chmod 666-rw-rw-rw-
chmod 666 gives everyone — owner, group, and public — read and write access but nobody execute access.
Owner
You, the file's creator
Group
Everyone in the file's group
Public
Everyone else on the system
Special permissions(advanced, rarely needed)
Common presets
On a file, 666 means any user on the system can view or overwrite its contents, but nobody can run it as a program. There's no meaningful protection left against modification by anyone with shell access.
666 isn't a meaningful directory permission: without an execute bit for anyone, the directory can't be entered or traversed by any account, including its owner. It's essentially never an intentional choice for a directory.
When to use chmod 666
Legitimate uses are rare — occasionally you'll see 666 on a shared scratch or lock file that multiple local processes and users need to both read and write. Even then, a dedicated group with 664 is usually the safer way to express the same intent.
Security notes
Anyone with shell access on the host can overwrite a 666 file's contents outright, so it should never sit on a shared or internet-facing system holding anything that matters. If you're using 666 to work around an upload or write failure, fix the process's owner or group instead of opening the file to everyone.