Read write execute permission linux

Unix / Linux — File Permission / Access Modes

In this chapter, we will discuss in detail about file permission and access modes in Unix. File ownership is an important component of Unix that provides a secure method for storing files. Every file in Unix has the following attributes −

Owner permissions − The owner’s permissions determine what actions the owner of the file can perform on the file.

Group permissions − The group’s permissions determine what actions a user, who is a member of the group that a file belongs to, can perform on the file.

Other (world) permissions − The permissions for others indicate what action all other users can perform on the file.

The Permission Indicators

While using ls -l command, it displays various information related to file permission as follows −

Here, the first column represents different access modes, i.e., the permission associated with a file or a directory.

The permissions are broken into groups of threes, and each position in the group denotes a specific permission, in this order: read (r), write (w), execute (x) −

The first three characters (2-4) represent the permissions for the file’s owner. For example, -rwxr-xr— represents that the owner has read (r), write (w) and execute (x) permission.

The second group of three characters (5-7) consists of the permissions for the group to which the file belongs. For example, -rwxr-xr— represents that the group has read (r) and execute (x) permission, but no write permission.

The last group of three characters (8-10) represents the permissions for everyone else. For example, -rwxr-xr— represents that there is read (r) only permission.

File Access Modes

The permissions of a file are the first line of defense in the security of a Unix system. The basic building blocks of Unix permissions are the read, write, and execute permissions, which have been described below −

Grants the capability to read, i.e., view the contents of the file.

Write

Grants the capability to modify, or remove the content of the file.

Execute

User with execute permissions can run a file as a program.

Directory Access Modes

Directory access modes are listed and organized in the same manner as any other file. There are a few differences that need to be mentioned −

Access to a directory means that the user can read the contents. The user can look at the filenames inside the directory.

Write

Access means that the user can add or delete files from the directory.

Execute

Executing a directory doesn’t really make sense, so think of this as a traverse permission.

A user must have execute access to the bin directory in order to execute the ls or the cd command.

Changing Permissions

To change the file or the directory permissions, you use the chmod (change mode) command. There are two ways to use chmod — the symbolic mode and the absolute mode.

Using chmod in Symbolic Mode

The easiest way for a beginner to modify file or directory permissions is to use the symbolic mode. With symbolic permissions you can add, delete, or specify the permission set you want by using the operators in the following table.

Adds the designated permission(s) to a file or directory.

Removes the designated permission(s) from a file or directory.

Sets the designated permission(s).

Here’s an example using testfile. Running ls -1 on the testfile shows that the file’s permissions are as follows −

Then each example chmod command from the preceding table is run on the testfile, followed by ls –l, so you can see the permission changes −

Here’s how you can combine these commands on a single line −

Using chmod with Absolute Permissions

The second way to modify permissions with the chmod command is to use a number to specify each set of permissions for the file.

Each permission is assigned a value, as the following table shows, and the total of each set of permissions provides a number for that set.

Sr.No.	Chmod operator & Description
1

Number	Octal Permission Representation	Ref
0	No permission	—
1	Execute permission	—x
2	Write permission	-w-
3	Execute and write permission: 1 (execute) + 2 (write) = 3	-wx
4	Read permission	r—
5	Read and execute permission: 4 (read) + 1 (execute) = 5	r-x
6	Read and write permission: 4 (read) + 2 (write) = 6	rw-
7	All permissions: 4 (read) + 2 (write) + 1 (execute) = 7	rwx

Here’s an example using the testfile. Running ls -1 on the testfile shows that the file’s permissions are as follows −

Then each example chmod command from the preceding table is run on the testfile, followed by ls –l, so you can see the permission changes −

Changing Owners and Groups

While creating an account on Unix, it assigns a owner ID and a group ID to each user. All the permissions mentioned above are also assigned based on the Owner and the Groups.

Two commands are available to change the owner and the group of files −

chown − The chown command stands for «change owner» and is used to change the owner of a file.

chgrp − The chgrp command stands for «change group» and is used to change the group of a file.

Changing Ownership

The chown command changes the ownership of a file. The basic syntax is as follows −

The value of the user can be either the name of a user on the system or the user id (uid) of a user on the system.

The following example will help you understand the concept −

Changes the owner of the given file to the user amrood.

NOTE − The super user, root, has the unrestricted capability to change the ownership of any file but normal users can change the ownership of only those files that they own.

Changing Group Ownership

The chgrp command changes the group ownership of a file. The basic syntax is as follows −

The value of group can be the name of a group on the system or the group ID (GID) of a group on the system.

Following example helps you understand the concept −

Changes the group of the given file to special group.

SUID and SGID File Permission

Often when a command is executed, it will have to be executed with special privileges in order to accomplish its task.

As an example, when you change your password with the passwd command, your new password is stored in the file /etc/shadow.

As a regular user, you do not have read or write access to this file for security reasons, but when you change your password, you need to have the write permission to this file. This means that the passwd program has to give you additional permissions so that you can write to the file /etc/shadow.

Additional permissions are given to programs via a mechanism known as the Set User ID (SUID) and Set Group ID (SGID) bits.

When you execute a program that has the SUID bit enabled, you inherit the permissions of that program’s owner. Programs that do not have the SUID bit set are run with the permissions of the user who started the program.

This is the case with SGID as well. Normally, programs execute with your group permissions, but instead your group will be changed just for this program to the group owner of the program.

The SUID and SGID bits will appear as the letter «s» if the permission is available. The SUID «s» bit will be located in the permission bits where the owners’ execute permission normally resides.

For example, the command −

Shows that the SUID bit is set and that the command is owned by the root. A capital letter S in the execute position instead of a lowercase s indicates that the execute bit is not set.

If the sticky bit is enabled on the directory, files can only be removed if you are one of the following users −

• The owner of the sticky directory
• The owner of the file being removed
• The super user, root

To set the SUID and SGID bits for any directory try the following command −

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Understanding Linux File Permissions

Although there are already a lot of good security features built into Linux-based systems, one very important potential vulnerability can exist when local access is granted – – that is file permission based issues resulting from a user not assigning the correct permissions to files and directories. So based upon the need for proper permissions, I will go over the ways to assign permissions and show you some examples where modification may be necessary.

Basic File Permissions

Permission Groups

Each file and directory has three user based permission groups:

• owner – The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users.
• group – The Group permissions apply only to the group that has been assigned to the file or directory, they will not effect the actions of other users.
• all users – The All Users permissions apply to all other users on the system, this is the permission group that you want to watch the most.

Permission Types

Each file or directory has three basic permission types:

• read – The Read permission refers to a user’s capability to read the contents of the file.
• write – The Write permissions refer to a user’s capability to write or modify a file or directory.
• execute – The Execute permission affects a user’s capability to execute a file or view the contents of a directory.

Viewing the Permissions

You can view the permissions by checking the file or directory permissions in your favorite GUI File Manager (which I will not cover here) or by reviewing the output of the “ls -l” command while in the terminal and while working in the directory which contains the file or folder.

The permission in the command line is displayed as: _rwxrwxrwx 1 owner:group

1. User rights/Permissions
   1. The first character that I marked with an underscore is the special permission flag that can vary.
   2. The following set of three characters (rwx) is for the owner permissions.
   3. The second set of three characters (rwx) is for the Group permissions.
   4. The third set of three characters (rwx) is for the All Users permissions.
2. Following that grouping since the integer/number displays the number of hardlinks to the file.
3. The last piece is the Owner and Group assignment formatted as Owner:Group.

Modifying the Permissions

When in the command line, the permissions are edited by using the command chmod. You can assign the permissions explicitly or by using a binary reference as described below.

Explicitly Defining Permissions

To explicity define permissions you will need to reference the Permission Group and Permission Types.

The Permission Groups used are:

The potential Assignment Operators are + (plus) and – (minus); these are used to tell the system whether to add or remove the specific permissions.

The Permission Types that are used are:

• r – Read
• w – Write
• x – Execute

So for an example, lets say I have a file named file1 that currently has the permissions set to _rw_rw_rw, which means that the owner, group and all users have read and write permission. Now we want to remove the read and write permissions from the all users group.

To make this modification you would invoke the command: chmod a-rw file1
To add the permissions above you would invoke the command: chmod a+rw file1

As you can see, if you want to grant those permissions you would change the minus character to a plus to add those permissions.

Using Binary References to Set permissions

Now that you understand the permissions groups and types this one should feel natural. To set the permission using binary references you must first understand that the input is done by entering three integers/numbers.

A sample permission string would be chmod 640 file1, which means that the owner has read and write permissions, the group has read permissions, and all other user have no rights to the file.

The first number represents the Owner permission; the second represents the Group permissions; and the last number represents the permissions for all other users. The numbers are a binary representation of the rwx string.

You add the numbers to get the integer/number representing the permissions you wish to set. You will need to include the binary permissions for each of the three permission groups.

So to set a file to permissions on file1 to read _rwxr_____, you would enter chmod 740 file1.

Owners and Groups

I have made several references to Owners and Groups above, but have not yet told you how to assign or change the Owner and Group assigned to a file or directory.

You use the chown command to change owner and group assignments, the syntax is simplechown owner:group filename, so to change the owner of file1 to user1 and the group to family you would enter chown user1:family file1.

Advanced Permissions

The special permissions flag can be marked with any of the following:

• _ – no special permissions
• d – directory
• l– The file or directory is a symbolic link
• s – This indicated the setuid/setgid permissions. This is not set displayed in the special permission part of the permissions display, but is represented as a s in the read portion of the owner or group permissions.
• t – This indicates the sticky bit permissions. This is not set displayed in the special permission part of the permissions display, but is represented as a t in the executable portion of the all users permissions

Setuid/Setgid Special Permissions

The setuid/setguid permissions are used to tell the system to run an executable as the owner with the owner’s permissions.

Be careful using setuid/setgid bits in permissions. If you incorrectly assign permissions to a file owned by root with the setuid/setgid bit set, then you can open your system to intrusion.

You can only assign the setuid/setgid bit by explicitly defining permissions. The character for the setuid/setguid bit is s.

So do set the setuid/setguid bit on file2.sh you would issue the command chmod g+s file2.sh.

Sticky Bit Special Permissions

The sticky bit can be very useful in shared environment because when it has been assigned to the permissions on a directory it sets it so only file owner can rename or delete the said file.

You can only assign the sticky bit by explicitly defining permissions. The character for the sticky bit is t.

To set the sticky bit on a directory named dir1 you would issue the command chmod +t dir1.

To some users of Mac- or Windows-based computers you don’t think about permissions, but those environments don’t focus so aggressively on user based rights on files unless you are in a corporate environment. But now you are running a Linux-based system and permission based security is simplified and can be easily used to restrict access as you please.

So I will show you some documents and folders that you want to focus on and show you how the optimal permissions should be set.

• home directories– The users’ home directories are important because you do not want other users to be able to view and modify the files in another user’s documents of desktop. To remedy this you will want the directory to have the drwx______ (700) permissions, so lets say we want to enforce the correct permissions on the user user1’s home directory that can be done by issuing the command chmod 700 /home/user1.
• bootloader configuration files– If you decide to implement password to boot specific operating systems then you will want to remove read and write permissions from the configuration file from all users but root. To do you can change the permissions of the file to 700.
• system and daemon configuration files– It is very important to restrict rights to system and daemon configuration files to restrict users from editing the contents, it may not be advisable to restrict read permissions, but restricting write permissions is a must. In these cases it may be best to modify the rights to 644.
• firewall scripts – It may not always be necessary to block all users from reading the firewall file, but it is advisable to restrict the users from writing to the file. In this case the firewall script is run by the root user automatically on boot, so all other users need no rights, so you can assign the 700 permissions.

Other examples can be given, but this article is already very lengthy, so if you want to share other examples of needed restrictions please do so in the comments.

If you have anything to add or want to make a comment or correction please do so in the comments. I look forward to your feedback and wish you the best in your future with Linux-based systems.

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