usermod
Motivation
On CentOS, I need to grant a user root privilege, which was created before.
Introduction1
The usermod command modifies the system account files2 to reflect the changes that are specified on the command line.
Useful Options
-a, --append
Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace.
Example:
usemod -a -G wheel <user name>
would grant a specified user root priviledge.
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From man page.
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What files are system account files?