[CentOS] UID/GID CentOS 6 to CentOS 7

Thu Oct 22 13:06:22 UTC 2020
Simon Matter <simon.matter at invoca.ch>

> Hi,
> we are upgrading some servers from C6 to C7 with a lot of user accounts
> on them (UID>=500).
> CentOS 7 has MIN_UID/MIN_GID 1000, Centos 6 has 500 in login.defs.
> Can I change in /etc/login.defs MIN_UID/MIN_GID to 500 for C7? So I
> could just grep the users out from passwd/shadow/group files and append
> them to the Centos7 passwd/shadow/group files.
> Can this do any damage to CentOS7 later on? Thinking about updates....

When I did an upgrade from CentOS 5 to 7 I found that even a standard
install of CentOS 7 already used a number of GIDs in the range of 500-999.

In the end I decided to rearrange all users to new UIDs/GIDs and converted
all storage with a script.

The tricky part was to find a way which doesn't take ages to convert
storage. Doing so with find.... wasn't possible for performance reasons.

Attached script was used to convert every user. It was the fastest way I
found. The script was started in background for every user.