usermod - Modify a user account

usermod [-c comment] [-d home_dir [-m]]
[-e expire_date] [-f inactive_days]
[-g initial_group] [-G group [,...]]
[-l login_name] [-p passwd]
[-s shell] [-u uid [-o]] [-L|-U] login

The usermod command modifies the system account files to reflect the changes that are specified on the command line. The options which apply to
the usermod command are:

-c comment
The new value of the user's password file comment field. It is normally modified using the chfn(1) utility.

-d home_dir
The user's new login directory. If the -m option is given the contents of the current home directory will be moved to the new home direc-
tory, which is created if it does not already exist.

-e expire_date
The date on which the user account will be disabled. The date is specified in the format YYYY-MM-DD.

-f inactive_days
The number of days after a password expires until the account is permanently disabled. A value of 0 disables the account as soon as the
password has expired, and a value of -1 disables the feature. The default value is -1.

-g initial_group
The group name or number of the user's new initial login group. The group name must exist. A group number must refer to an already exist-
ing group. The default group number is 1.

-G group,[...]
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. The groups are subject to the same restrictions as the group given with the -g option. If the user is currently a member of a
group which is not listed, the user will be removed from the group

-l login_name
The name of the user will be changed from login to login_name. Nothing else is changed. In particular, the user's home directory name
should probably be changed to reflect the new login name.

-o When used with the -r option, this option allows to change the user ID to a non-unique value.

-p passwd
The encrypted password, as returned by crypt(3).

-s shell
The name of the user's new login shell. Setting this field to blank causes the system to select the default login shell.

-u uid The numerical value of the user's ID. This value must be unique, unless the -o option is used. The value must be non-negative. Values
between 0 and 99 are typically reserved for system accounts. Any files which the user owns and which are located in the directory tree
rooted at the user's home directory will have the file user ID changed automatically. Files outside of the user's home directory must be
altered manually.

-L Lock a user's password. This puts a '!' in front of the encrypted password, effectively disabling the password. You can't use this option
with -p or -U.
-o Allows changing the user ID to a non-unique value when used with -u option.

-U Unlock a user's password. This removes the '!' in front of the encrypted password. You can't use this option with -p or -L.

usermod will not allow you to change the name of a user who is logged in. You must make certain that the named user is not executing any pro-
cesses when this command is being executed if the user's numerical user ID is being changed. You must change the owner of any crontab files
manually. You must change the owner of any at jobs manually. You must make any changes involving NIS on the NIS server.

/etc/passwd - user account information
/etc/shadow - secure user account information
/etc/group - group information