Renumber a batch of users' uids and gids, and fix file ownerships

massrenumber.sh

#!/bin/bash

while IFS="," read -r name olduid oldgid newuid newgid
do
    /usr/sbin/groupmod $name -g $newgid
    /usr/sbin/usermod  $name -u $newuid -g $newgid
    /bin/chown --changes --silent --no-dereference --recursive \
       --from=:$oldgid :$name /home/$name
done << END
tinker,8001,8001,701,701
evans,8002,8005,702,702
chance,8003,8006,703,703
END

This is not for random fun (at least not on a production machine). This is for when you need to put a legacy machine onto LDAP, AD, or another system that has been assigned a block of UIDs/GIDs and you have existing users and groups in that range. Do research to futureproof your decisions, and then it helps to make a spreadsheet and export the values as CSV to be added into the data section.

This works in Centos 6, where usermod automatically changes file permissions but groupmod does not. YMMV