Use useradd in linux to create the user's Command Format

In linux, use useradd to create the user's command format-Linux general technology-Linux technology and application information. The following is a detailed description. [Root @ linux ~] # Useradd [-u UID] [-g initial_group] [-G other_group] \
>-[Mm] [-c description bar] [-d home] [-s shell] username
Parameters:
-U: UID followed by a group of numbers. Specify a specific UID for this account;
-G: The group name we followed is the initial group we mentioned above.
The group ID (GID) is placed in the fourth column of/etc/passwd.
-G: The group name that follows is the group that this account can support.
This parameter will modify relevant information in/etc/group!
-M: force! Do not create a user's home directory
-M: force! Create a user's home directory!
-C: This is the description in the Fifth Column of/etc/passwd ~ You can set it as needed.
-D: specify a directory as the Home Directory instead of the default value;
-R: Create a system account. The UID of this account is limited (/etc/login. defs)
-S: Followed by a shell. The default is/bin/bash.

Example:

Example 1: create a user named vbird1 Based on the preset values.
[Root @ linux ~] # Useradd vbird1
[Root @ linux ~] # Ls-l/home
Drwxr-xr-x 3 vbird1 vbird1 4096 Aug 30 vbird1
[Root @ linux ~] # Grep vbird1/etc/passwd/etc/shadow/etc/group
/Etc/passwd: vbird1: x: 502: 502:/home/vbird1:/bin/bash
/Etc/shadow: vbird1 :!! : 13025: 0: 99999: 7 :::
/Etc/group: vbird1: x: 502:
# This example is just for you to understand. In fact, the system has standardized some parameters for new users!
# Therefore, when we use useradd, the system will take the initiative to modify/etc/passwd and/etc/shadow,
# The reference values of the corresponding columns in these two archives will be standardized with the content of Some preset files!
# Note that when useradd is used to add a user, the user's/etc/shadow
# The password Column cannot be logged on (!! ). Therefore, passwd must be used.
# The vbird1 password is added!

Example 2: I know that there is a group named users in my system and UID 700 does not exist,
Use these two parameters to create an account for vbird2!
[Root @ linux ~] # Useradd-u 700-g users vbird2
[Root @ linux ~] # Ls-l/home
Drwxr-xr-x 3 vbird2 users 4096 Aug 30 vbird2
[Root @ linux ~] # Grep vbird2/etc/passwd/etc/shadow/etc/group
/Etc/passwd: vbird2: x: 700: 100:/home/vbird2:/bin/bash
/Etc/shadow: vbird2 :!! : 13025: 0: 99999: 7 :::
# Check that UID and initial group are actually changed to what we need!

Example 3: Create a system account named vbird3
[Root @ linux ~] # Usradd-r vbird3
[Root @ linux ~] # Grep vbird3/etc/passwd/etc/shadow/etc/group
/Etc/passwd: vbird3: x: 101: 102:/home/vbird3:/bin/bash
/Etc/shadow: vbird3 :!! : 13025 ::::::
/Etc/group: vbird3: x: 102:
# It's important! You will find that the UID is actually 101, and what is GID? It will be 102,
# What's the relationship with/etc/group! Is there a lot of difference between-r!