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

[Andy Chu]

On Wed, Feb 22, 2017 at 5:01 PM, Ryan Marples <address@hidden>
wrote:

> Hi,
>
> I’ve seen bash statements like the following: if `cmd` …
>
> This appears to run the body of the if when cmd exits successfully. But I
> don’t understand the mechanics of why. My understanding is that `cmd`
> (assuming in this case cmd prints nothing to either stdout nor stderr but
> exits successfully) should evaluate to an empty string, and then you’d be
> saying if “” … which should be false.
>

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.

If you want to test if a value is empty, you can use

if test -n ""

or as a shortcut

if test ""

Or any of these:

[ -n "" ]
[ "" ]
[[ -n "" ]]
[[ "" ]]


 is an alias for test and [[ is a special shell construct that behaves
somewhat like test / [.

Andy