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.\" $OpenBSD: expr.1,v 1.18 2009/02/08 17:15:09 jmc Exp $
.\" $NetBSD: expr.1,v 1.9 1995/04/28 23:27:13 jtc Exp $
.\"
.\" Written by J.T. Conklin <jtc@netbsd.org>.
.\" Public domain.
.\"
.Dd $Mdocdate: February 8 2009 $
.Dt EXPR 1
.Os
.Sh NAME
.Nm expr
.Nd evaluate expression
.Sh SYNOPSIS
.Nm expr
.Ar expression
.Sh DESCRIPTION
The
.Nm
utility evaluates
.Ar expression
and writes the result on standard output.
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 & 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 "{=, >, >=, <, <=, !=}" 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
.Ql \&:
operator matches
.Ar expr1
against
.Ar expr2 ,
which must be a basic regular expression.
The regular expression is anchored
to the beginning of the string with an implicit
.Ql ^ .
.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, returns 0.
.Pp
Note: the empty string cannot be matched using
.Bd -literal -offset indent
expr '' : '$'
.Ed
.Pp
This is because the returned number of matched characters
.Pq zero
is indistinguishable from a failed match, so
.Nm
returns failure
.Pq 0 .
To match the empty string, use a structure such as:
.Bd -literal -offset indent
expr X'' : 'X$'
.Ed
.El
.Pp
Parentheses are used for grouping in the usual manner.
.Sh EXAMPLES
.Li $ a=`expr $a + 1`
.Pp
Add 1 to the variable
.Va a .
.Pp
.Li $ expr "//$a" \&: '.*/\e(.*\e)'
.Pp
Return the filename portion of a pathname stored
in variable
.Va a .
The
.Ql //
characters act to eliminate ambiguity with the division operator.
.Pp
.Li $ expr $a \&: '.*'
.Pp
Return the number of characters in variable
.Va a .
.Sh DIAGNOSTICS
The
.Nm
utility exits with one of the following values:
.Pp
.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 >2
An error occurred (such as memory allocation failure).
.El
.Sh SEE ALSO
.Xr test 1 ,
.Xr re_format 7
.Sh STANDARDS
The
.Nm
utility is compliant with the
.St -p1003.1-2008
specification.
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