Re: [Help-bash] Awkward behavior of empty arrays

[Chet Ramey]

On 8/31/17 5:51 AM, Andy Chu wrote:
> The problems you point out are indeed subtle, but if you follow a few
> rules, you can avoid them:
> 
> 1. Never use anything that isn't in parentheses on the RHS of an array
> initialization.

This is essentially an attempt to avoid the implicit use of subscript 0.

> 2. Never reference an array with anything but "address@hidden" or "${A[i]}" 
> where
> i is an integer (double quotes are mandatory in both cases).
> 
> Examples 2 and 3 break this rule -- address@hidden doesn't have double quotes.

It's not bad as long as you understand that the difference between 
address@hidden
and "address@hidden" is the same as the difference between $@ and "$@". If you
are prepared to deal with the different word splitting behavior, both can
be useful.

> 
> also invalid:
> "${A}"  # this is an implicit "address@hidden" but is confusing

It's not; it's an implicit "${A[0]}".

> "${A[*]}"
> ${A[*]}
These are useful in the same way that $* and "$*" are useful.


> In particular, don't expect to compare arrays with [[.
> 
> $ declare -a A=(A B C D)
> $ declare -a B=('A B' 'C D')
> 
> $ echo "address@hidden"
> 4
> $ echo "address@hidden"
> 2
> 
> # arrays compare equal because they're coerced to strings before comparison
> $ [[ "address@hidden" == "address@hidden" ]]; echo $?
> 0

Because the operands in [[ commands don't undergo word splitting.

-- 
``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/