Using Linux:Managing Users and Groups






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Managing Groups with gpasswd

To add users to an existing group, log in as the root user and enter the following command:



[root@insoc /root]# gpasswd -a loginname groupname


where loginname is the login name of the user you want to add to the group groupname.
For example, to add the user vector to the group ittf, you would use the command:


[root@insoc /root]# gpasswd -a vector ittf


To remove a user from an existing group, log in as the root user and enter the following command:



[root@insoc /root]# gpasswd -d loginname groupname


where loginname is the login that you want to remove from the group groupname.
For example, if you wanted to remove the user yoko from the group beatles, you would use the command:


[root@insoc /root]# gpasswd -d yoko beatles


For a full list of command-line options available for the gpasswd command, see the related man page.
Using the chgrp Command

To change the group of a file, use the chgrp command. Log in as the root user and enter the following command:


[root@insoc /root]# chgrp groupname filename


where groupname is the name of the group you want to change the file’s group setting to, and filename is the name of the file for which you want the group changed.
For example, if you wanted to change the group for the file index.html to www, you would use chgrp as follows:


[root@insoc /root]# chgrp www index.html


To change the group of a directory and all its subdirectories and files, you can use the chgrp command with the -R option. For example, to change the group on all the files in the htdocs directory to www, you would use:


[root@insoc /root]# chgrp -R www htdocs


Changing File Ownership and Permissions
Two programs in the command-line arsenal help change file ownership and permissions. chown, or Change Ownership, lets you change a file (or group of files) to another owner. chmod lets you change the access permissions to individual files.
Using the chown Command

To change the owner of a file, you use the chown command. Log in as the root user and enter the following command:


[root@insoc /root]# chown ownername filename


where ownername is the login name of the user you want to change the file’s owner setting to, and filename is the name of the file for which you want the owner changed.
For example, if you wanted to change the owner for the file index.html to sshah, you would use chown as follows:


[root@insoc /root]# chown sshah index.html


To change the owner of a directory and all its subdirectories and files, you can use the chown command with the -R option. For example, to change the owner on all the files in the htdocs directory to sshah, you would use:


[root@insoc /root]# chown -R sshah htdocs


Using the chmod Command

Before we can explain the usage of the chmod command, you need to first understand file permissions.
In Linux, every file and directory has three sets of access permissions: those applied to the owner of the file, those applied to the group the file has, and those of all users in the system. You can see these permissions when you do an ls -lg. For example:


drwxr-xr-x 2 sshah sysadmin 1024 Feb 14 15:49
wedding_plans
-rw------- 1 sshah sysadmin 2465 Feb 5 19:22
index.html


The first column of the listing is the permissions of the file. The first character represents the type of file (ëd’ means directory, ël’ means symbolic link, and so on), and the next nine characters are the permissions. The first three characters represent the permissions held by the file’s owner, the second three are for the group the file is in, and the last three represent the world permissions.

The following letters are used to represent permissions:

Letter
Meaning

r
Read

w
Write

x
Execute

Each permission has a corresponding value. The read attribute is equal to 4, the write attribute is equal to 2, and the execute attribute is equal to 1. When you combine attributes, you add their values. See the following examples.

The most common groups of three and their meanings are:

Permission
Values
Meaning

---
0
No permissions

r--
4
Read only

rw-
6
Read and write

rwx
7
Read, write, and execute

r-x
5
Read and execute

--x
1
Execute only

Although other combinations do exist (for example: -wx), they are nonsensical, and the likelihood you’ll ever run across them is almost nil.




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