Nov 17, 2013

Why Shebang?

Frequently the initial two characters on the initial line of a script are: #!.

Why Shebang? Well, simple reason, because shell needs to know which interpreter to use when executing the script.

The sha-bang (#!) at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte magic number, a special marker that designates a file type, or in this case an executable shell script (type man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (the line following the sha-bang line), and ignoring comments. - Starting Off With a Sha-Bang

The shebang line is usually ignored by the interpreter because the # character is a comment marker in many scripting languages. Even the actual script is written with a different commenting character, such as in JavaScript:

#!/usr/bin/env node
// JavaScript stuff starts here.

Because the first line is interpreted by the shell, and the rest is passed into JavaScript interpreter, in this case, Node.

The syntax of shebang:

#! interpreter [optional-arg]

Whether there is a space between shebang character and the interpreter or not, it does not matter.

The interpreter must usually be an absolute path to a program that is not itself a script. The following are example interpreters:

#! /bin/sh
#! /bin/bash

More examples are via head -n 1 /etc/init.d/*.

The optional‑arg should either not be included or it should be a string that is meant to be a single argument (for reasons of portability, it should not contain any whitespace). - Shebang_(Unix)

We also frequently see the following shebang:

#!/usr/bin/env bash

That has to do with portability. Not every system install node or bash command in the same path. /usr/bin/env will search through user’s $PATH, and correctly locate the executable.

To conclude for Node, here is the format I am using:

#! /usr/bin/env node

// Title
// =====
//
// Markdown style description starts here.

'use strict';