Chmod Calculator

Each digit is the sum of read (4), write (2) and execute (1) for one class of user. 755 gives the owner 7 (read, write, execute) and the group and everyone else 5 (read, execute). In symbolic form it is rwxr-xr-x. It is the usual mode for scripts, programs and directories that other users need to open but not change.

Only the execute bit. 644 (rw-r--r--) lets the owner edit and everyone read, which suits documents, images, HTML and config files. 755 adds execute for all three classes, which a file needs to run as a program and a directory needs before anyone can enter it or reach the files inside.

Rarely. 777 lets every account on the machine, including a compromised web server process, rewrite or replace the file. When something only works at 777, the real fix is usually to change the file's owner or group with chown so the right user gets access, then use 755 or 775 for directories and 644 or 664 for files.

They are the optional fourth (leading) digit. Setuid (4) makes an executable run as its owner, which is how passwd can update a root-owned file. Setgid (2) on a directory makes new files inherit the directory's group, handy for shared project folders. The sticky bit (1) on a directory, as on /tmp with mode 1777, lets users delete only their own files. A capital S or T in the symbolic string means the special bit is set without the underlying execute bit.

On a directory, read lets you list the names inside, but execute is what lets you enter it and open the files within. A directory set to 644 shows names in a listing but refuses access to every file in it, which is why folders are normally 755 or 700.