Three digits, three permission sets, one symbolic string
Owner 7, group 5, other 5 produces octal 755 and symbolic rwxr-xr-x -- the owner gets full read, write, and execute (7 = 4+2+1), while group and other each get read and execute but not write (5 = 4+1).
Each octal digit is a sum of three bit values: 4 for read, 2 for write, 1 for execute. That's why 7 (4+2+1) means all three permissions, 5 (4+1) means read and execute without write, and 6 (4+2) would mean read and write without execute -- every possible combination from 0 to 7 maps to a unique set of the three permission bits.
755 is one of the most common permission sets for executable scripts and directories: the owner can modify the file, while everyone else can read and execute (or, for a directory, list and traverse) it but not modify it -- a sensible default for shared, non-writable content.
What this doesn't cover
Three-digit ugo model. Special bits (setuid, setgid, sticky) are omitted. Beyond the standard owner/group/other three digits, Unix permissions support special bits (setuid, setgid, and the sticky bit), expressed as an optional fourth leading digit -- these enable more advanced behaviors like running a program with the file owner's privileges, and aren't part of this calculator's standard three-digit model.
This calculates the permission bits themselves -- it doesn't account for how those permissions interact with the file's actual owner and group assignments, umask settings at file-creation time, or ACLs (access control lists), which can add finer-grained permissions beyond the basic owner/group/other model.
The same octal-to-symbolic logic applies identically whether the target is a file or a directory, though the practical effect of each permission differs slightly -- execute on a directory means the ability to traverse into it and access its contents by name, not literally 'run' the directory.
Related developer reference tools
For quickly finding the right git command for a common task (a similarly structured cheat-sheet lookup), the Git Command Cheatsheet Builder covers a different but similarly frequent developer reference need.
Frequently Asked Questions (FAQ)
How does the octal digit 7 translate to 'rwx'?
Each permission is a bit value: read = 4, write = 2, execute = 1. Summing all three (4+2+1 = 7) means all three permissions are granted, which the symbolic notation shows as rwx.
Why does 5 mean read and execute but not write?
5 is the sum of read (4) and execute (1), with write (2) not included in the sum -- there's no way to reach 5 by including the write bit, so a digit of 5 always means read and execute without write.
What are the special bits this calculator doesn't cover?
Three-digit ugo model. Special bits (setuid, setgid, sticky) are omitted. Setuid and setgid let a program run with the file's owner or group privileges rather than the invoking user's, and the sticky bit (commonly used on shared directories like /tmp) restricts file deletion to the file's owner. These are expressed as an optional fourth digit, not part of this standard three-digit model.
Does execute permission mean the same thing for a file and a directory?
Not exactly. On a file, execute permission allows running it as a program or script. On a directory, execute permission allows traversing into it and accessing its contents by name -- a related but distinct meaning of the same permission bit.
Why is 755 such a common permission setting?
It gives the owner full control (read, write, execute) while letting everyone else read and execute (or traverse, for directories) without being able to modify the content -- a sensible default for shared scripts, executables, and directories that shouldn't be altered by other users.
