Linux current user permissions

How to manage Linux permissions for users, groups, and others

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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

Linux security

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.

Note: You would use the passwd user01 command to set the user’s password.

In your home directory, create a new directory named 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?

Use the mkdir command to create directories. The touch command is one of many ways to create files.

How do I create a directory named Resources ?

How do I create a directory path (a series of directories that don’t yet exist)?

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 file1 ?

How do I create several files at once?

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?

The 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.

How do I change the user/owner associated with file1 ?

How do I change the group associated with file1 ?

How do I change the owner and group at the same time for 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 ?

How do I change the user/group for a directory and all of its contents?

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.

How do I manage permissions?

The change mode or chmod command sets permissions. The syntax is straight-forward:

Here are two examples of manipulating permissions for 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
Read	4
Write	2
Execute	1

Each identity (user, group, others) has a position:

Identity	Position
User	First or left-most
Group	Middle
Others	Last or right-most

More Linux resources

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?

The three permissions values are associated with identities:
ugo
740

• 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 file2 .

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 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.

How do I set permissions for the Resources directory and all of its contents by using absolute mode?

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:

Access level	Symbol
Read	r
Write	w
Execute	x

Each identity has a symbol:

Identity	Symbol
User	u
Group	g
Others	o

There are also operators to manipulate the permissions:

Task	Operator
Grant a level of access	+
Remove a level of access	—
Set a level of access	=

The general chmod command syntax is the same:

Here is an example:

How do I remove the read permissions from others for file2 by using symbolic mode?

This example removes ( — ) the read ( r ) permission from others ( o ) for file2 .

Here’s another simple example:

How do I grant the read and write permissions to the group for file2 ?

This one gives ( + ) read and write ( rw ) to the group ( g ) for file2 .

How do I set permissions for a directory and all of its contents by using symbolic mode?

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 file1 , while user02 can be granted r— to file1 . Here is a great article on ACLs.

Wrap up

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.

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Linux current user permissions

Last updated on: 2019-03-07

Authored by: Jered Heeschen

This article explains how to use the ls command to check Linux® file permissions. Being able to check the permissions on a file is useful, especially for troubleshooting. You can ensure that a user can read a particular file, for example, or examine a directory structure to ensure that users can follow the hierarchy to the files that they need.

For a more in depth discussion on Linux file permissions, see Linux file permission concepts.

ls command

Use the ls command (the first letter is a lowercase L) to see what files are in a directory. When run by itself, ls returns a list of the current working directory. You can also specify a directory to list. The following example shows a list of the first few files in the /etc directory on a Gentoo system.

The -h option changes the way file sizes are displayed. When you use the -h option, files sizes are displayed in the human-readable format of kilobytes, megabytes, and so on, rather than in raw bytes. Other linux tools such as df also support this flag. The command df -h shows current disk usage in a easier to read format.

To display hidden files (files with names that start with a period), use the -a option. For example, if you use only ls to look at the root home directory on a clean Linux installation, no files are returned:

However, if you add the -a option, the ls command returns a list of files:

Files that start with a period are often system files and application settings files, and you usually don’t want them included in directory lists. But it’s important to know that they’re there and how to see them. The .bashrc file is especially useful to know about because it contains user environment settings that you can change.

If you combine the -a option with the -l option (see the next section) into -la , you get all the details of the hidden files:

Consider the single period and double period in both directory lists:

The single period (.) refers to the directory itself. This is convenient if you want it to run a command and reference your current directory (for example, when you want to copy a file there).

The double period (..) refers to the parent directory. If you type cd .. the directory changes to the one above the one you’re in, in the file system hierarchy. For example, if your current directory is /root , typing cd .. would take you to / , the very top of the hierarchy.

To get more information about the files in a directory, use the -l option with ls, as shown in the following example.

The file names are on the far right side of each line, and the file details precede the names. The necessary details to check file permissions are (1) the series of letters and dashes on the far left of each line, and (2) the two columns that have root in them (in the preceding example). The rest of this article explains how to interpret and use these details.

Permission details

This section explains the series of letters and dashes that define the file permissions.

The first character: file type

In the preceding examples, the first character in each list was either a dash (-) or the letter d .

A dash (-) indicates that the file is a regular file.

The letter d indicates that the file is a directory, which is basically a special kind of file.

A special file type that you might see is a symlink, sometimes called a soft link. It begins with a lowercase L , as shown in the following example:

A symlink is a pointer to another location in the file system.

Permissions abbreviations

Permissions for files are represented by the following letters.

• r refers to the read permission.
• w refers to the write permission.
• x refers to the execute permission.

The permissions characters

Consider the following example:

The first trio of letters after the file type in a file list ( rwx ) shows the permissions for the user , or file owner.

The next trio of characters (also rwx ) shows the permissions for the group category.

The last trio of characters ( r-x ) shows the permissions for the final category, other . In this example, users who are neither the file owner nor in the group have read and execute permissions but not write, as indicated by the dash (-) in the middle position.

Notice the specific order to the permissions in a trio: read, write, execute. A dash in place of a letter for a permission means that category doesn’t have that permission.

The first number

The number listed after the permissions indicates the link count of a file or the number of contained directory entries, for a directory. This number is not relevant for permissions.

Owner and group

After the number of links, two names are listed. In the preceding example, the names are root and mail .

The first name indicates the owner of the file. The user permissions apply to the owner of the file, so in this case, the user ‘root’ has read, write, and execute permissions for this directory.

The second name is the file’s group. The group permissions apply to any user in the same group as the file, so in this case, those permissions apply to anyone in the mail group.

The rest

The remainder of the file details are the size of the file, the date and time that the file was created or last modified, and the file name.

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