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

[Chet Ramey]

On 2/23/17 12:23 PM, Greg Wooledge wrote:
> On Thu, Feb 23, 2017 at 09:10:52AM -0800, Seth David Schoen wrote:
>> $ if `true`; then echo foo; fi
>> foo
>> $ if `false`; then echo foo; fi
>> $
> 
> First of all, and I cannot possibly stress this enough, this is shitty
> code and you should never EVER write code like this.
> 
> Right, so.  Apparently bash treats a command substitution that returns an
> empty string in some special-snowflake way, rather than as a syntax error.

Please. Don't overthink this.

First, why should bash treat a command substitution that returns no output
as anything different from a variable expansion that has no value?

Consider `something`.  That's a simple command, with a single word, that
undergoes word expansion.  It can either expand to some text, in which
case that text undergoes word splitting and globbing and is executed as a
command, or nothing.

If it returns nothing, it falls into this Posix-standardized and historical
shell behavior category:

"If there is no command name, but the command contained a command
substitution, the command shall complete with the exit status of the last
command substitution performed."

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01

Virtually the same language appears in the bash manual.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/