The single bracket
[ is actually an alias for the
test command, it's not syntax.
One of the downsides (of many) of the single bracket is that if one or more of the operands it is trying to evaluate return an empty string, it will complain that it was expecting two operands (binary). This is why you see people do
[ x$foo = x$blah ], the
x guarantees that the operand will never evaluate to an empty string.
The double bracket
[[ ]], on the other hand, is syntax and is much more capable than
[ ]. As you found out, it does not have the "missing operand" issue and it also allows for more C-like syntax with
>, <, >=, <=, !=, ==, &&, || operators.
My recommendation is the following: If your interpreter is
#!/bin/bash, then always use
It is important to note that
[[ ]] is not supported by all POSIX shells, however many shells do support it such as
ksh in addition to