Linux File Permissions: What chmod 644, 755, and 777 Mean and How to Use Them

We’re always setting permissions on folders and files in Linux with chmod 777. Those 777 digits are the permissions — just three numbers expressing three kinds of permission for three kinds of role. Let’s break them down one by one.

Binary Bits

Those three digits need to be taken apart: each digit represents three permissions for one role. Take 7 as our example.

Linux is cleverly designed — it uses three binary bits to mark the three permissions. Here’s the binary view:

• 000: 0 in decimal — no permissions at all
• 001: 1 in decimal — executable
• 010: 2 in decimal — writable
• 100: 4 in decimal — readable

That gives us four basic permissions:

• 0: no permission
• 1: execute
• 2: write
• 4: read

Which then combine freely into these decimal numbers:

• 1+2=3: execute, write
• 1+4=5: execute, read
• 2+4=6: write, read
• 1+2+4=7: execute, write, read

As you can see, 7 covers read, write, and execute — the maximum permission value.

Permissions by Role

There are three digits when setting permissions. Take 750 as an example; left to right:

• 7: permissions for the file owner (User)
• 5: permissions for same-group users (Group)
• 0: permissions for other users (Other)

So 750 means: the owner has full permissions; group users have execute and read; everyone else has nothing.

Letter Notation

Sometimes you’ll see rwx instead of numbers. Same thing; here’s the mapping:

• - = 0, no permission
• x = 1, execute
• w = 2, write
• r = 4, read

Combining per the above:

• w-x = 3, execute and write
• -rx = 5, execute and read
• wr- = 6, write and read
• wrx = 7, execute, write, and read

From that we can write 750 in letters as wrx-rx----. Simple, right?

The chmod Command

Instead of chmod 771 we can also use letters. Following the logic above, 771 breaks down as: user read-write-execute, group read-write-execute, other execute. So you can use:

chmod ug=rwx,o=x file

u means user, g means group, and o means other.