.TH SH-EXPR 1
.SH NAME
expr, ntest \- shell module for simple arithmetic.
.SH SYNOPSIS
.B load expr

.B ${expr
[
-r
.I radix
]
[
.I arg...
]
.B }
.br
.B ntest
.I num
.br
.SH DESCRIPTION
.I Expr
is a loadable module for
.IR sh (1)
that provides support for simple integer arithmetic.
It provides one command,
.IR ntest ,
which performs a simple boolean test
on its integer argument, and the
substitution operator
.IR expr ,
which takes an expression in Reverse Polish
notation, and yields its result.
.PP
.I Ntest
returns true if its argument
.I num
is non-zero,
and false otherwise.
.PP
.I Expr
evaluates each
.I arg
in turn; if it is an integer it gets pushed onto
the stack; otherwise it should name
one of the operators below, whereupon
the appropriate number of operands are
popped off the stack, evaluated as arguments
to the operator, and the result pushed back onto
the stack. Arguments are passed to the operator
first-pushed first, so, for instance,
.B ${expr 2 1 -}
yields 1, not -1.
Alternative names are given for some operators;
this is to avoid the necessity of quoting operators
that contain
.IR sh (1)
metacharacters. All operations use 64-bit signed
integers; integers are given in the same form acceptable
to Limbo. The relational operators yield either
1 (true) or 0 (false). If the
.B -r
option is given,
.I radix
specifies an output base for numbers yielded by
.IR expr .
Numbers are printed in a form suitable for re-interpretation
by
.IR expr .
.PP
When all its arguments have been evaluated,
.B expr
yields all the values remaining on its stack, first pushed
first. The operators supported by expr are as follows (the number
of operands required in is given parentheses):
.TP 15
.BR + \ (2)
Addition
.TP
.BR - \ (2)
Subtraction
.TP
.BR x ,\  * \ (2)
Multiplication
.TP
.BR / \ (2)
Division. Division by zero raises a
.B divide by zero
exception.
.TP
.BR % \ (2)
Modulus. A zero modulus will cause a
.B divide by zero
exception to be raised.
.TP
.BR and \ (2)
Bitwise-and.
.TP
.BR or \ (2)
Bitwise-or.
.TP
.BR xor \ (2)
Bitwise-xor.
.TP
.BR ~ \ (1)
Bitwise-complement..
.TP
.BR _ \ (1)
Unary minus.
.TP
.BR << ,\  shl \ (2)
Shift left.
.TP
.BR >> ,\  shr \ (2)
Shift right.
.TP
.BR = ", " == ", " eq " (2)"
Equality.
.TP
.BR != ", " neq " (2)"
Inequality.
.TP
.BR > ", " gt " (2)"
Greater than.
.TP
.BR < ", " lt " (2)"
Less than.
.TP
.BR <= ", " le " (2)"
Less than or equal to.
.TP
.BR >= ", " ge " (2)"
Greater than or equal to.
.TP
.BR ! ", " not " (1)"
Logical negation.
.TP 
.BI rep \ \f1(\fPn\f1)\fP
.B Rep
repeats the last operation (which must
have been a two-operand operation other
than
.BR seq )
until the values in the stack are exhausted.
.TP
.BR seq \ (2)
.B Seq
pushes on the stack a sequence of numbers ranging
numerically from its first argument up to and including
its second argument. If its second argument is
less than its first, the sequence will descend.
.SH SOURCE
.B /appl/cmd/sh/expr.b
.SH SEE ALSO
.IR sh (1),
.IR sh-std (1),
.IR sh-tk (1)
.SH BUGS
Postfix notation can be confusing.
Any operators that contain shell metacharacters (e.g. ``*'', ``>'')
must be quoted to avoid interpretation by the shell.