The Unix/Linux command “usermod” is a useful one to alter the already created user via command line.
The command Useradd or adduser is for creating a user in Linux system/server from its command prompt. The basic syntax for creting a user is simply type the username after the command useradd.
It will create a user with defined settings. In some situations we need to alter the parameters of a user, in this case we can use the command usermod. See the syntax explained below.
User information are stored in /etc/passwd file.
The important user information are Username, UID(User ID), GID(Group ID), a comment, home directory location, shell details etc. With the command usermod, a lot of switches are available to change those parameters.
Example, you can change the comment of a user in the file /etc/passwd by using the switch “-c” similarly you can change the home directory by using the switch “d”.
Syntax
# usermod [options] USERNAME
See the default values to compare the usermod output
[root@localhost ~]# useradd me
[root@localhost ~]# grep -w me /etc/passwd
me:x:502:502::/home/me:/bin/bash
Switches with example
1, -a, –append
Add the user to the supplementary group(s). Use only with the -G option.
2, -c, –comment COMMENT
The new value of the user´s password file comment field.
Example:
[root@localhost ~]# usermod -c "I'm usermod" me
[root@localhost ~]# grep -w me /etc/passwd
me:x:502:502:I'm usermod:/home/me:/bin/bash
Comment field changed to “I’m usermod”
3, -d, –home 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.
Example:
[root@localhost ~]# usermod -d /home/meusermod me
[root@localhost ~]# grep -w me /etc/passwd
me:x:502:502:I'm usermod:/home/meusermod:/bin/bash
4, -e, –expiredate EXPIRE_DATE
The date on which the user account will be disabled. The date is specified in the format YYYY-MM-DD.
5, -f, –inactive INACTIVE
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.
6, -g, –gid GROUP
The group name or number of the user´s new initial login group. The group must exist.
Example:
[root@localhost ~]# usermod -g 0 me
[root@localhost ~]# grep -w me /etc/passwd
me:x:502:0:I'm usermod:/home/meusermod:/bin/bash
7, -G, –groups GROUP1[,GROUP2,…[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Separated from the next by a comma without space. 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. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list.
Example:
[root@localhost ~]# usermod -G crybit me [root@localhost ~]# groupmems -g crybit -l me
8, -l, –login NEW_LOGIN
The name of the user will be changed from LOGIN to NEW_LOGIN. In particular, the user´s home directory name should probably be changed manually to reflect the new login name.
Example:
[root@localhost ~]# usermod -l namechange me
usermod: warning: /var/spool/mail/me not owned by me
[root@localhost ~]# grep -w me /etc/passwd
[root@localhost ~]# tail -n2 /etc/passwd
crybit:x:501:501::/home/crybit:/bin/bash
namechange:x:502:0:I'm usermod:/home/meusermod:/bin/bash
Login name changed to “namechange”
9, -L, –lock
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.
Note: if you wish to lock the account (not only access with a password), you should also set the EXPIRE_DATE to 1.
Example:
[root@localhost ~]# usermod -L me
[root@localhost ~]# grep -w me /etc/shadow
me:!$6$E6bjJlI1$Phbo3rhsWNoHBwkRyGgPZV0BBM8.okQyJZifbHNzrGsmBsREmcxfNLv9TrKIX34zA7e3Hv/J0LxzxEIOTTt7n/:16101:0:99999:7:::
10, -U, –unlock
Unlock a user´s password. This removes the ´!´ in front of the encrypted password. You can´t use this option with -p or -L.
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).
[root@localhost ~]# usermod -U me
[root@localhost ~]# grep -w me /etc/shadow
me:$6$E6bjJlI1$Phbo3rhsWNoHBwkRyGgPZV0BBM8.okQyJZifbHNzrGsmBsREmcxfNLv9TrKIX34zA7e3Hv/J0LxzxEIOTTt7n/:16101:0:99999:7:::
11, -m, –move-home
Move the content of the user´s home directory to the new location. This option is only valid in combination with the -d (or –home) option.
12, -o, –non-unique
When used with the -u option, this option allows to change the user ID to a non-unique value.
13, -p, –password PASSWORD
The encrypted password, as returned by crypt(3).
Not recommended. The password (or encrypted password) will be visible by users listing the processes. You should make sure the password respects the system´s password policy.
14, -s, –shell SHELL
The name of the user´s new login shell. Setting this field to blank causes the system to select the default login shell.
Example:
[root@localhost ~]# usermod -s /bin me
[root@localhost ~]# grep -w me /etc/passwd
me:x:502:0:I'm usermod:/home/meusermod:/bin
15, -u, –uid UID
The new numerical value of the user´s ID. Set uniq number. The value must be non-negative. The UID 0 to 999 are system reserved.
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.
Change ownerships outside of the user ´s home directory manually.
[root@localhost ~]# usermod -u 100000 me
[root@localhost ~]# grep -w me /etc/passwd
me:x:100000:0:I'm usermod:/home/meusermod:/bin
16, -Z, –selinux-user SEUSER
The SELinux user for the user´s login. The default is to leave this field the blank, which causes the system to select the default SELinux user.
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