Red Hat RHCSARHCE 7 Cert Guide（四）

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5./etc/passwd and /etc/shadow
        GID：primary group
        The /sbin/nologin command is a specific command that silently denies access to users
        You can create a file with the name /etc/nologin.txt that contains a message that will be displayed when a user who has /sbin/
                nologin as its shell tries to log in.
6./etc/shadow is that no one except the superuser root and processes running with root permissions
7.userdel -r to remove a user, including the complete user environment.
8.vipwd:
        vipw . This command opens an editor interface on your configuration files, and more important,
                it sets the appropriate locks on the configuration files to prevent corruption. It does not check syntax
9.vipwd -s(vigr):
        If you want to use this tool to modify the /etc/shadow file, use vipw -s . To edit the contents of the
/etc/group file where groups are defined, a similar command with the name vigr exists.
10.useradd -m -u 1201 -G sales,ops linda #create a user linda who is a member of the groups sales and ops with UID 1201 and add a home directory to the user account as well
11.useradd -m #the content of the “skeleton” directory is copied to the user home directory. The skeleton directory is /etc/skel
12.usermod has an option -p that tells you to “use encrypted password for the new password
13.When working with tools as useradd, some default values are assumed(/etc/login.defs and /etc/default/useradd)
        the /etc/default/useradd file contains some default values that are applied when using useradd
        In the file /etc/login.defs, different login-related variables are set.
        USERGROUPS_ENAB: Set to yes to create a private group for all new
                users. That means that a new user has a group with the same name as the user
                as its default group. If set to no, all users are made a member of the group
                (users).
14.passwd -n 30 -w 3 -x 90 linda   #sets the password for user linda to a minimal usage period of 30 days and an expiry after 90 days, where a warning is generated 3 days before expiry
        chage -E 2015-12-31 bob #have the account for user bob expire on December 31, 2015
        chage -l linda
15.
        ■ /etc/profile: Used for default settings for all users when starting a login shell
        ■ /etc/bashrc: Used to define defaults for all users when starting a subshell
        ■ ~/.profile: Specific settings for one user applied when starting a login shell
        ■ ~/.bashrc: Specific settings for one user applied when starting a subshell
16.for i in lisa lori bob; do useradd $i; done
17.Linux users can be a member of two different kinds of groups. First, there is the primary group
        The users primary group membership is defined in /etc/passwd; the group itself is stored in the /etc/group configuration file.
        Secondary groups are important to get access to files. If the group a user is a member of has access to specific files, the user will get access to
                these files also.
        The group configuration files can be modified directly using vigr(/etc/group) or the commandline utility groupadd
        Members: Here you find the names of users that are a member of this group as a secondary group. Note that it does not show users that are a member of this group as their primary group
18.groupadd :
        -g:-g which allows you to specify a group ID when creating the group.
        groupmod:To manage group properties, groupmod is available
        usermod -aG will add users to new groups that will be used as their secondary group
        groupmems -g sales -l to see which users are a member of the group sales
        groupadd -aG sales linda
        groupadd -aG sales lisa
        groupadd -aG account lori
        groupadd -aG account bob
        (add users linda and lisa to the group sales, and lori and bob to the group account:) 组名在前