Bash is the GNU shell, compatible with the Bourne shell and incorporating many useful features from other shells. When the shell is started, it reads its configuration files. The most important are:
Bash behaves different when in interactive mode and also has a POSIX compliant and a restricted mode.
Shell commands can be split up in three groups: the shell functions, shell built-ins and existing commands in a directory on your system. Bash supports additional built-ins not found in the plain Bourne shell.
Shell scripts consist of these commands arranged as shell syntax dictates. Scripts are read and executed line per line and should have a logical structure.