Linux file permissions.

In Ubuntu (and other Linux systems), file permissions are an essential part of managing file security and access. Permissions define who can read, write, and execute a file or directory. Understanding these permissions is crucial for system administration, security, and software development.

File Permission Overview

Each file or directory in Ubuntu has three types of permissions:

1. Read (r): Allows viewing or reading the contents of a file. For a directory, it means listing its contents.
2. Write (w): Allows modifying or editing the contents of a file. For a directory, it allows adding or deleting files within it.
3. Execute (x): Allows running or executing a file (e.g., shell scripts or binary files). For a directory, it allows traversing into it (changing into that directory).

Permission Levels: User, Group, and Others

There are three categories of users for each file and directory:

1. User (u): The owner of the file. This is usually the person who created the file.
2. Group (g): A group of users who share the same permissions.
3. Others (o): All other users who are not the owner or part of the group.

Permissions for each category are represented in the following format:

Example: -rwxr-xr--

This example shows typical permissions of a file in a ls -l listing:

-rwxr-xr--

Breaking Down the Example

• -: Indicates the file type. - is a regular file, d is a directory, and l is a symbolic link.
• rwx: User permissions (owner). The user has read (r), write (w), and execute (x) permissions.
• r-x: Group permissions. The group has read (r) and execute (x), but not write (-) permission.
• r--: Other users’ permissions. Other users have read-only (r) permission.

Changing Permissions: The chmod Command

You can change file permissions using the chmod command. There are two ways to use chmod:

1. Symbolic Mode: Uses letters to represent permissions (r, w, x).
2. Numeric Mode: Uses numbers (e.g., 755, 644) to set permissions.

Symbolic Mode: Examples

• chmod u+x file.txt: Adds execute (x) permission for the user.
• chmod g-w file.txt: Removes write (w) permission for the group.
• chmod o+r file.txt: Adds read (r) permission for others.
• chmod u=rwx, g=rx, o=r file.txt: Sets rwx for user, rx for group, and r for others.

Numeric Mode: Examples

In numeric mode, each permission has a corresponding value:

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

The sum of these values determines the permissions. For example:

• rwx = 4 + 2 + 1 = 7
• rw- = 4 + 2 + 0 = 6
• r-- = 4 + 0 + 0 = 4

Setting Permissions Using Numbers

When using numbers, you provide three digits, where each digit represents a category:

1. First Digit: Permissions for the user (owner).
2. Second Digit: Permissions for the group.
3. Third Digit: Permissions for others.

Example:

• chmod 755 file.txt: This sets permissions as follows:

  • 7 (User): rwx
  • 5 (Group): r-x
  • 5 (Others): r-x

Thus, rwxr-xr-x.

• chmod 644 file.txt: This sets permissions as:

  • 6 (User): rw-
  • 4 (Group): r--
  • 4 (Others): r--

Thus, rw-r--r--.

Ownership: The chown Command

Besides permissions, each file has an owner and a group. You can change the owner or group using the chown command:

• chown user:group file.txt: Changes the owner and group of the file.
• chown user file.txt: Changes only the owner.
• chown :group file.txt: Changes only the group.

Example:

• chown alice:developers script.sh changes the owner to alice and the group to developers.

File Types in Permission Listings

When you list files with ls -l, you’ll see the file type indicated as the first character:

• -: Regular file
• d: Directory
• l: Symbolic link
• c: Character device file (e.g., terminals, serial ports)
• b: Block device file (e.g., disk drives)

Example Output of ls -l:

drwxr-xr-x  2 alice developers 4096 Oct  3 10:45  directory_name
-rw-r--r--  1 bob   users      1024 Oct  3 10:30  file_name.txt

In this example:

1. Directory (d): directory_name has drwxr-xr-x permissions.
2. Regular file (-): file_name.txt has rw-r--r-- permissions.

Practical Permission Management Tips

1. Setting Permissions for Scripts: Use chmod +x script.sh to make shell scripts executable.
2. Limiting Access: Set sensitive files to chmod 600 file.txt so only the owner can read/write.
3. Directory Permissions: Use chmod 755 for directories you want others to traverse but not modify.

Summary of Common chmod Values

• 777: rwxrwxrwx – Everyone can read, write, and execute (dangerous for sensitive files).
• 755: rwxr-xr-x – User can read, write, execute; others can only read and execute.
• 644: rw-r--r-- – User can read, write; others can only read.