Chmod Calculator

755 grants the owner read, write and execute (7 = 4+2+1), and everyone else read and execute (5 = 4+1). In symbolic form: rwxr-xr-x. It's the standard permission for executables and directories that others may enter but not modify — which is why web servers expect 755 on directories and 644 on files.

Each digit is a sum of read=4, write=2, execute=1, and the three digits apply to the file's owner, its group, and everyone else, in that order. So 640 = owner rw (4+2), group r (4), others nothing (0). Once you internalise 4-2-1, you can read any mode at a glance.

777 lets every user on the system modify and execute the file — any compromised process or account can replace your script with a malicious one, and web servers will happily run it. It's usually applied out of frustration when something "doesn't work"; the correct fix is almost always changing the owner (chown) or group instead of opening the file to the world. Practically no production scenario needs 777.

On directories the bits change meaning: read lists the filenames, execute lets you enter the directory and access files inside (traverse), and write lets you create, rename and delete entries. A directory with execute but not read acts as a pass-through: you can open files you know the names of, but can't list what's there. That's why directories usually need 755 rather than 644.

They're a fourth, leading octal digit (setuid=4, setgid=2, sticky=1). Setuid makes an executable run with the file owner's privileges (how passwd edits a root-owned file). Setgid on a directory makes new files inherit the directory's group — the standard trick for shared project folders. The sticky bit on a world-writable directory (like /tmp, mode 1777) means only a file's owner can delete it.

Numeric mode (chmod 644 file) sets all bits at once. Symbolic mode (chmod u+x file, chmod go-w file) adjusts specific bits relative to what's already there: u/g/o/a selects owner/group/others/all, +/-/= adds, removes or sets exactly. Symbolic is safer for surgical changes (adding execute without touching anything else), numeric for asserting a known-good state.

No — it trades one visible error for a silent security hole. Diagnose instead: ls -l shows who owns the file, id shows who you are; the usual fixes are chown -R to the service user, adding your user to the right group, or 755/644 with correct ownership. If a vendor's install guide says chmod 777, treat it as a red flag about the vendor.