ChmodTest — Free Linux Permissions Calculator & Umask Tester

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Permission Breakdown

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Guides

umask Explained chmod Special Bits and Directories

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Frequently Asked Questions

Is my data safe on chmodtest.com?

Yes. The permission calculator runs entirely in your browser. Nothing you type is sent to a server, stored, or logged.

How do I convert octal 755 to rwxr-xr-x?

Each octal digit encodes one set of permissions: 4 for read, 2 for write, 1 for execute. 7 = 4+2+1 gives rwx for the owner, and 5 = 4+1 gives r-x for group and others. Enter 755 in the calculator to see all three classes expanded.

What umask gives me 644 files and 755 directories?

A umask of 022. Directories start at 777 and regular files at 666, and the umask removes permissions: 777 minus 022 is 755 for directories, 666 minus 022 is 644 for files. Use the umask calculator page to work it out, or to find the umask for a mode you want.

🧭 Pro Tips & Gotchas

Reading the modes

Three octal digits, three rwx slots each: owner, group, other. r=4, w=2, x=1, summed per slot — so 755 is rwxr-xr-x.
The tool decomposes the octal into per-class symbol strings, which is the fastest way to spot the accidental 777.

Permission gotchas

Four-digit modes carry the special bits: 4xxx setuid, 2xxx setgid, 1xxx sticky. chmod 755 on a file that needed 4755 silently drops the setuid bit — and chmod without the leading digit is the usual reason a SUID binary stops working after maintenance.
x on a directory means "traverse/enter", not "execute". A directory at 644 cannot be cd'd into even though its listing looks readable; conversely --x (711) hides listings while allowing access, which is exactly what you want for shared home directories.
Web servers want 755 dirs and 644 files. 777 is the reflex fix that opens a write-anywhere hole; the real cause is usually ownership, not mode — fix it with chown -R instead.
SSH refuses keys and configs that are group/world-writable: 600 for private keys, 700 for ~/.ssh, 644 for authorized_keys. The error message names the wrong file surprisingly often.
umask is applied at create time and is inverted: umask 022 yields 644 files, umask 077 yields 600. It never affects chmod, which is why "I chmod 644 and it came out 600" means something else rewrote the file.
ACLs (setfacl) and SELinux contexts override POSIX modes entirely — ls -l will show 755 while the effective access is nothing. Check getfacl/ls -Z before concluding a mode is wrong.