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Full Discussion: Scripts without shebang
Top Forums Shell Programming and Scripting Scripts without shebang Post 302694923 by alister on Friday 31st of August 2012 04:56:59 PM
Old 08-31-2012
Quote:
Originally Posted by 244an
Code:
# enter sh:
[user@server ~/test]$ sh
[2 user@server /usr/home/user/test]$ ./tfs
[[: not found
no
# enter bash:
[2 user@server /usr/home/user/test]$ bash
[3 user@server ~/test]$ ./tfs
yes

... <snip> ....

Can it be that it's only when the file is a foreign binary that it falls back to _PATH_BSHELL?
No. The code in the libc functions (and the kernel syscall they invoke) does not make that distinction.

I can replicate your observation on an old debian system using bash 3.1.17. Bash must not be using any of those libc functions. If it were, /bin/sh would have been called. Bash is sidestepping them, either by invoking execve(2) directly or by using an exec*(3) variant which does not have any of the /bin/sh fallback semantics, so that it can use itself to interpret executable files that aren't recognized by the kernel.

This is allowed by POSIX shell command search and execution, but it is not required.

Like Corona688, I too think it's best to use the shebang. At the very least, it serves as documentation of the author's intentions. At the very most, it's a weak guarantee that the script will be interpreted by a compatible interpreter. Just because it's there, does not mean that it will be used. If the script is sourced by an interpreter, or fed on standard input, or passed as a command line argument -- in other words, when a shell is reading the script -- the shebang is treated as a comment and ignored. The shebang is only relevant when the kernel itself, in execve(2), is reading the script.

Regards,
Alister
 

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LEARN ABOUT FREEBSD

atf-sh

ATF-SH(1)						    BSD General Commands Manual 						 ATF-SH(1)

NAME

     atf-sh [-s shell] -- interpreter for shell-based test programs

SYNOPSIS

     atf-sh script

DESCRIPTION

     atf-sh is an interpreter that runs the test program given in script after loading the atf-sh(3) library.

     atf-sh is not a real interpreter though: it is just a wrapper around the system-wide shell defined by ATF_SHELL.  atf-sh executes the inter-
     preter, loads the atf-sh(3) library and then runs the script.  You must consider atf-sh to be a POSIX shell by default and thus should not
     use any non-standard extensions.

     The following options are available:

     -s shell	  Specifies the shell to use instead of the value provided by ATF_SHELL.

ENVIRONMENT

     ATF_LIBEXECDIR    Overrides the builtin directory where atf-sh is located.  Should not be overridden other than for testing purposes.
     ATF_PKGDATADIR    Overrides the builtin directory where libatf-sh.subr is located.  Should not be overridden other than for testing purposes.
     ATF_SHELL	       Path to the system shell to be used in the generated scripts.  Scripts must not rely on this variable being set to select a
		       specific interpreter.

EXAMPLES

     Scripts using atf-sh(3) should start with:

	   #! /usr/bin/env atf-sh

     Alternatively, if you want to explicitly choose a shell interpreter, you cannot rely on env(1) to find atf-sh.  Instead, you have to hardcode
     the path to atf-sh in the script and then use the -s option afterwards as a single parameter:

	   #! /path/to/bin/atf-sh -s/bin/bash

ENVIRONMENT

     ATF_SHELL	  Path to the system shell to be used in the generated scripts.

SEE ALSO

     atf-sh(3)

BSD
								September 27, 2014							       BSD
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