[CentOS] What is /etc/subuid ?

Wed Oct 9 19:47:19 UTC 2019
Valeri Galtsev <galtsev at kicp.uchicago.edu>

Dear Experts,

Could someone enlighten me about the following file:


? This file appears to be owned by "setup" package. This is CentOS 7 
system, and until now these files if existed were never changed. Today I 
have added user quite routine way, by doing

/usr/sbin/groupadd -g 4500 [username]
/usr/sbin/useradd -g [username] -u 4500 -c "User Name, email at domain" 

And the file /etc/subuid changed and user was added into it:


Nothing like that was happening before. This is first time I create 
account after update done on Oct 3, 2019. I checked several CentOS 7 
machines, basically doing this:

  # grep subuid /usr/sbin/useradd
Binary file /usr/sbin/useradd matches

And CentOS 7 machines indeed may have that file name in the useradd 
binary. None of CentOS 6 machines has that.

I tried to do FreeBSD-ism:

man /etc/subuid

came empty, and realized that I'm doing FreeBSD-ism.

I tried to do search on the web (did not "google", I use duckduckgo... 
so I "did search"), and came pretty much empty.

Is it just me, or indeed something in CentOS 7 indeed changed? And what 
is it?

Another question on the same note: how do we find out what the file is 
about and is used for in Linux, apart from searching on the web. (When 
there are surprises like the one I had today, one does like to know what 
this particular file is used for).

Thanks in advance for your answers.


Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247