shebang

A shebang, also called a hashbang, is the two-character sequence “#!” at the very start of a script file. It signals the operating system to use a specific interpreter to execute the file, making the script object executable as a program on Unix-like systems without invoking the interpreter manually.

Mechanism and usage: When a file marked as executable begins with “#!”, the kernel reads the rest of that first line to determine the interpreter’s path and any optional arguments. The kernel then starts the interpreter and passes the script file as its first argument. This allows scripts written in languages such as Bash, Python, Perl, or Ruby to be run directly. A common pattern is to specify the interpreter explicitly, e.g., “#!/bin/bash”, or to use a portable form such as “#!/usr/bin/env python3” to locate the interpreter via PATH.

Limitations and portability: The shebang line is a feature of the operating system, not the scripting language. Its presence is highly dependent on Unix-like execution semantics; Windows does not rely on shebangs in the same way, though some environments provide support. There are practical limits on the length of the line (and thus the interpreter path) on many systems, with overly long shebang lines potentially failing to execute. The script must also have executable permission for the kernel to apply the shebang mechanism.

Examples: A Python script might begin with “#!/usr/bin/env python3” to locate Python 3 in PATH, followed by the script’s code. A Bash script might start with “#!/bin/bash” to use Bash as the interpreter.