A Beginner's Guide to File Permissions and Security

When you use chmod to change permissions on a file or folder, you’re deciding what three types of people can do with it:

• Owner (the person who made the file)

• Group (people in the same club as the owner)

• Others (everyone else)

Each of these three gets a set of “permissions”—read, write, and execute.

• Read (r) means they can look at what’s inside.

• Write (w) means they can change or delete it.

• Execute (x) means they can run it if it’s a program.

Permissions are shown with three letters, such as rwx or rw-. A dash means you do not have that permission.

The numbers in chmod are a shortcut:

• 4 means read (r)

• 2 means write (w)

• 1 means execute (x)

If you forget what the numbers mean, remember this:

• r is 4 (for Read)

• w is 2 (for Write)

• x is 1 (for eXecute)

You add the numbers up for each person:

• 7 (4+2+1) = read, write, and execute (rwx)

• 6 (4+2) = read and write (rw-)

• 5 (4+1) = read and execute (r-x)

• 4 (just 4) = read only (r--)

• 0 means no permission at all

For example, chmod 764 file means:

• The owner gets 7—all permissions: read, write, and execute.

• The group gets 6—read and write.

• Others get 4—only read.