Shebang – Linux kernel

The shebang (#!) is a special directive read and interpreted by the Linux kernel (and other Unix-like operating systems) to determine which interpreter should be used to execute a script. The kernel’s built-in mechanism (specifically, the execve() system call) processes this directive.

How the Linux Kernel Handles the Shebang
When a user attempts to execute a file with the executable permission set (using chmod +x filename), the Linux kernel follows a process to run it:

File Examination: The kernel opens the file and examines the very first two bytes (the “magic numbers”).

Shebang Detection: If these bytes are #! (hexadecimal 0x23 0x21), the kernel identifies the file as a script.

Interpreter Extraction: The kernel reads the rest of the first line to find the path to the required interpreter (e.g., /bin/bash, /usr/bin/python3).

Execution: The kernel then executes the specified interpreter program, passing the original script’s filename as an argument to the interpreter.

For example, if you run an executable script named myscript.py that starts with #!/usr/bin/python3, the kernel effectively runs the command /usr/bin/python3 myscript.py.

Key Points

Kernel Feature: It is the kernel, not the shell from which the script is launched, that interprets the shebang line.

Location: The shebang must be the absolute first line of the file, with no preceding characters or blank space.

Purpose: It allows scripts to be run directly like any other executable program without explicitly calling the interpreter on the command line.

Fallback: If a script without a shebang is executed, the shell typically defaults to using its own interpreter to run the commands, which may lead to errors if the script was written for a different shell or language.