MacbookAir1:so1 palfvin$ a='[]'
MacbookAir1:so1 palfvin$ echo $a

MacbookAir1:so1 palfvin$ echo "$a"
[]
MacbookAir1:so1 palfvin$ echo '[]'
[]
MacbookAir1:so1 palfvin$

MacbookAir1:~ palfvin$ echo "$IFS" | od -c
0000000       t  n  n
0000004
MacbookAir1:~ palfvin$ echo "$IFS" | od -h
0000000      0920    0a0a
0000004

$ a='[]'
$ echo $a
[]
$ shopt -s nullglob
$ echo $a

$

$ echo []
[]
$ x=[]
$ echo $x
[]
$ IFS='[]'
$ echo $x

$