pgrep -f it will search that additional information to match your query as opposed to what is displayed when you just use ps or ps -e

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