Why do you need to put #!/bin/bash at the beginning of a script file?

Question

I have made Bash scripts before and they all ran fine without #!/bin/bash at the beginning.

What's the point of putting it in? Would things be any different?

Also, how do you pronounce #? I know that ! is pronounced as "bang."

How is #! pronounced?

Answer 1

It's a convention so the *nix shell knows what kind of interpreter to run.

For example, older flavors of ATT defaulted to sh (the Bourne shell), while older versions of BSD defaulted to csh (the C shell).

Even today (where most systems run bash, the "Bourne Again Shell"), scripts can be in bash, python, perl, ruby, PHP, etc, etc. For example, you might see #!/bin/perl or #!/bin/perl5.

PS: The exclamation mark (!) is affectionately called "bang". The shell comment symbol (#) is sometimes called "hash".

PPS: Remember - under *nix, associating a suffix with a file type is merely a convention, not a "rule". An executable can be a binary program, any one of a million script types and other things as well. Hence the need for #!/bin/bash.

Answer 2

To be more precise the shebang #!, when it is the first two bytes of an executable (x mode) file, is interpreted by the execve(2) system call (which execute programs). But POSIX specification for execve don't mention the shebang.

It must be followed by a file path of an interpreter executable (which BTW could even be relative, but most often is absolute).

A nice trick (or perhaps not so nice one) to find an interpreter (e.g. python) in the user's $PATH is to use the env program (always at /usr/bin/env on all Linux) like e.g.

 #!/usr/bin/env python 

Any ELF executable can be an interpreter. You could even use #!/bin/cat or #!/bin/true if you wanted to! (but that would be often useless)