I'm trying to determine which group(s) a running child process has inherited. I want to find all groups the process is in given its uid. Is there a way to determine this via the /proc filesystem?

  • Are you looking for the groups associated to the process's UID (which one?), or for the groups that the process is in? The two are usually the same, but they might not be (e.g. if the process was run with sudo -g, or if the group database changed since the user logged in). May 17, 2018 at 15:58

4 Answers 4


For the effective group id, real group id and supplementary group ids (as used for access control):

ps -o gid,rgid,supgid -p "$pid"

gid and rgid are fairly portable, supgid less so (all 3 would be available with the ps from procps as typically found on Linux-based systems).

group, rgroup and supgrp can be used to translate group ids to group names, but note that for group ids that have several corresponding group names, only one of them will be shown (same as for ls -l vs ls -n or anything that deals with user or group names based on ids).

For the process group id (as used for terminal job control):

ps -o pgid -p "$pid"

To store it into a variable:

pgid=$(($(ps -o pgid= -p "$pid")))
  • For parsing, it's helpful to omit the output of the header; -o pgid= will do that. -p is optional and can be left out. There's still leading whitespace in the output, so do this to capture the PGID: pgid=$(ps -o pgid= "$pid" | grep -o '[0-9]\+') Sep 1, 2018 at 9:16
  • @Ingo, -p is not optional per POSIX. Neither -o nor \+ are standard either. Here, you could use pgid=$(($(ps -o pgid= -p "$pid"))) portably if you wanted to store that pid in a variable (gives 0 if $pid is not found but preserves the exit status of ps). Sep 1, 2018 at 9:24
  • Thanks for adding the parsing part into your answer, and for the notes on POSIX compatibility; that $((...)) evaluation is a really neat trick (and also slightly more efficient that using grep). I'm learning so much from your great answers!!! Sep 1, 2018 at 9:34

The list of groups is given under Groups in /proc/<pid>/status; for example,

$ grep '^Groups' /proc/$$/status
Groups: 4 24 27 30 46 110 115 116 1000

The primary group is given under Gid:

$ grep '^Gid' /proc/$$/status
Gid:    1000    1000    1000    1000

ps is also capable of showing the groups of a process, as the other answers indicate.

  • best-practice is to use ps command when possible instead of opening/reading/parsing files under /proc/pid/ because the files and contents of files under /proc/pid can change whenever the kernel version changes. for example: i had a script that worked on kernel version 3.x and then the same script failed on 5.x kernel version. i changed the script to use ps and now it works in both kernel version 3.x and 5.x. Jan 9 at 15:29

Using ps:

$ ps -o group,supgrp $$
muru     adm,cdrom,sudo,dip,www-data,plugdev,lpadmin,mlocate,sambashare,lxd,libvirtd,docker,muru

From man ps, the output columns used for -o:

   egid        EGID      effective group ID number of the process as a
                         decimal integer.  (alias gid).

   egroup      EGROUP    effective group ID of the process.  This will be
                         the textual group ID, if it can be obtained and
                         the field width permits, or a decimal
                         representation otherwise.  (alias group).

   gid         GID       see egid.  (alias egid).

   group       GROUP     see egroup.  (alias egroup).

   supgid      SUPGID    group ids of supplementary groups, if any.  See

   supgrp      SUPGRP    group names of supplementary groups, if any.  See

On a UNIX system derived from SVr4, you may call:

pcred <prcess-id>

Note that the official procfs is not ASCII but binary.

  • The question is about Linux, which is not (much) derived from SVr4, so this doesn't answer the question. Also there is no such thing as “the official procfs”: each Unix variant has its own implementation of it, or doesn't have one. May 17, 2018 at 16:00
  • You seem to miss that there was a paper on procfs and another one on procfs-2. Linux has not much more in common with that paper than the name procfs. Since this portal is about UNIX, it is obvious that there are people who like to know how things work on UNIX even thought the question might have been Linux specific.
    – schily
    May 17, 2018 at 20:09

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