What Do Permissions 755 and 644 Mean on Linux
How the numbers are calculated and when to use each combination.
If you already know the basic concepts of permissions, these numbers are the shorthand way of expressing exactly the same thing.
How each number is calculated
| Value | Means |
|---|---|
| 4 | Read (r) |
| 2 | Write (w) |
| 1 | Execute (x) |
They're added up for each level: 4+2+1=7 (everything allowed), 4+2=6 (read and write, no execute), 4=read only.
755: the typical one for programs and folders
7 (owner: everything) 5 (group: read and execute, 4+1) 5 (others: same as group). The owner controls everything; everyone else can use it but not modify it.
644: the typical one for regular documents
6 (owner: read and write, 4+2) 4 (group: read only) 4 (others: read only). Nobody but the owner can modify the file.
Applying it
chmod 755 script.sh
chmod 644 document.txt
Warning: avoid using chmod 777
(everything allowed for everyone) unless you know exactly why you need
it — it gives full permission to any user on the system, something
rarely needed and a potential real security risk.
Frequently asked questions
Why is 755 common for scripts but 644 for documents?
755 includes execute permission (needed for a script to work as a program), while 644 only allows reading and writing, suitable for documents that don't need to be executed.
Is it safe to use chmod 777?
It generally isn't recommended — it gives full permission to any user on the system over that file, which is rarely needed and can pose a security risk.