The options which apply to the usermod command are:
-c, --comment COMMENT
-d, --home HOME_DIR
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, --expiredate EXPIRE_DATE
-f, --inactive INACTIVE
A value of 0 disables the account as soon as the password has expired, and a value of -1 disables the feature.
-g, --gid GROUP
Any file from the user's home directory owned by the previous primary group of the user will be owned by this new group.
The group ownership of files outside of the user's home directory must be fixed manually.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
If the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list.
-l, --login NEW_LOGIN
Note: if you wish to lock the account (not only access with a password), you should also set the EXPIRE_DATE to 1.
This option is only valid in combination with the -d (or --home) option.
usermod will try to adapt the ownership of the files and to copy the modes, ACL and extended attributes, but manual changes might be needed afterwards.
-p, --password PASSWORD
Note: This option is not recommended because the password (or encrypted password) will be visible by users listing the processes.
The password will be written in the local /etc/passwd or /etc/shadow file. This might differ from the password database configured in your PAM configuration.
You should make sure the password respects the system's password policy.
-s, --shell SHELL
-u, --uid UID
This value must be unique, unless the -o option is used. The value must be non-negative. Values between 0 and 999 are typically reserved for system accounts.
The user's mailbox, and any files which the user owns and which are located in the user's home directory will have the file user ID changed automatically.
The ownership of files outside of the user's home directory must be fixed manually.
Note: if you wish to unlock the account (not only access with a password), you should also set the EXPIRE_DATE (for example to 99999, or to the EXPIRE value from /etc/default/useradd).
-Z, --selinux-user SEUSER
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, the user's name, or the user's home directory is being changed. usermod checks this on Linux, but only check if the user is logged in according to utmp on other architectures.
You must change the owner of any crontab files or at jobs manually.
You must make any changes involving NIS on the NIS server.
The following configuration variables in /etc/login.defs change the behavior of this tool:
The MAIL_DIR and MAIL_FILE variables are used by useradd, usermod, and userdel to create, move, or delete the user's mail spool.
The default value is 0, meaning that there are no limits in the number of members in a group.
This feature (split group) permits to limit the length of lines in the group file. This is useful to make sure that lines for NIS groups are not larger than 1024 characters.
If you need to enforce such limit, you can use 25.
Note: split groups may not be supported by all tools (even in the Shadow toolsuite). You should not use this variable unless you really need it.
chfn(1), chsh(1), passwd(1), crypt(3), gpasswd(8), groupadd(8), groupdel(8), groupmod(8), login.defs(5), useradd(8), userdel(8).