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.\"	$NetBSD: expr.1,v 1.36 2016/08/23 20:34:23 sevan Exp $
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.Dd August 23, 2016
.Dt EXPR 1
.Os
.Sh NAME
.Nm expr
.Nd evaluate expression
.Sh SYNOPSIS
.Nm
.Ar expression
.Sh DESCRIPTION
The
.Nm
utility evaluates
.Ar expression
and writes the result on standard output.
.Pp
All operators are separate arguments to the
.Nm
utility.
Characters special to the command interpreter must be escaped.
.Pp
Operators are listed below in order of increasing precedence.
Operators with equal precedence are grouped within { } symbols.
.Bl -tag -width indent
.It Ar expr1 Li \&| Ar expr2
Returns the evaluation of
.Ar expr1
if it is neither an empty string nor zero;
otherwise, returns the evaluation of
.Ar expr2 .
.It Ar expr1 Li \*[Am] Ar expr2
Returns the evaluation of
.Ar expr1
if neither expression evaluates to an empty string or zero;
otherwise, returns zero.
.It Ar expr1 Li "{=, \*[Gt], \*[Ge], \*[Lt], \*[Le], !=}" Ar expr2
Returns the results of integer comparison if both arguments are integers;
otherwise, returns the results of string comparison using the locale-specific
collation sequence.
The result of each comparison is 1 if the specified relation is true,
or 0 if the relation is false.
.It Ar expr1 Li "{+, -}" Ar expr2
Returns the results of addition or subtraction of integer-valued arguments.
.It Ar expr1 Li "{*, /, %}" Ar expr2
Returns the results of multiplication, integer division, or remainder of integer-valued arguments.
.It Ar expr1 Li \&: Ar expr2
The
.Dq \&:
operator matches
.Ar expr1
against
.Ar expr2 ,
which must be a regular expression.
The regular expression is anchored
to the beginning of the string with an implicit
.Dq ^ .
.Pp
If the match succeeds and the pattern contains at least one regular
expression subexpression
.Dq "\e(...\e)" ,
the string corresponding to
.Dq "\e1"
is returned;
otherwise the matching operator returns the number of characters matched.
If the match fails and the pattern contains a regular expression subexpression
the null string is returned;
otherwise 0.
.It "( " Ar expr No " )"
Parentheses are used for grouping in the usual manner.
.El
.Pp
Additionally, the following keywords are recognized:
.Bl -tag -width indent
.It length Ar expr
Returns the length of the specified string in bytes.
.El
.Pp
Operator precedence (from highest to lowest):
.Bl -enum -compact -offset indent
.It
parentheses
.It
length
.It
.Dq \&:
.It
.Dq "*" ,
.Dq "/" ,
and
.Dq "%"
.It
.Dq "+"
and
.Dq "-"
.It
compare operators
.It
.Dq \*[Am]
.It
.Dq \&|
.El
.Sh EXIT STATUS
The
.Nm
utility exits with one of the following values:
.Bl -tag -width Ds -compact
.It 0
the expression is neither an empty string nor 0.
.It 1
the expression is an empty string or 0.
.It 2
the expression is invalid.
.It \*[Gt]2
an error occurred (such as memory allocation failure).
.El
.Sh EXAMPLES
.Bl -enum
.It
The following example adds one to variable
.Dq a :
.Dl a=`expr $a + 1`
.It
The following example returns zero, due to subtraction having higher precedence
than the
.Dq \*[Am]
operator:
.Dl expr 1 '\*[Am]' 1 - 1
.It
The following example returns the filename portion of a pathname stored
in variable
.Dq a :
.Dl expr "/$a" Li : '.*/\e(.*\e)'
.It
The following example returns the number of characters in variable
.Dq a :
.Dl expr $a Li : '.*'
.El
.Sh COMPATIBILITY
This implementation of
.Nm
internally uses 64 bit representation of integers and checks for
over- and underflows.
It also treats
.Dq /
(the division mark) and option
.Dq --
correctly depending upon context.
.Pp
.Nm
on other systems (including
.Nx
up to and including
.Nx 1.5 )
might not be so graceful.
Arithmetic results might be arbitrarily
limited on such systems, most commonly to 32 bit quantities.
This means such
.Nm
can only process values between -2147483648 and +2147483647.
.Pp
On other systems,
.Nm
might also not work correctly for regular expressions where
either side contains
.Dq /
(a single forward slash), like this:
.Bd -literal -offset indent
expr / : '.*/\e(.*\e)'
.Ed
.Pp
If this is the case, you might use
.Dq //
(a double forward slash)
to avoid confusion with the division operator:
.Bd -literal -offset indent
expr "//$a" : '.*/\e(.*\e)'
.Ed
.Pp
According to
.St -p1003.2 ,
.Nm
has to recognize special option
.Dq -- ,
treat it as a delimiter to mark the end of command
line options, and ignore it.
Some
.Nm
implementations do not recognize it at all; others
might ignore it even in cases where doing so results in syntax
error.
There should be same result for both following examples,
but it might not always be:
.Bl -enum -compact -offset indent
.It
expr -- : .
.It
expr -- -- : .
.El
Although
.Nx
.Nm
handles both cases correctly, you should not depend on this behavior
for portability reasons and avoid passing a bare
.Dq --
as the first
argument.
.Sh STANDARDS
The
.Nm
utility conforms to
.St -p1003.2 .
The
.Ar length
keyword is an extension for compatibility with GNU
.Nm .
.Sh HISTORY
An
.Nm
utility first appeared in the Programmer's Workbench (PWB/UNIX).
A public domain version of
.Nm
written by
.An Pace Willisson
.Aq pace@blitz.com
appeared in
.Bx 386 0.1 .
.Sh AUTHORS
Initial implementation by
.An Pace Willisson Aq Mt pace@blitz.com
was largely rewritten by
.An -nosplit
.An J.T. Conklin Aq Mt jtc@NetBSD.org .
It was rewritten again for
.Nx 1.6
by
.An -nosplit
.An Jaromir Dolecek Aq Mt jdolecek@NetBSD.org .
.Sh NOTES
The empty string
.Do Dc
cannot be matched with the intuitive:
.Bd -literal -offset indent
expr '' : '$'
.Ed
.Pp
The reason is that the returned number of matched characters (zero)
is indistinguishable from a failed match, so this returns failure.
To match the empty string, use something like:
.Bd -literal -offset indent
expr x'' : 'x$'
.Ed