Groups can also have passwords, which you set with the
gpasswdcommand, but it is not used at all!
usermodcommand to modify a user account. we can use most of the options that you use with
useradd, except that you cannot create or populate a new home directory for the user.
groupdelcommand to delete a group requires only the group name; it has no options. You cannot delete any group that is the primary group of a user.
Note: If root deletes a group with members, people wont be deleted! They will just wont be the members of that group anymore.
There are some users with /sbin/nologin shell, They are actually system accounts that run a service and no one can interactively login using them. Some times it has been set to /bin/false.
Note: !! means user can not log in with any passwords. Most of service accounts are like this.
$id$salt$hashed, The $id is the algorithm used On GNU/Linux as follows: $1$ is MD5 $2a$ is Blowfish $2y$ is Blowfish $5$ is SHA-256 $6$ is SHA-512
chage -d 0 user-namewill force user to change his password in next login.
passwd can also change or reset the account's validity period — how much time can pass before the password expires and must be changed.
its format is :
Group name:Encrypted password:Group administrators: Group members
! :groups can have passwords but it have never been used in any distribution!