chmod 775: a directory the owner and its group can both write

What chmod 775 means

In 775 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, write and execute, and others get read and execute.

ls -l shows -rwxrwxr-x. The two 7s are 4 + 2 + 1 and the 5 is 4 + 1. On a directory, group write is the whole point: any group member can create, rename and delete entries inside it, not only the owner. It is also what mkdir produces under a 002 umask.

775 versus 755

The only difference is the middle digit. 755 lets the group read and enter; 775 also lets the group change the contents. Use 755 when one account maintains the files and everybody else consumes them, and 775 when several people or services need to add and edit files in the same place, such as a shared upload area, a build cache or a project checkout on a team server.

For files, the same split is 644 versus 664. A script the whole group may edit and run can be 775; plain data files in a 775 directory are usually 664.

Make the group stick with 2775

On its own, 775 only helps while files belong to the shared group. A new file normally takes the creating user's primary group, so after a week half the tree belongs to personal groups and the sharing quietly breaks. Setting the directory to 2775 adds the setgid bit, which makes new entries inherit the directory's group instead. The usual recipe is chgrp -R team dir, find dir -type d -exec chmod 2775 {} +, and umask 002 for the people and services that write there.

Pitfalls

775 is only as narrow as the group. Check membership with getent group <name> before relying on it, and avoid broad groups such as users. The last digit still lets every local account list and read the contents, so drop to 770 if outsiders should see nothing. As with every directory mode, do not apply it with chmod -R to a mixed tree, or every file becomes executable; split directories and files with find.