test <expr>

Condition command.
Returns zero if <expr> is true, else returns non-zero.

<expr> primitives -

-r <file>	- true if readable file.
-w <file>	- true if writable file.
-f <file>	- true if non-dir file.
-d <file>	- true if dir file.
-s <file>	- true if non-empty file.
-t <fildes>	- true if <fildes> is a terminal.

-z <str>	- true if <str> len is zero.
-n <str>	- true if <str> len is non-zero.
<s1> = <s2>	- true if <s1> = <s2>.
<s1> != <s2> - true if <s1> != <s2>.
<str>		- true if <str> is not the null string.

<n1> <rel_op> <n2>	- relational op on two numerical operands.
			rel_ops are -eq -ne -gt -ge -lt -le


Primitives can be combined with following operators -
!	- unary negation.
-a	- binary and (higher precedence than '-o').
-o	- binary or.
(expr)	- grouping (must be escaped).
