Should I use a Shebang with Bash scripts?

Question

I am using Bash

$ echo $SHELL
/bin/bash

and starting about a year ago I stopped using Shebangs with my Bash scripts. Can I benefit from using #!/bin/sh or #!/bin/bash?

Update: In certain situations a file is only treated as a script with the Shebang, example

$ cat foo.sh
ls

$ cat bar.sh
#!/bin/sh
ls

$ file foo.sh bar.sh
foo.sh: ASCII text
bar.sh: POSIX shell script, ASCII text executable

Answer 1

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.