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root@system:~/# x='abc'
root@system:~/# y=''
root@system:~/# [[ $(echo $x) != '' ]] && echo true
root@system:~/# [[ $(echo $y) != '' ]] && echo true
sh: : unknown operand
In Ubuntu the last line returns nothing (as expected).  Any ideas why it's throwing an error in Yocto?
                I'm betting that sh is not the same on those two platforms and that on Yocto it's not as fully implemented, I was actually surprised that sh implemented [[ at all, Yocto or otherwise
– Eric Renouf
                Dec 13, 2015 at 1:07
                @eric-renouf Any idea exactly what the problem can be though, considering it works in Yocto for $x?
– geotheory
                Dec 13, 2015 at 12:02
                I'm guessing the problem is how it handles "empty" args, so as long as you have a value on each side of != it will work.  You might just try quoting the command substitutions like [[ "$(echo $y)" != '' ]]... and see if that helps
– Eric Renouf
                Dec 13, 2015 at 12:25
The problem seems to be that $(echo $y) is expanding to an empty string, and then [[ isn't handling it correctly.  The solution to that would be to quote the command substitution like
[[ "$(echo "$y")" != '' ]] && echo true
though it's probably better still to use printf than echo so you might do it as
[[ "$(printf '%s' "$y")" != '' ]] && echo true
just in case $y might end up with special characters that can trip up echo or similar
Apparently, busybox ash has a rather simplistic implementation of [[. It is the same as [ except that it expects a ]] instead of ] final argument. This misses the point of why [[ can be useful at all: [[ is supposed to be a keyword with special parsing and using it looks more beautiful and avoids various pitfalls (while adding some of its own). I guess they added it so a few more bash scripts run unmodified on busybox ash.
To avoid confusion, I recommend not using [[ in busybox at all. Use [ and quote all command substitutions and parameter expansions.
        
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