difference between braces and normal brackets in bash?

Question

what is the difference between braces and normal brackets in bash?

FYI... I did not get it in the related questions list and also not able to locate it in questions search. Please point me to the question if it has been already asked.

Answer 1

Braces are used for parameter expansion (${foo%123}), brace expansion in either alternate ({foo,bar}) or sequence forms ({1..25}), or in blocks of code ({ foo ; bar ; }).

Square brackets are used as comparison commmands ([ "$foo" -lt 3 ], [[ $bar =~ ^123 ]]), As a range or character class in a glob (ba[rz], foo[[:alnum:]], qu[[=u=]]x), as part of an array assignment (foo=([2]=3 4 5), foo[42]=bar), or in parameter expansion when dealing with an array (${foo[@]}).

In other words, they're completely different.

Answer 2

http://ss64.com/bash/syntax-brackets.html