Single square brackets delimit a "test" statement.
Double square brackets delimit a Conditional Expression.

Historically both invoked external commands but nowadays both are built into the shell (for example: ksh, POSIX sh). In ancient Bourne shell "[[" is actually a program.

There is some commonality in the syntax for a "test" and a "conditional expression" but there is also much unique syntax so it is important to know which one to use. The two have separate sections in a shell manual.


The "type" command can tell you how your shell treats square brackets.
For example in ksh:

Code:

type [
[ is a shell builtin.

type [[
[[ is a keyword.

type test
test is a shell builtin.