About Chmod Calculator

Convert between symbolic and numeric Unix file permissions. Part of the DevTools Surf developer suite. Browse more tools in the Generators collection.

Use Cases

• Calculating deployment script permissions for CI/CD pipelines
• Setting correct permissions for SSH key files (600 or 400)
• Converting symbolic permissions from ls -la output to numeric
• Configuring web server file permissions for uploads directories

Tips

• Toggle permission checkboxes to see the numeric value update
• Convert between symbolic notation like rwxr-xr-x and 755
• Verify directory permissions separately from file permissions

Fun Facts

• Unix file permissions were designed by Ken Thompson and Dennis Ritchie for the first version of Unix at Bell Labs in 1971.
• The sticky bit (1000) was originally used to keep programs in swap space for faster loading; today it prevents file deletion in shared directories.
• The SUID bit (4000) allows a program to run with the permissions of the file owner, which is how the passwd command can modify /etc/shadow.

FAQ

Symbolic vs numeric — which should I use?

Numeric (755, 644) is unambiguous and the common form in scripts. Symbolic (u+x, g-w) is safer for incremental changes — avoids accidentally clearing other bits.

What do 755, 644, 600 mean?

755: owner all, group+others read+execute (typical for directories and executables). 644: owner rw, others r (typical for data files). 600: owner rw, others nothing (private keys).

Does it show the effect visually?

Yes — an rwxrwxrwx grid lights up showing which bits are set. Much faster to verify than mentally decoding 3 octal digits.

What about setuid and sticky bits?

The 4-digit form (4755, 1777) covers these. Setuid (4), setgid (2), sticky (1). The tool handles both 3-digit and 4-digit input.