Tests

test

test expression

[ expression ]

Test returns 0 (zero!) if expression is true and >0 if it is false.

Testing for empty string:

test -z "$variable"

[ -z "$variable" ]

[ x$variable != x ]

"[" is a command like any other --> "]" is a argument to "[" and must be separated by a whitespace from the penultimate argument.
what would happen if the quotes in the first two example were ommitted?
the third example is actually an example to write the test without quotes

test if two strings are equal

test "$variable" = "hello world"

test if two numbers are equal

test $number -eq 3

test if $filename exists and is a regular file

test -f $filename

test if $filename exists and is a regular file AND is not executable

test -f $filename -a ! -x $filename

test if $number is greater than 7

test $number -gt 7