Re: d-i repo at dillon

[Ian Campbell <ijc@hellion.org.uk>]

On Sat, 2018-06-16 at 10:17 +0100, Ian Campbell wrote:
> On Sat, 2018-06-16 at 08:36 +0200, Holger Wansing wrote:
> > > The original/final lines are a bit strange, though, instead of
> having:
> > > 
> > >   if $($git foo bar); then … fi
> > > 
> > > I suppose it should only be:
> > > 
> > >   if $git foo bar; then … fi
> > 
> > However, with this simplified variant it fails. So I left it as is
> for now.
> 
> It seems there is an interesting (and new to me, or at least I'd
> never
> fully appreciated the behaviour) corner case of the `if $(foo); then`
> syntax, which is that if `foo` exits producing no output then its
> exit
> code is apparently used for the condition. If `foo` does produce
> output
> then the shell will attempt to execute that and use the resulting
> exit
> code.
> 
> These just run true or false and take the output:

Should be "These just run true or false and use the exit code".

BTW, it's worth mentioning that `true` and `false` here are actually
`/bin/{true,false}` i.e. literal commands which return the appropriate
exit code that the shell `fork`s and `exec`s. There's no shell syntax
magic[*] going on here where `true` and `false` are somehow parsed
specially.

Ian.

[*] technically `true` and `false` might be shell builtins for
performance reasons (and it looks like with `dash` `true` is but
`false` isn't). However logically they can be treated as external
commands without special handling. To be unambiguous you could rerun
all the examples using the explicit /bin/true etc versions directly.

>    $ dash -c 'if true ; then echo YES ; else echo NO ; fi'
>    YES
>    $ dash -c 'if false ; then echo YES ; else echo NO ; fi'
>    NO

Ian.