Rename or Change User Name and UID on Linux Systems.

We will see and use the usermod command to change a username in Unix/Linux distributions. usermod is used to modify or change any attributes of an already created user account via command line. It is similar to the useradd or adduser command only here the login is granted to an existing user.

PS: do not edit /etc/passwd file by hand or by using a text editor such as nano or vi.

Rename/ change by user name

First: You need to log out from the account you are trying to rename. Either by simply logging out or by killing all the processes running for that user. Execute the following command:

$ sudo pkill -u USER
or 
sudo pkill -9 -u USER

now, back to our previous example.
The syntax is as follows:

$ usermod -l new_username old_username

ie. If you have a user named 'batman' and want to rename it to 'snubmonkey', execute the following command from your terminal.

$ usermod -l batman snubmonkey

Remember; this will only change the username; everything else, like group, home directory, and UID will remain the same.

Change user UID (user ID)

The syntax is as follows:

$ usermod -u UID username

where, the numerical value of the user’s ID (UID). 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, in 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.

In the example below, you see that the user account snubmonkey holds the UID of 1003, now we want to change it to 777 as a new UID.

$ id snubmonkey
uid=1003(snubmonkey) gid=1003(snubmonkey) groups=1003(snubmonkey),4(adm),24(cdrom),
27(sudo),30(dip),46(plugdev),116(lxd),1004(XX),
1005(PIC),1006(RNC),1008(Q),123(lpadmin)

Now, let’s change the UID for user snubmonkey using -u (uid) option and verify the changes.

$ usermod -u 777 snubmonkey
uid=777(snubmonkey) gid=1003(snubmonkey) groups=1003(snubmonkey),4(adm),24(cdrom),27(sudo),
30(dip),46(plugdev),116(lxd),1004(XX),1005(PIC),
1006(RNC),1008(Q),123(lpadmin)

the
Rename and change the primary groupname

To rename the group from 'king' to 'queen', we will use the groupmod command.
Use the following command to rename the group:

$ ugroupmod -n king queen

To use a name that already exists, just apply the following command:

$ ugroupmod -o -n king queen

...with this, we end this tutorial on how to rename users in Linux.