Re: [Help-bash] Evaluations of backticks in if statements

[Andy Chu]

On Thu, Feb 23, 2017 at 9:10 AM, Seth David Schoen <address@hidden>
wrote:

> Andy Chu writes:
>
> > Shell doesn't work this way.  The syntax is "if command", and
> >
> > if `command`
> >
> > is a particular case of that which is not special in any way (and as you
> > mention it's also not useful).  There's no such thing as
> >
> > if ""
> >
> > where "" is a string/variable value -- the "" is interpreted a command
> name.
>
> I agree with this interpretation, but I see behavior that I don't know
> how to account for.  Presumably the shell should be running the output
> of the substituted command and then checking the return code of _that_,
> which accounts for
>
> $ if `echo true`; then echo foo; fi
> foo
> $ if `echo false`; then echo foo; fi
> $
>
> And it also accounts for
>
> $ if `echo wow`; then echo foo; fi
> wow: command not found
>
> But since neither true(1) nor false(1) outputs anything, there shouldn't
> be a difference between
>
> $ if `true`; then echo foo; fi
> foo
> $ if `false`; then echo foo; fi
> $
>
> and indeed both of them should presumably complain about the unknown
> command with an empty name -- but neither does, and they exhibit
> different behavior from one another.  So is there a special case after
> all?  I can't find a justification for this in the man page's
> documentation of "if" or command substitution.
>


Yes, this is surprising to me:

$ if ''; then echo TRUE; else echo FALSE; fi
: command not found
FALSE

$ if `true`; then echo TRUE; else echo FALSE; fi
TRUE

$ if `false`; then echo TRUE; else echo FALSE; fi
FALSE

So basically the exit code of the subshell command IS checked, and it's not
just treated as a word.  There is some special interaction between if and
subshell.

Moreover I tested bash, dash, zsh, and mksh, and they all exhibit this
behavior!  But I don't see it in POSIX.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

Digging through the source should probably reveal this as a special case.

I have a test framework running against all shells and mine is the only one
that's "wrong" here!

Andy