The difference between a single middle bracket and a double bracket in a shell

The difference between a single middle bracket and a double bracket in a shell

1.[
"[" is an executable program, the path is "/usr/bin/["
He is equivalent to the executable program "test".

Example:
If [3-eq 2]; Then
echo "= =";
Else
echo "!=";
Fi

Equivalent to:
if/usr/bin/[3-eq 2]; Then
echo "= =";
Else
echo "!=";
Fi

Equivalent to:
if test 3-eq 2; Then
echo "= =";
Else
echo "!=";
Fi

2.[[
"[[" is a keyword for the script interpreter (BASH,SH, etc.)
Example:
if [[2 = 2]]; Then
echo "= =";
Else
echo "!=";
Fi

Recommended use of [[].