Which character is used in Linux to define the shell as a bin/bash shell that reads the program?

The character used in Linux to define the shell for a script, specifically indicating that the script should be executed using the bash shell, is the hash symbol (#), which is part of the shebang (#!) notation. When a script starts with #!, followed by the path to the interpreter, such as /bin/bash, it tells the operating system to execute the script using that specific interpreter.

In this context, # is considered the leading character in the shebang line, although it pairs with the exclamation point to form the shebang. Other options listed do not fulfill this role: the exclamation point on its own does not indicate a shell interpreter; the semicolon is used to separate commands; and the pipe symbol is used for piping output from one command to another. Thus, the correct answer highlights the importance of the shebang in script execution and its specific designation of the bash shell as the interpreter.