Decoding Numeric Permissions in Linux: Understanding chmod’s Numeric Representation

When working with file permissions in Linux, the numeric representation used by the chmod command can seem cryptic at first. However, understanding how the numeric permissions work is essential for effectively managing file access rights. Let’s decode the numeric representation used by chmod:

1. Numeric Values:
   The numeric representation consists of a three-digit number, where each digit represents the permission set for a specific user category: owner, group, and others.
2. Permission Assignments:
   Each digit is composed of a combination of three permission types: read (r), write (w), and execute (x). These permissions are assigned specific numeric values:

• Read (r): Assigned a value of 4.
• Write (w): Assigned a value of 2.
• Execute (x): Assigned a value of 1.

1. Permission Combinations:
   To assign permissions using numeric representation, you add the values corresponding to the desired permissions. For example:

• Read and write permissions (r+w) are assigned a value of 4 (read) + 2 (write) = 6.
• Read and execute permissions (r+x) are assigned a value of 4 (read) + 1 (execute) = 5.
• Read, write, and execute permissions (r+w+x) are assigned a value of 4 (read) + 2 (write) + 1 (execute) = 7.

1. Assigning Numeric Permissions:
   When using chmod with the numeric representation, you specify the desired permission values for each category: owner, group, and others. For example:

• chmod 755 file.txt: Gives the owner read, write, and execute permissions (7), and read and execute permissions to the group (5) and others (5).
• chmod 644 file.txt: Gives the owner read and write permissions (6), and read-only permissions to the group (4) and others (4).

Remember that permissions are applied based on the user’s relationship to the file: owner, group, or others. By using the numeric representation, you can quickly assign permissions without relying on the symbolic representation (e.g., u, g, o) of chmod.

Understanding numeric permissions helps you precisely define access rights for various user categories, ensuring proper security and control over your files and directories.