mjhjmtu

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

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.

Using ps:

$ ps -o group,supgrp $$
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
 getgroups(2).

 supgrp SUPGRP group names of supplementary groups, if any. See
 getgroups(2).

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"

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

pcred <prcess-id>

Note that the official procfs is not ASCII but binary.