To add a new user account, you can run either of the following two commands as root.

adduser tom
useradd jerry

[root@machine1 ~]# cat /etc/passwd | grep tom
tom:x:501:501::/home/tom:/bin/bash
[root@machine1 ~]# cat /etc/passwd | grep jerry
jerry:x:502:502::/home/jerry:/bin/bash
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View a List of All Groups
To view all the groups on the system, you can just use the groups command:

groups
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To set the password for user

[root@machine1 ~]# passwd tom
Changing password for user tom.
New password:
BAD PASSWORD: it is WAY too short
BAD PASSWORD: is too simple
Retype new password:
passwd: all authentication tokens updated successfully
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When a new user account is added to the system, the following operations are performed.
1. His/her home directory is created (/home/username by default).
2. The following hidden files are copied into the user’s home directory, and will be used to provide environment variables for his/her user session.
.bash_logout
.bash_profile
.bashrc
3. A mail spool is created for the user at /var/spool/mail/username.
4. A group is created and given the same name as the new user account.
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su(switch user) is the command to switch to another user
If use su username(U will be changed to another user but the pwd(present working directory) where u are before will be same)
If use su – username

[root@machine1 ~]# su – tom
[tom@machine1 ~]$ ls -la
total 28
drwx——. 4 tom  tom  4096 Jan 28 00:41 .
drwxr-xr-x. 5 root root 4096 Jan 28 00:41 ..
-rw-r–r–. 1 tom  tom    18 Aug 17 03:49 .bash_logout
-rw-r–r–. 1 tom  tom   176 Aug 17 03:49 .bash_profile
-rw-r–r–. 1 tom  tom   124 Aug 17 03:49 .bashrc
drwxr-xr-x. 2 tom  tom  4096 Jul 14  2010 .gnome2
drwxr-xr-x. 4 tom  tom  4096 Oct 24 05:24 .mozilla
[tom@machine1 ~]$ logout
[root@machine1 ~]# su – jerry
[jerry@machine1 ~]$ ls -la
total 28
drwx——. 4 jerry jerry 4096 Jan 28 00:41 .
drwxr-xr-x. 5 root  root  4096 Jan 28 00:41 ..
-rw-r–r–. 1 jerry jerry   18 Aug 17 03:49 .bash_logout
-rw-r–r–. 1 jerry jerry  176 Aug 17 03:49 .bash_profile
-rw-r–r–. 1 jerry jerry  124 Aug 17 03:49 .bashrc
drwxr-xr-x. 2 jerry jerry 4096 Jul 14  2010 .gnome2
drwxr-xr-x. 4 jerry jerry 4096 Oct 24 05:24 .mozilla
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Understanding /etc/passwd

The full account information is stored in the /etc/passwd file. This file contains a record per system user account and has the following format (fields are delimited by a colon).

[username]:[x]:[UID]:[GID]:[Comment]:[Home directory]:[Default shell]

Fields [username] and [Comment] are self explanatory.
The x in the second field indicates that the account is protected by a shadowed password (in /etc/shadow), which is needed to logon as [username].
The [UID] and [GID] fields are integers that represent the User IDentification and the primary Group IDentification to which [username] belongs, respectively.
The [Home directory] indicates the absolute path to [username]’s home directory, and
The [Default shell] is the shell that will be made available to this user when he or she logins the system.
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Understanding /etc/group

Group information is stored in the /etc/group file. Each record has the following format.

[Group name]:[Group password]:[GID]:[Group members]
[Group name] is the name of group.
An x in [Group password] indicates group passwords are not being used.
[GID]: same as in /etc/passwd.
[Group members]: a comma separated list of users who are members of [Group name
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After adding an account, you can edit the following information (to name a few fields) using the usermod command, whose basic syntax of usermod is as follows.
# usermod [options] [username]

Setting the expiry date for an account
Use the –expiredate flag followed by a date in YYYY-MM-DD format.
# usermod –expiredate 2014-10-30 tom
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Adding the user to supplementary groups
Use the combined -aG, or –append –groups options, followed by a comma separated list of groups.
# usermod –append –groups root,users tom

Changing the default location of the user’s home directory
Use the -d, or –home options, followed by the absolute path to the new home directory.
# usermod –home /tmp tom
# usermod -d /tmp tom
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Changing the shell the user will use by default
Use –shell, followed by the path to the new shell.
# usermod –shell /bin/sh tom
# usermod  -s /bin/sh tom
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Displaying the groups an user is a member of
# groups tom
# id tom
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Now let’s execute all the above commands in one go.
# usermod –expiredate 2014-10-30 –append –groups root,users –home /tmp –shell /bin/sh tom
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[root@machine1 ~]# chage  -l tom
Last password change : Jan 28, 2016
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7
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[root@machine1 ~]# cat /etc/passwd | grep tom
tom:x:501:501::/home/tom:/bin/bash

[root@machine1 ~]# usermod –home /tmp tom
[root@machine1 ~]# cat /etc/passwd | grep tom
tom:x:501:501::/tmp:/bin/bash

[root@machine1 ~]# cat /etc/group | grep tom
tom:x:501:

[root@machine1 ~]# cat /etc/passwd | grep jerry
jerry:x:502:502::/home/jerry:/bin/bash

[root@machine1 ~]# usermod –expiredate 2016-10-30 tom
[root@machine1 ~]# chage  -l tom
Last password change : Jan 28, 2016
Password expires : never
Password inactive : never
Account expires : Oct 30, 2016
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7

[root@localhost ~]# tail -n1 /etc/shadow

mohammed:$1$jUXt.n.l$Jop9u6TlkRT.I6OT7zFq50:16984:0:99999:7:::

Username : It is your login name.

Password : It is your encrypted password. The password should be minimum 6-8 characters long including special characters/digits and more.

Last password change (lastchanged) : Days since Jan 1, 1970 that password was last changed

Minimum : The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password

Maximum : The maximum number of days the password is valid (after that user is forced to change his/her password)

Warn : The number of days before password is to expire that user is warned that his/her password must be changed

Inactive : The number of days after password expires that account is disabled

Expire : days since Jan 1, 1970 that account is disabled i.e. an absolute date specifying when the login may no longer be used.

The last 6 fields provides password aging and account lockout features. You need to use the chage command to setup password aging. According to man page of shadow – the password field must be filled. The encrypted password consists of 13 to 24 characters from the 64 character alphabet a through z, A through Z, 0 through 9, \. and /. Optionally it can start with a “$” character. This means the encrypted password was generated using another (not DES) algorithm. For example if it starts with “$1$” it means the MD5-based algorithm was used. Please note that a password field which starts with a exclamation mark (!) means that the password is locked. The remaining characters on the line represent the password field before the password was locked.