chmod Permission Calculator

Each digit is the sum of read (4), write (2) and execute (1) for one class. 755 gives the owner 7 (rwx) and the group and others 5 (r-x): they can read and execute but not write. It is a common mode for directories and executable scripts.

644 gives the owner read and write (6 = 4 + 2) and the group and others read only (4). It is a common mode for ordinary files such as web pages and configuration.

777 lets every user on the system read, write and execute the file. On a shared or internet-facing server that is rarely necessary and a security risk. Grant the narrowest permission that works, usually by fixing the owner or group instead.

They are the optional fourth leading digit: 4 is setuid (run as the file’s owner), 2 is setgid (run as the file’s group, or inherit the directory’s group) and 1 is the sticky bit (on a directory, only a file’s owner can delete it, as in /tmp). In the rwx string they appear as s or S and t or T in the execute position.

A lowercase s or t means the special bit is set and execute is also set for that class. A capital S or T means the special bit is set but execute is not, which is usually a sign of a misconfiguration.