Unix file permissions and chmod explained in 5 minutes

In Unix-based systems, files and directories have access permissions.

If you open a terminal and run:

ls -l

You will get something like:

1. drwx------: file type and permissions. d indicates that it is a directory. The following nine characters represent the permissions for the file or directory. In this example, rwx------ indicates that the owner of the directory (aristot) has read, write, and execute permissions, while others have no permissions.

2. 3: number of links to the file or directory. In this case, the directory Applications has three links associated with it.

3. aristot: owner of the file or directory.

4. staff: group associated with the file or directory.

5. 96: file size in bytes. For directories, the size generally represents the amount of disk space used by the directory’s metadata.

6. Feb 15 10:20: date and time of the last modification to the file or directory.

7. Applications: name of the file or directory.

More about file permissions permissions:

• d: directory. Files don’t have this.
• r: read
• w: write
• x: execute

The reason there are 9 characters apart from d, is that each file/directory has permissions on three levels:

• user: file owner
• group: members of file’s group
• others: everyone else

Examples:

• rwx------: file owner can read, write and execute the file, and everyone else can do nothing.
• r-xr-x--x: file owner and group can read and execute. Everyone else can only execute.

Each permission is associated with a number: r: 4, w: 2, x: 1. This results in a unique sum for each permission set:

• rwx: 4+2+1 = 7
• rw-: 4+2 = 6
• r-x: 4+1 = 5
• -wx: 2+1 = 3

chmod is just a command-line tool that allows you to change permissions:

• chmod 755 /some/file sets the permissions to rwx for owner and r-x for group and others.
• chmod +x /some/file adds execute permission to all.
• chmod -w /some/file removes write permission from all.