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I have a script which does ps -o comm= -p $PPID.

The explanation says this gets the parent process name.

From the man page I found out -o means user defined format, comm means command and -p means select the process by the given PID - in this case $PPID, which means parent PID.

  • What does comm= -p $PPID mean?
  • How does this command work?

1 Answer 1

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  • -o comm= means user output should be the command name only, but without any column title. E.g. if you do -o comm=COMMAND, it will print you a column title COMMAND:

     $ ps -o comm= -p $PPID
     xterm
     $ ps -o comm=COMMAND -p $PPID
     COMMAND
     xterm
    
  • -p $PPID selects the process by the given parent's PID, the PPID.

That means -o comm= -p $PPID are two independent options.

So your command essentially does give you the name of the parent process by its PPID.

E.g. if I start tmux, it has the PID of 1632. Now I start several bash in each pane, which each have the PPID of 1632, but have their own PID.

You can learn more in What are PID and PPID?

I am not sure, but ps might look at /proc/$PPID/comm to determine the parent's command name.

In my case, executing this command gives you the name of the parent's process, without using ps:

$ cat /proc/$PPID/comm
tmux
$ cat /proc/1632/comm
tmux
2
  • I just meant the formatting is done on top of the output of original ps ?
    – user881300
    Jul 21, 2014 at 20:58
  • Yeah, I guess internally ps just has a switch case that prints out given columns, if the -o option is given, and a header for each column that has a string following the =.
    – polym
    Jul 21, 2014 at 21:02

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