json配列を1行ずつに分割・それを戻す

a.txt

$ cat <<EOD | jq -c '.[]'
> [ [ 11, 12, 13 ], [ 21, 22, 23 ], [ 31, 32, 33 ] ]
> EOD
[11,12,13]
[21,22,23]
[31,32,33]
$
$ cat <<EOD | jq -c '.[]' | jq -s 'reduce .[] as $i ([]; .+[$i])'
> [ [ 11, 12, 13 ], [ 21, 22, 23 ], [ 31, 32, 33 ] ]
> EOD
[
  [
    11,
    12,
    13
  ],
  [
    21,
    22,
    23
  ],
  [
    31,
    32,
    33
  ]
]