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Re: question about Dash

On Fri, Aug 07, 2009 at 11:20 +0300, Andrei Popescu wrote:
> On Thu,06.Aug.09, 23:24:18, Paul E Condon wrote:
> > Recent mentions of dash on this list prompted me to try to learn more
> > about it. I googled and learned that it has a different convention for
> > indicating that a file is a script that should be processed by
> > dash. This affects only the first line of a script.
> The first line in a script *should* indicate what program is to be used 
> to interpret it. The most common script interpreter on a GNU/Linux 
> system is a "Bourne shell" (sh) *clone*. Both bash and dash are such 
> clones.  The downside to bash (the "Bourne again shell") is that it has 
> a *lot* of extensions, and even worse, it's using them even if called as 
> 'sh' (yes, it can tell if it was invoked as 'sh' or 'bash').
> Dash ("Debian Almquist SHell") also has two or three extensions, but 
> these are specifically allowed by Debian Policy, it is about 10 times 
> smaller than bash and significantly faster.
> Of course, you probably won't notice the speed difference on a three 
> line script.
> > Is this the only difference in coding scripts? I know it is said to be
> > smaller and faster, which is good, but are there things that need to
> > be changed in a bash script, after the first line, in order to make it
> > into a proper dash script? What are they? Where can I learn what to
> > change in my scripts? Or is it just smaller and faster with no
> > conversion pain?
> If your scripts do not use bash extensions (bashisms) and the shebang is
> 	#!/bin/sh
> then just install dash, let it take over the /bin/sh symlink (run 
> 'dpkg-reconfigure dash' if you're on lenny) and enjoy the additional 
> speed. I've been doing this for years without ill effects. You will also 
> notice if any 'sh' scripts are using bash extensions ;)
> A safer way to do this is to use the 'checkbashisms' script in the 
> package 'devscripts' first. Beware though that 'checkbashisms' is not 
> 100% sure. In the end you won't know unless you check the scripts 
> yourself (if you know what to look for) or just try to run them with 
> dash:
> /bin/dash my_custom_script.sh
> If you do find a script that doesn't work with dash (but does with bash) 
> you can:
> 1. try to rewrite it in 'sh'
> 2. change the shebang to point to 'bash'
> Option 1. is good if you want to make your scripts more portable.
> Option 2. is easier, especially on complex scripts.
> BTW, this talk about 'dash' vs. 'bash' is NOT about the user shell (the 
> shell that is specified in /etc/passwd and started on login to the 
> console or when you open a terminal emulator). As a user shell dash is 
> NOT apropiate, unless you are running some embedded system with very 
> little storage/memory.
> Regards,
> Andrei
> -- 
> If you can't explain it simply, you don't understand it well enough.
> (Albert Einstein)

I fully agree with Andrei in what he is saying (including his .sig :)
If interested, you might also whant to look at:


for historical background and more (even Bourne's macros making C look
like algol68).


Please don't Cc: me when replying, I might not see either copy.
               or:                bsb-at-psycho-dot-i21k-dot-de
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