Question 1

How do I convert chmod 755 to symbolic notation?

Accepted Answer

Each digit of 755 maps to a class — 7 = owner, 5 = group, 5 = others. Each digit is the sum of read (4), write (2) and execute (1). 7 = 4+2+1 = rwx, 5 = 4+1 = r-x. So 755 becomes rwxr-xr-x: owner can read/write/execute, group and others can read and execute but not write. This is the most common permission set for executable files and directories (files get 644: rw-r--r--). This tool converts both directions instantly and shows the bit breakdown for each class.

Question 2

What is the difference between chmod 644 and 755?

Accepted Answer

644 (rw-r--r--) gives the owner read/write and everyone else read-only. It is the standard for regular files — source code, configs, documents. 755 (rwxr-xr-x) adds execute for all classes and is the standard for executables and directories, which need the execute (search) bit to be traversable. The practical rule: files 644, executables 755, directories 755. Using 777 (rwxrwxrwx) on files is a security smell — anyone can modify them — and many CI/CD scanners flag it.

Question 3

What are setuid, setgid and sticky bits?

Accepted Answer

The special bits appear as a fourth digit (4000, 2000, 1000) or as s/S/t in symbolic notation. Setuid (4): a program runs with the file owner's privileges — /usr/bin/passwd uses it so any user can change their password. Setgid (2): a program runs with the group's privileges, and on directories, new files inherit the directory's group instead of the creator's. Sticky (1): on directories, only the owner can delete their own files even if the directory is world-writable — /tmp is the classic example. The uppercase S/T means the setuid/setgid/sticky bit is set but execute is not, which is almost always a mistake.

Question 4

How do I use chmod with symbolic notation?

Accepted Answer

Symbolic chmod has three parts: who (u=user, g=group, o=others, a=all), operator (+ add, - remove, = set exactly), and permissions (r, w, x, X, s, t). Examples: chmod +x script.sh adds execute for all classes; chmod u=rwx,g=rx,o=rx file sets exactly rwxr-xr-x; chmod -R u+rwX dir adds read/write for the owner recursively and execute only where a bit of execute already exists (the capital X). This calculator accepts symbolic input and converts it to the numeric equivalent.

Numeric	Symbolic	Use case
644	rw-r--r--	Regular files (source, configs, docs)
755	rwxr-xr-x	Executables & directories (web roots)
600	rw-------	Private keys, secrets, .env
700	rwx------	Private directories
664	rw-rw-r--	Team-shared files (group writable)
775	rwxrwxr-x	Team-shared directories
rwsr-xr-x	rwsr-xr-x	Setuid executables (passwd, sudo)
rwxrwxrwt	rwxrwxrwt	World-writable + sticky (/tmp)

Chmod Calculator

🟢📖 Common Permission Presets

🟢📖 The Bit Math