Managing access to resources is a fundamental task for sysadmins. This responsibility consists of three components: identities, resources, and permissions. This article covers several user, group, and file management commands to control access to resources. The article uses a "How do I…?" format, and it assumes you have a few resources to work with. Specifically, I cover the following topics:
- Creating directories and files
- Managing ownership and associated groups
- Setting permissions with absolute and symbolic modes
Setting up a playground
I've been in IT for about 25 years, and most of that time was spent as a technical trainer. That means that the things that I write are usually structured as some sort of lab or other hands-on opportunity. It's just how I cover material. With that in mind, I'll assume you have a couple of identities and resources to experiment with as you read the rest of the article. You can use the following commands to set up a playground. It's best to do this on a virtual machine rather than your personal Linux box, but these tasks are relatively harmless.
Create two new users and two new groups to work with. Note that you do not need to configure passwords for the users in this exercise, as you won't log on with those accounts.
# useradd user01 # useradd user02 # groupadd groupA # groupadd groupB
Note: You would use the
passwd user01 command to set the user's password.
[ Want to test your sysadmin skills? Take a skills assessment today. ]
In your home directory, create a new directory named
# mkdir playground
Change into the
~/playground directory by using the
cd command. You are ready to work with the commands and concepts below.
When you've completed the article and learned the techniques I've covered, delete the two user accounts, the groups, and the playground directory. Use
rm -fR /playground,
userdel user01, and
groupdel groupA to remove the resources.
How do I create directories and files?
mkdir command to create directories. The
touch command is one of many ways to create files.
How do I create a directory named
# mkdir Resources
How do I create a directory path (a series of directories that don't yet exist)?
# mkdir -p Resources/data/2020data
Note: The goal here is to create the
2020data directory, but the given path's
data directory does not yet exist. The
-p option creates parent directories as needed to complete the path.
How do I create a file named
# touch file1
How do I create several files at once?
# touch file2 file3 file4
How do I manage ownership and groups?
In the playground directory, display the current owner and group associated with the
Resources directory and the files.
How do I display permission, owners, and groups?
# ls -l
ls -l command displays directory contents in long format. The long format contains both permissions and ownership. You can see that the user account that created the resources also owns those resources. The group association is also that user's primary group.
[ Free download: Advanced Linux commands cheat sheet. ]
How do I change the user/owner associated with
# chown user02 file1
How do I change the group associated with
# chown :groupA file1
How do I change the owner and group at the same time for
# chown user02:groupA file2
There is a specific
chgrp command, but I prefer only to memorize one command (
chown) and apply it to both functions (user and group associations) rather than
chown for the user and then have to recall
chgrp for the group.
So how do I use
# chgrp groupB file1
How do I change the user/group for a directory and all of its contents?
# chown -R user01:groupA Resources
The above task provides a recursive configuration. Technically, recursive commands are repeated on each specified object. Effectively, recursive means "this and everything in it." In the above example, you are configuring the related user/group for the
Resources directory and everything in it. Without the
-R option, you would only affect the
Resources directory itself, but not its contents.
[ Readers also liked: An introduction to Linux Access Control Lists (ACLs) ]
How do I manage permissions?
The change mode or
chmod command sets permissions. The syntax is straight-forward:
chmod permissions resource-name
Here are two examples of manipulating permissions for
# chmod 740 file2 # chmod u=rwx,g=r,o-rwx file2
But wait! Those appear to be radically different examples (they're not, actually). What are all those letters and numbers?
We need to discuss absolute mode and symbolic mode.
How do I use absolute mode?
Absolute mode is one of two ways of specifying permissions. I've seen this mode referred to as octal or numeric mode, but the term I learned was absolute. That term also makes the most sense to me because it's an absolute statement of the desired permissions. I always told my students that this seemed like the most complex of the two modes but is actually the simplest. Usually, they agreed.
Each access level (read, write, execute) has an octal value:
|Access level||Octal value|
Each identity (user, group, others) has a position:
|User||First or left-most|
|Others||Last or right-most|
The absolute mode syntax states the desired permissions from left to right.
How do I grant the user (owner) read, write, and execute, the group read-only, and all others no access to
file2 by using absolute mode?
# chmod 740 file2
The three permissions values are associated with identities:
- The 7 is assigned to the user and is the sum of 4+2+1 or read+write+execute (full access)
- The 4 is assigned to the group and is the sum of 4+0+0 (read-only)
- The 0 is assigned to others and is the sum of 0+0+0 (no access)
In this example, the user has rwx, the group has r only, and all others have no access to
Let's look at one more example.
How do I grant the user (owner) read and write, the group read-only, and all others read-only to
# chmod 644 file2
- The user has 6 (read and write)
- The group has 4 (read-only)
- All others have 4 (read-only)
I find this easier because there are no calculations involved. I'm not concerned with adding or subtracting specific permissions based on the current settings. Instead, I say, "set the permissions to be this," and that's the end result I get. It's an absolute statement.
[ Free cheat sheet: Get a list of Linux utilities and commands for managing servers and networks. ]
How do I set permissions for the
Resources directory and all of its contents by using absolute mode?
# chmod -R 744 Resources
How do I use symbolic mode?
Symbolic mode uses more symbols, but the symbols are simpler to understand. That's attractive to sysadmins that are new to standard Linux permissions.
Each access level has a symbol:
Each identity has a symbol:
There are also operators to manipulate the permissions:
|Grant a level of access||+|
|Remove a level of access||-|
|Set a level of access||=|
chmod command syntax is the same:
command permissions directory/file
Here is an example:
How do I remove the read permissions from others for
file2 by using symbolic mode?
# chmod o-r file2
This example removes (
-) the read (
r) permission from others (
Here's another simple example:
How do I grant the read and write permissions to the group for
# chmod g+rw file2
This one gives (
+) read and write (
rw) to the group (
How do I set permissions for a directory and all of its contents by using symbolic mode?
# chmod -R o=rwx,g+rw,o-rwx Resources
Special permissions and Access Control Lists
The above discussion covers standard Linux permissions—applying rwx to the user, group, and all others. Linux has far more flexibility, however. Special permissions permit users to run applications with other credentials, control the inheritance of group associations, and keep files from being changed accidentally. Check out this great article on special permissions.
Linux also has a way of enforcing different permissions for different users and groups. Access Control Lists (ACLs) permit sysadmins to define permissions for more than just one user and one group, which adds a great deal more flexibility to standard permissions. For example,
user01 can be granted rw- to
user02 can be granted r-- to
file1. Here is a great article on ACLs.
[ Free course: Red Hat Satellite Technical Overview. ]
Creating resources, managing users, and setting permissions are fundamental tasks for Linux users. My goal was to provide a quick and easy guide based on common questions or tasks that we must all accomplish regularly. If you're new to Linux, having a solid grasp of the eight commands discussed above will make your sysadmin life much easier.