Linux and Unix usermod command
Enables a super user or root user to modify a users account.
usermod [-c comment] [-d home_dir [ -m]] [-e expire_date] [-f inactive_time] [-g initial_group] [-G group[,...]] [-l login_name] [-p passwd] [-s shell] [-u uid [ -o]] [-L|-U] login
|-c comment||The new value of the user's password file comment field. It is normally modified using the chfn 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 directory, 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_time||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 existing 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.|
|-p passwd||The encrypted password, as returned by crypt.|
|-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.|
|-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.|
Tip: For these commands to work you must have super user rights or be logged in as root.
usermod -d /home/newperson newperson
In the above example the usermod command would modify the home directory for the newperson account to "/home/newperson"
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 processes 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.