Unix in a Nutshell, 4th Edition

Name

test

Synopsis

    test expression
            or
    [ expression ]

Evaluate an expression and, if its value is true, return a zero exit status ; otherwise, return a nonzero exit status. In shell scripts, you can use the alternate form [ expression ]. The brackets are typed literally and must be separated from expression. Generally, this command is used with conditional constructs in shell programs. See Chapter 4 for more information on test.