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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 DEBIAN

checkbashisms

CHECKBASHISMS(1)					      General Commands Manual						  CHECKBASHISMS(1)

NAME

       checkbashisms - check for bashisms in /bin/sh scripts

SYNOPSIS

       checkbashisms script ...
       checkbashisms --help|--version

DESCRIPTION

       checkbashisms, based on one of the checks from the lintian system, performs basic checks on /bin/sh shell scripts for the possible presence
       of bashisms.  It takes the names of the shell scripts on the command line, and outputs warnings if possible bashisms are detected.

       Note that the definition of a bashism in this context roughly equates to "a shell feature that is not required to be supported  by  POSIX";
       this means that some issues flagged may be permitted under optional sections of POSIX, such as XSI or User Portability.

       In  cases  where  POSIX	and  Debian  Policy  disagree, checkbashisms by default allows extensions permitted by Policy but may also provide
       options for stricter checking.

OPTIONS

       --help, -h
	      Show a summary of options.

       --newline, -n
	      Check for "echo -n" usage (non POSIX but required by Debian Policy 10.4.)

       --posix, -p
	      Check for issues which are non POSIX but required to be supported by Debian Policy 10.4 (implies -n).

       --force, -f
	      Force each script to be checked, even if it would normally not be (for instance, it has a bash or non POSIX shell shebang or appears
	      to be a shell wrapper).

       --extra, -x
	      Highlight lines which, whilst they do not contain bashisms, may be useful in determining whether a particular issue is a false posi-
	      tive which may be ignored.  For example, the use of "$BASH_ENV" may be preceded by checking whether "$BASH" is set.

       --version, -v
	      Show version and copyright information.

EXIT VALUES

       The exit value will be 0 if no possible bashisms or other problems were detected.  Otherwise it will be the sum of the following error val-
       ues:

       1      A possible bashism was detected.

       2      A file was skipped for some reason, for example, because it was unreadable or not found.	The warning message will give details.

SEE ALSO

       lintian(1).

AUTHOR

       checkbashisms  was originally written as a shell script by Yann Dirson <dirson@debian.org> and rewritten in Perl with many more features by
       Julian Gilbey <jdg@debian.org>.

DEBIAN
								 Debian Utilities						  CHECKBASHISMS(1)
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