chmod modifies Linux file permissions. Three sets of permissions Owner of the file Member's of the file groups Everyone else Permitting or preventing a file from being read, modified, or execution if it is a program or script. For a directory, the permissions govern who can cd into the directory and who can create, or modify files within the directory. 2^2 - Read 2^1 - Write 2^0 - Execute permission. Therefore, for example, 7 is used to allow for reading, writing, and executing together. Helpful Website