Linux User

Each Linux user is identified to the system with a user name because that is a multi-user system

To this name the system associates a unique number (UID), a password, your full name, a boot directory (the home user) and a login shell (there are different shell programs, and choose between one or the other according to the characteristics of the user or the type of shell you prefer)

All these data saved the system in the /etc/passwd and /etc/shadow. The /etc/shadow file contains the encrypted password and data over the same (such as the validity time, in days to warn before expiry, every time you have to force your change, etc). The creation of this file came as a matter of safety, as the file passwd must be accessible to all users

Structure

The structure of the file passwd is the following:

/etc/passwd

user:x:uid:gid:full name:home directory:command shell

Some of the most common can be found in the following table:

Add

The command to add a user is useradd

useradd -g users -G ftp -s /bin/shell -p password-d /home/patxi -m patxi

In the example we created the patxi user who belongs to the users group and ftp, has as shell /bin/shell, we assign the password key and their home folder will be /home/patxi

This command supports the following parameters:

In the file /etc/default/useradd saved default settings for the useradd command and in the directory /etc/skel are the files and directories by default will be in the user's home directory when you create it

Edit

The command to modify users is usermod

usermod -G server patxi

In the example we have modified the user patxi who belonged to the group users and ftp, now his main group is a server, although it still belongs to the groups users and ftp

This command supports the following parameters:

Delete

The command to delete users is userdel

userdel -r patxi

In the example deletion by user patxi and in addition we have forced to delete the contents of your home when you use the-r parameter, if we had not used, would still exist

Change the key

The command to change the key of a user is the passwd command. Keep in mind that you must have sufficient permissions to be able to make the change, either because of who is running the command is the user or the superuser root

$ passwd <usuario>
Enter existing password
Enter new password
Enter new password again (to validate)

If we want to change the password of the root user, and we have sufficient permissions, can be done by leaving the parameter empty user

shadow

The structure of the file shadow is the following:

/etc/shadow

user:encrypted password:1:2:3:4:5:6

Groups Linux

The users are grouped into groups. The groups are also identified by a name and the system associates an identification number (GID). Every user has a primary group, but can belong to multiple groups, which inherits its privileges. Groups allow you to give the rights and restrict options globally to multiple users

The relationship of the groups is saved in the file /etc/group. There is also the file /etc/gshadow for encrypted passwords of group, although it is not very used

Structure

The structure of the group file is the following:

/etc/group

group:x:gid:list members

Some of the most common groups can be found in the following table:

Add

The command to add groups groupadd

groupadd -g 500 server

In the example we have set the group server with a gid of 500, in case you do not put anything you would use the greater than 500 is not used by another group. You can use the-g parameter to set the gid manually

If you want to add existing users to the newly created group will use the gpasswd command

gpasswd -patxi server

In the example we added the user patxi with the parameter -a to the group server

Edit

The command to change groups is groupmod

groupmod -g 501 server

In the example we have modified the gid to 501 of the group server

This command supports the following parameters:

This command only makes changes in the file /etc/group, so the rest of
modifications involved in changing the gid of a group, you have to do manually in the files /etc/passwd and /etc/gshadow

Delete

The command to delete a group groupdel

groupdel server

In the example deleted the group server

You cannot delete a group that is a initial group for any user, without first deleting the user

The command does not traverse the file system looking for files owned by the group. If
we want to remove them, we have manually or with command-deletion of files

If we want to delete existing users to the newly created group will use the gpasswd command

gpasswd -d patxi server

In the example we have removed the user patxi with the-d parameter of the group server. But this will not delete the home directory of the group