chmod 755: the standard mode for directories and executables

What chmod 755 means

In 755 the first digit is the owner, the second the group and the third everyone else: the owner gets read, write and execute, the group gets read and execute, and others get read and execute.

In ls -l it appears as -rwxr-xr-x. The 7 is 4 + 2 + 1 and each 5 is 4 + 1, read plus execute. On a directory, read lets a user list the names inside and execute lets them traverse it to open files by path, so 755 is a directory everyone can browse but only the owner can add to or remove from.

Where 755 is the right choice

Most of /usr/bin is 755, owned by root: every user can run ls or git, nobody but root can replace them. The same applies to your own scripts in ~/bin and to web document roots, where the server must enter each directory but should not be able to rewrite the site.

It is also what mkdir produces under the common 022 umask, because new directories start from 777 and the umask removes group and other write. When a directory "looks normal" in a listing, it is usually 755.

Pitfalls

755 on a regular data file is harmless but misleading: it marks a config file or an image as executable. Use 644 for files that are only read. And 755 is readable by every account on the host, so it is the wrong mode for anything holding a secret — a directory of credentials wants 700 or 750.

Do not apply it with chmod -R 755 across a mixed tree; every file becomes executable. Use find dir -type d -exec chmod 755 {} + for directories and set files separately, or the symbolic form chmod -R u=rwX,go=rX dir, where capital X adds execute only to directories and to files that were already executable.

Related values

755 sits between two neighbours. 750 removes every bit from others, so only the owner and one group can enter; 700 goes further and keeps the directory to the owner alone. For files that are read but never run, 644 is the counterpart that drops execute everywhere. If you are tempted by 777 because a process cannot write, the problem is ownership, and 755 with the right owner is usually the answer.