Should I use a Shebang with Bash scripts?

On UNIX-like systems, you should always start scripts with a shebang line. The system call execve (which is responsible for starting programs) relies on an executable having either an executable header or a shebang line.

From FreeBSD's execve manual page:

 The execve() system call transforms the calling process into a new process.  The new process is constructed from an ordinary file, whose name is pointed to by path, called the new process file. [...] This file is either an executable object file, or a file of data for an interpreter. [...] An interpreter file begins with a line of the form:       #! interpreter [arg] When an interpreter file is execve'd, the system actually execve's the specified interpreter.  If the optional arg is specified, it becomes the first argument to the interpreter, and the name of the originally execve'd file becomes the second argument

Similarly from the Linux manual page:

execve() executes the program pointed to by filename. filename must be either a binary executable, or a script starting with a line of the form:

#! interpreter [optional-arg]

In fact, if a file doesn't have the right "magic number" in it's header, (like an ELF header or #!), execve will fail with the ENOEXEC error (again from FreeBSD's execve manpage):

[ENOEXEC] The new process file has the appropriate access permission, but has an invalid magic number in its header.

If the file has executable permissions, but no shebang line but does seem to be a text file, the behaviour depends on the shell that you're running in.

Most shells seem to start a new instance of themselves and feed it the file, see below.

Since there is no guarantee that the script was actually written for that shell, this can work or fail spectacularly.

From tcsh(1):

   On  systems which do not understand the `#!' script interpreter conven‐   tion the shell may be compiled to emulate it;  see  the  version  shell   variable.  If so, the shell checks the first line of the file to see if   it is of the form `#!interpreter arg ...'.  If it is, the shell  starts   interpreter  with  the  given args and feeds the file to it on standard   input.

From FreeBSD's sh(1):

If the program is not a normal executable file (i.e., if it     does not begin with the “magic number” whose ASCII representation is     “#!”, resulting in an ENOEXEC return value from execve(2)) but appears to     be a text file, the shell will run a new instance of sh to interpret it.

From bash(1):

   If this execution fails because the file is not in  executable  format,   and  the file is not a directory, it is assumed to be a shell script, a   file containing shell commands.  A subshell is spawned to  execute  it.

You cannot always depend on the location of a non-standard program like bash. I've seen bash in /usr/bin, /usr/local/bin, /opt/fsf/bin and /opt/gnu/bin to name a few.

So it is generally a good idea to use env;

#!/usr/bin/env bash

If you want your script to be portable, use sh instead of bash.

#!/bin/sh

While standards like POSIX do not guarantee the absolute paths of standard utilities, most UNIX-like systems seem to have sh in /bin and env in /usr/bin.

Scripts should always begin with a shebang line. If a script doesn't start with this, then it may be executed by the current shell. But that means that if someone who uses your script is running a different shell than you do, the script may behave differently. Also, it means the script can't be run directly from a program (e.g. the C exec() system call, or find -exec), it has to be run from a shell.

You might be interested in an early description by Dennis M Ritchie (dmr) who invented the #! :

From uucp Thu Jan 10 01:37:58 1980

.>From dmr Thu Jan 10 04:25:49 1980 remote from research

The system has been changed so that if a file being executed begins with the magic characters #! , the rest of the line is understood to be the name of an interpreter for the executed file. Previously (and in fact still) the shell did much of this job; it automatically executed itself on a text file with executable mode when the text file's name was typed as a command. Putting the facility into the system gives the following benefits.

1) It makes shell scripts more like real executable files, because they can be the subject of 'exec.'

2) If you do a 'ps' while such a command is running, its real name appears instead of 'sh'. Likewise, accounting is done on the basis of the real name.

3) Shell scripts can be set-user-ID.

4) It is simpler to have alternate shells available; e.g. if you like the Berkeley csh there is no question about which shell is to interpret a file.

5) It will allow other interpreters to fit in more smoothly.

To take advantage of this wonderful opportunity, put

   #! /bin/sh

at the left margin of the first line of your shell scripts. Blanks after ! are OK. Use a complete pathname (no search is done). At the moment the whole line is restricted to 16 characters but this limit will be raised.

Hope this helps