A shell is a command-line interface that allows users to interact with the operating system. It is a program that provides a way for users to execute commands and run scripts on their computer.
In a Unix-based operating system, file permissions determine which users can access, modify or execute a file. The three types of permissions are: read, write, and execute. Permissions are set for three groups of users: owner, group, and others.
In a Unix-based operating system, shell redirection is a way to redirect the input or output of a command or a script.
In a Unix-based operating system, shell init files are scripts that are executed when a shell is started. These scripts can be used to customize the shell environment, set environment variables, and define shell functions.