Shebang (#!)

Thu, 04 May 2023 12:30:00 +0530

What is the shebang?

The shebang (#!) is placed at the top of a script and is used to tell the kernel which interpreter to execute to read the script.

Syntax:

#!/path/to/interpreter [parameters (optional)]

The name shebang or hashbang comes from # (SHArp or HASH) and ! (bang).

Using a shebang like #!/usr/bin/echo will output just the filename (linux echo command), similarly #!/usr/bin/cat will output the contents of the file (linux cat command).

Note: Shebangs max out at 127 characters.

#!/usr/bin/env name

The /usr/bin/env command looks for and runs the first executable it finds in the user’s $PATH in a separate shell instance.

The #!/usr/bin/env shebang is used to increase portability of code.
It is also used in cases where the absolute path to the interpreter is unknown or may vary (such as when handling different versions of node with the Node Version Manager (nvm)).

#!/usr/bin/env node

Note: Arguments cannot be passed on to the interpreter when using #!/usr/bin/env.

Why `#!`?

The character sequence #!is encoded to the bytes 23 21, which is the magic number of an executable script.¹

A magic number or file signature is a sequence of bytes at the beginning of a file that allows to identify the type of file. For example, a ZIP file has a file signature of 50 4B 03 04.²

Ref:

What the #! shebang really does by meleu
what is a shebang (#!)? by anthonywritescode

bash on Siddharth Arumugam - Blog

Shebang (#!)

What is the shebang?

#!/usr/bin/env name

Why #!?

Why `#!`?