1.diff

--- /home/yuri/_/1.sh	2019-03-31 17:56:18.380352272 +0300
+++ /home/yuri/_/2.sh	2019-03-31 17:56:05.463685315 +0300
@@ -13,11 +13,11 @@
 3
 ]
 ...'
-sed -ne '                    ;# "-n" suppresses autoprint of pattern space
-    /^\[/!d                  ;# skip lines that donot begin a fresh block
-    :a;$!N;/\n]/!ba          ;# keep appending lines to pattern space until end of block
+sed -Ene '                   ;# "-n" suppresses autoprint of pattern space
+    /^\[$/!d                 ;# skip lines that donot begin a fresh block
+    :a;N;/\n]$/!ba           ;# keep appending lines to pattern space until end of block
     G                        ;# append our makeshift counter (hold space) to pattern spc
-    s/\n\{2\}$//p            ;# only when counter had 2 chars in it, we print block
+    s/\n{2}$//p              ;# only when counter had 2 chars in it, we print block
     /\n$/!q                  ;# target block, 2nd block has been printed so quit 
     x;s/$/\n/;x              ;# target block not reached, so increment
 ' <<<"$file"

1.sh

#!/usr/bin/env bash
set -eu
file='...
[
1
]
...
[
2
]
...
[
3
]
...'
sed -Ene '                   ;# "-n" suppresses autoprint of pattern space
    /^\[$/!d                 ;# skip lines that donot begin a fresh block
    :a;N;/\n]$/!ba           ;# keep appending lines to pattern space until end of block
    G                        ;# append our makeshift counter (hold space) to pattern spc
    s/\n{2}$//p              ;# only when counter had 2 chars in it, we print block
    /\n$/!q                  ;# target block, 2nd block has been printed so quit 
    x;s/$/\n/;x              ;# target block not reached, so increment
' <<<"$file"