Re: @K transformation

[Léa Gris]

Le 21/08/2021 à 00:59, Greg Wooledge écrivait :
> The fact that "${a[@]@K}" expands to a single word is surprising to me.
> I know someone else already mentioned it in this thread (sorry, I forgot
> who it was), but it would be nice if there were a similar one that gave
> a list of multiple words.
>
> unicorn:~$ printf '<%s> ' "${a[@]@Q}"; echo
> <'1'> <'2'> <'3'>
> unicorn:~$ printf '<%s> ' "${a[@]@U}"; echo
> <1> <2> <3>
> unicorn:~$ printf '<%s> ' "${a[@]@L}"; echo
> <1> <2> <3>
> unicorn:~$ printf '<%s> ' "${a[@]@E}"; echo
> <1> <2> <3>
> unicorn:~$ printf '<%s> ' "${a[@]@K}"; echo
> <0 "1" 1 "2" 2 "3">
>
> It really sticks out.

Not all expansion transformers returns same number of arguments when 
expanding an array

Lets test with this script:

> #!/usr/bin/env bash
>
> oper=(U u L Q E P A K a)
>
> arr=(foo bar baz qux)
>
> mapfile -t expansions < <(
>   printf '"${arr[@]@%s}"\n' "${oper[@]}"
> )
>
> for exp in "${expansions[@]}"; do
>   eval "set -- $exp"
>   if [ $# -gt 1 ]; then
>     printf '%s expands into %d argumsnts:\n' "$exp" "$#"
>     for ((i = 1; $#; i++)); do
>       printf '%d.\t%s\n' $i "$1"
>       shift
>     done
>   else
>     printf '%s expands into a single argument:\n1.\t%s\n' "$exp" "$1"
>   fi
>   printf \\n
> done

The output is:

> "${arr[@]@U}" expands into 4 argumsnts:
> 1.      FOO
> 2.      BAR
> 3.      BAZ
> 4.      QUX
>
> "${arr[@]@u}" expands into 4 argumsnts:
> 1.      Foo
> 2.      Bar
> 3.      Baz
> 4.      Qux
>
> "${arr[@]@L}" expands into 4 argumsnts:
> 1.      foo
> 2.      bar
> 3.      baz
> 4.      qux
>
> "${arr[@]@Q}" expands into 4 argumsnts:
> 1.      'foo'
> 2.      'bar'
> 3.      'baz'
> 4.      'qux'
>
> "${arr[@]@E}" expands into 4 argumsnts:
> 1.      foo
> 2.      bar
> 3.      baz
> 4.      qux
>
> "${arr[@]@P}" expands into 4 argumsnts:
> 1.      foo
> 2.      bar
> 3.      baz
> 4.      qux
>
> "${arr[@]@A}" expands into 3 argumsnts:
> 1.      declare
> 2.      -a
> 3.      arr=([0]="foo" [1]="bar" [2]="baz" [3]="qux")
>
> "${arr[@]@K}" expands into a single argument:
> 1.      0 "foo" 1 "bar" 2 "baz" 3 "qux"
>
> "${arr[@]@a}" expands into 4 argumsnts:
> 1.      a
> 2.      a
> 3.      a
> 4.      a

Now @K would have been useful if it expanded into individual arguments 
for each entry rather than an eval expression.

The @A suffers from the same weirdness expanding into an eval expression 
that actually duplicates the feature from declare -p.

The @a expands the attribute from the container array for each element, 
which is as strange, because all array elements don't have attributes of 
their own.

--
Léa Gris