Linux File Permissions and the chmod Command

Overview of Permissions in Linux File System

• The chmod command is used to change permissions of files and directories in Linux.
• Understanding permissions is vital for file security and access control.

Viewing Permissions

• Use ls -lha to view detailed file listings
• -l: long listing format
• -h: human-readable file sizes
• -a: show all files, including hidden files
• Hidden files begin with a period (.)

Permission String Breakdown

• The permission string consists of four sections:

1. File Type:
   • -: signifies a normal file
   • d: signifies a directory
2. User Permissions: Three characters indicating the permissions for the file owner. (e.g., rwx)
3. Group Permissions: Three characters for the group that owns the file.
4. Other (World) Permissions: Three characters for all other users.

Permission Characters and Their Meanings

• r: read
• w: write
• x: execute
• A hyphen - indicates that a permission is not granted.

Example Permissions Structure

• For a permission string like -rwxr-xr--:
• rwx for the user (read, write, execute)
• r-x for the group (read, execute)
• r-- for others (read only)

Understanding Ownership

• Ownership of files: User (often root), Group.
• Example: root owns ansiblebootstrap.sh.

Changing Permissions with chmod

1. Basic Command Structure: chmod [options] [permissions] [filename]
2. Adding Permissions: Use +

• Example: chmod g+w ansiblebootstrap.sh (adds write permission for group)

1. Removing Permissions: Use -

• Example: chmod g-w ansiblebootstrap.sh (removes write permission for group)

1. Copying Permissions: Use = to set permissions equal to another category.

• Example: chmod g=u ansiblebootstrap.sh (group permissions set equal to user permissions)

1. Combining Commands: Multiple operations can be applied in one command.

• Example: chmod g-w,u-x,o-r ansiblebootstrap.sh

Numeric Representation of Permissions

• Permissions can also be set using a three-digit numerical system:
• Values:
  • r = 4
  • w = 2
  • x = 1
• Example: chmod 644 ansiblebootstrap.sh
• First digit for user: 6 (4+2) = rw-
• Second digit for group: 4 = r--
• Third digit for others: 4 = r--

File vs. Directory Permissions

• Files:

• r: View contents, read the file.

• w: Modify contents (write).

• x: Execute the file if it's a script or program.

• Directories:

• r: List contents of the directory.

• w: Add or remove files in the directory.

• x: Navigate into the directory (using cd). Without it, you get a "Permission denied" error.

Summary

• Understanding and modifying file permissions is essential for effective file system management in Linux.
• The chmod command, using both symbolic and numeric methods, allows for flexible permission settings, promoting security and access management.