Interpreter directive
An interpreter directive is a computer language construct, that on some systems is better described as an aspect of the system's executable file format, that is used to control which interpreter parses and interprets the instructions in a computer program.

In Unix, Linux and other Unix-like operating systems, the first two bytes in a file can be the characters "#!", which constitute a magic number (hexadecimal 23 and 21, the ASCII values of "#" and "!") often referred to as shebang, prefix the first line in a script, with the remainder of the line being a command usually limited to a max of 14 (when introduced) up to usually about 80 characters in 2016. If the file system permissions on the script (a file) include an execute  permission bit for the user invoking it by its filename (often found through the command search path), it is used to tell the operating system what interpreter (usually a program that implements a scripting language) to use to execute the script's contents, which may be batch commands or might be intended for interactive use. An example would be #!/bin/bash, meaning run this script with the bash shell found in the /bin directory.

Other systems or files may use some other magic number as the interpreter directives.

See also 
 Architecture description language
 Bourne shell
 C shell
 Command-line interface#Command-line interpreter
 Filename extension#Command name issues
 Shebang (Unix)
 Unix shell

References 

Command shells
Computing terminology