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Date: Thu, 6 Apr 2023 18:18:59 +0100
To: Fergus Daly <fergusd84 AT outlook DOT com>
Cc: "'cygwin AT cygwin DOT com'" <cygwin AT cygwin DOT com>
Subject: Re: bash shell script: recently running, now failing
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From: Adam Dinwoodie via Cygwin <cygwin AT cygwin DOT com>
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On Thu, Apr 06, 2023 at 04:43:51AM +0000, Fergus Daly via Cygwin wrote:
> I have a "hash bang" bash shell script i.e. first line
> #! /bin/sh
> or equivalently
> #! /bin/bash
> For various reasons I want this file to be identified as binary so its second line
> is the single character null \x00 showing up in some editors e.g. nano as
>  ^@
> This does not prevent the script from running to a successful conclusion.
> Or not until recently. Now the script fails with
> /home/user/bin/file.old.sh: cannot execute binary file
> Q1 - was bash recently updated? Would this explain the changed behaviour?
> Q2 - if so, is this newly introduced "glitch" known and presumably intended? Or
> an unintended consequence that will be retracted in a later update? 
> I then altered the first line to
> #! /bin/dash
> whilst retaining the null character at line 2 and subsequent content also unaltered..
> The altered script file.new.sh runs as previously to a successful conclusion.
> Q3 - at 1/8 the size of bash and sh, I am not at all sure of the role and reach of dash.
> Should the edit (dash replacing bash/sh) be incorporated elsewhere or would this be a
> bad idea (and retained only locally in what is indeed an eccentric and one-off context)?

Dash is smaller and much less feature-rich than Bash.  Whether Dash is a
suitable replacement for Bash depends on how much (if at all) you're
relying on Bash-specific functions.  For very simple scripts, the only
difference is likely that Dash will be very slightly faster, but working
out whether your script is using any "Bashisms" isn't always a trivial
job.

(I have previously been involved work in migrating scripts between Ksh
and Bash, which is a similar-but-different problem, and there were *a
lot* of surprises in how the two differed.)

Depending on why you want the file to be identified as a binary, and how
that identification is being done, you could move your null byte later
in the file.  In particular, a pattern I've seen several times in Bash
is to have a normal Bash script, finishing with an explicit `exit`,
followed by an actual binary blob; this can be used to create things
like self-extracting bundles, where the binary blob is a tarball and the
script at the top of the file has the instructions for extracting the
tarball.

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