How can I map an OS group on linux to an administrative privilege in case it was not done at the time of install?
What did you not do, that you should have done?
Where is this documented in the Installation Guide?
While installing database 12c, I did not create OS groups corresponding to SYSDG, SYSBACKUP and SYSKM administrative privileges. Now I want create the same and map them to corresponding privileges ..
How can I do it now?
You can create groups (as root):
groupadd -g 54321 sysdg
groupadd -g 54322 sysbackup
groupadd -g 54323 syskm
and then assign there groups to the user oracle:
usermod -G sysdg,sysbackup,syskm oracle
uid=1100(oracle) gid=1001(oinstall) Gruppen=1001(oinstall),54321(sysdg),54322(sysbackup),54323(syskm)
Does adding a group with same name as privilege automatically a group to corresponding privilege?
You'll need to update your $ORACLE_HOME/rdbms/lib/config.c file and then relink.
That should give you enough pointers to sort out te detail.
That is a bt dangerous: your usermod command is removing the oracle user from all groups that he is in already, because you did not include the -a switch.
you are right: my secondary groups are lost: "-a" must be set