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File-Permission.md

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File Permissions in Linux/Unix

File permissions define who can read, write, or execute a file. In Linux/Unix-like systems, file permissions are set for three different groups of users: the owner of the file, the group associated with the file, and others.

Types of Permissions

  • Read (r): Allows the file to be opened and read.
  • Write (w): Allows modifying or deleting the file.
  • Execute (x): Allows executing the file as a program.

Categories of Users

  • User (u): The owner of the file.
  • Group (g): A set of users who share the same group.
  • Others (o): All other users on the system.

File Permission Format

File permissions are often displayed as a string of characters such as: --rwxr-xr--

This string consists of 10 characters, with the following breakdown:

  • The first character represents the file type:

    • - for a regular file
    • d for a directory
    • l for a symbolic link
    • b Block device (e.g., hard drives, USB drives)
    • c Character device (e.g., keyboards, mice)

  • The remaining 9 characters represent the permissions for user (owner), group, and others in the following order:

    • User (Owner): The first three characters
    • Group: The next three characters
    • Others: The last three characters

For example, rwxr-xr-- means: - User: rwx (read, write, and execute permissions) - Group: r-x (read and execute permissions) - Others: r-- (read permission)

Numeric Representation of Permissions

Permissions can also be represented by numeric values:

Permission Numeric Value
Read (r) 4
Write (w) 2
Execute (x) 1

Each permission type can be assigned a numeric value, and the sum of those values determines the permission setting. For example:

  • rwx = 4 + 2 + 1 = 7
  • r-x = 4 + 0 + 1 = 5
  • r-- = 4 + 0 + 0 = 4

Thus, the permission rwxr-xr-- corresponds to 755 in numeric form.

Changing File Permissions

You can use the chmod command to change file permissions.

Example Commands:

  • Add execute permission for user:
- Set permissions to `rwxr-xr--` (755):

- Set permission `rw-r--r--` (644):

- Set permission `rwx------` (700) (Only the owner can read, write, and execute):

- Set permission `rwxrwxrwx` (777) (Everyone has full access):

  • Symbolic: chmod u+x file.txt (adds execute permission to the user).

  • Numeric: chmod 755 file.txt (sets permissions to rwxr-xr--).

1.Checking File Permissions

To check file permissions, use the ls -l command:

ls -l /path/to/file

Example output:

drwxr-xr-x  2 user user  4096 Feb 9 10:30 my_folder
-rw-r--r--  1 user user  1024 Feb 9 10:30 file.txt
lrwxrwxrwx  1 user user    10 Feb 9 10:30 link -> file.txt

2.Changing File Permissions

Use the chmod command to modify file permissions:

Symbolic Method

chmod u+x file.txt   # Add execute permission for user
chmod g-w file.txt   # Remove write permission for group
chmod o+r file.txt   # Add read permission for others
chmod a+x file.txt   # Add execute permission for everyone

Numeric Method

chmod 644 file.txt   # rw-r--r--
chmod 755 script.sh  # rwxr-xr-x
chmod 777 public.sh  # rwxrwxrwx (full permissions)

3.Changing File Ownership

Use the chown command to change the owner and group of a file:

chown username file.txt        # Change owner
chown username:groupname file.txt  # Change owner and group

4. Default File Permissions (umask)

To check the current default permissions:

umask
To set a new default permission (e.g., 022 for 755 permissions):
umask 022