Introduction to File Permissions

What are File Permissions?

File permissions are a fundamental concept in Unix-like operating systems that determine who can read, write, or execute a file. These permissions are crucial for the security and integrity of the system, ensuring that only authorized users can access or modify files.

Types of Permissions

There are three types of permissions for files and directories:

• Read (r): Permission to read the contents of the file or directory.
• Write (w): Permission to modify the contents of the file or directory.
• Execute (x): Permission to execute the file or traverse the directory.

Understanding Permission Structure

Permissions are displayed using a combination of letters and dashes. For example:

This example breaks down as follows:

• -: File type (dash indicates a regular file).
• rwx: Owner's permissions (read, write, execute).
• r-x: Group's permissions (read, execute).
• r--: Others' permissions (read).

Changing File Permissions with chmod

The chmod command is used to change file permissions. There are two ways to use chmod: symbolic mode and numeric mode.

Symbolic Mode

In symbolic mode, you use symbols to modify permissions. For example:

This command adds execute permission for the user (owner) of example.sh.

Numeric Mode

In numeric mode, you use numbers to represent the permissions. Each permission type has a value:

• Read (r): 4
• Write (w): 2
• Execute (x): 1

These values are summed to set the permissions. For example:

This command sets the permissions to rwxr-xr-x:

• 7 (rwx): Owner can read, write, and execute.
• 5 (r-x): Group can read and execute.
• 5 (r-x): Others can read and execute.

Viewing File Permissions with ls -l

To view the permissions of a file, you can use the ls -l command:

-rwxr-xr-x 1 user group 1234 Jan 1 12:34 example.sh

This output shows the permissions, number of links, owner, group, size, modification date, and filename.

Special Permissions

In addition to the standard read, write, and execute permissions, there are special permissions that provide additional security features:

• Setuid (s): When set on an executable file, it allows the file to be executed with the permissions of the file's owner.
• Setgid (s): When set on a directory, it allows files created within the directory to inherit the group of the directory.
• Sticky Bit (t): When set on a directory, it restricts file deletion within the directory to the file's owner.

Conclusion

Understanding file permissions is essential for managing the security and integrity of your files and directories. By mastering the use of commands like chmod and ls -l, and comprehending the meaning of different permission settings, you can effectively control access to your system's resources.