Watch Out Where Bin Sh Is Pointing To

I recently helped a fellow on a discussion forum who had problems because his shell script failed:

the-script.sh: 29: Syntax error: "(" unexpected

I suggested that his shebang,#!/bin/sh was pointing to DASH instead of BASH, which indeed turned out to be the problem. I was then asked to explain the difference between the two. Thinking of it, this is quite confusing as/bin/sh used to always point on Linux sytems and I'll therefore explain it here as well, with the hope that it'll enlighten some frustratede BASH programmers out there :-)

My explanation to the above error message

“The language you have used to program your script is BASH (see man bash), whereas the program you ask the OS to interpret your script through (you can think of the interpretre as the equivalent of a compiler if you're used to languages such as C and Java), is DASH, which is a different language.

Both BASH and DASH inherit from the old Bourne shell (SH), but BASH is far more powerful, both as a shell and as a programming language. DASH is a much more light weight shell which is chosen as the default on Debian based operating systems (including Ubuntu) because it consumes less system resources.

This is why the link/bin/sh points to /bin/dash and not /bin/bash. If you want to make the OS interpret your script as a BASH script, you must therefore add/bin/bash to the interpreter header. Even better is defining it as:

1

#! /usr/bin/env bash

That way, the OS will find BASH independent of where on the file system it is installed. On most Linux distributions it's usually installed under /bin/bash, though. Still using#! /usr/bin/env is a good habit for all interpreted languages such as bash, python and perl.”