EXPR(P) 						     POSIX Programmer's Manual							   EXPR(P)

NAME

       expr - evaluate arguments as an expression

SYNOPSIS

       expr operand

DESCRIPTION

       The expr utility shall evaluate an expression and write the result to standard output.

OPTIONS

       None.

OPERANDS

       The  single  expression evaluated by expr shall be formed from the operands, as described in the EXTENDED DESCRIPTION section. The applica-
       tion shall ensure that each of the expression operator symbols:

	      (  )  |  &  =  >	>=  <  <=  !=  +  -  *	/  %  :

       and the symbols integer and string in the table are provided as separate arguments to expr.

STDIN

       Not used.

INPUT FILES

       None.

ENVIRONMENT VARIABLES

       The following environment variables shall affect the execution of expr:

       LANG   Provide a default value for the internationalization variables that  are	unset  or  null.  (See	the  Base  Definitions	volume	of
	      IEEE Std 1003.1-2001,  Section  8.2,  Internationalization  Variables  for  the precedence of internationalization variables used to
	      determine the values of locale categories.)

       LC_ALL If set to a non-empty string value, override the values of all the other internationalization variables.

       LC_COLLATE

	      Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements within regular  expres-
	      sions and by the string comparison operators.

       LC_CTYPE
	      Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to
	      multi-byte characters in arguments) and the behavior of character classes within regular expressions.

       LC_MESSAGES
	      Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.

       NLSPATH
	      Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS

       Default.

STDOUT

       The expr utility shall evaluate the expression and write the result, followed by a <newline>, to standard output.

STDERR

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       The formation of the expression to be evaluated is shown in the following table. The symbols expr, expr1, and expr2  represent  expressions
       formed  from  integer  and string symbols and the expression operator symbols (all separate arguments) by recursive application of the con-
       structs described in the table. The expressions are listed in order of  increasing  precedence,	with  equal-precedence	operators  grouped
       between horizontal lines. All of the operators shall be left-associative.

						Expression	 Description
						expr1 | expr2	 Returns the evaluation of expr1 if it is
								 neither null nor zero; otherwise,
								 returns the evaluation of expr2 if it is
								 not null; otherwise, zero.
						expr1 & expr2	 Returns the evaluation of expr1 if nei-
								 ther expression evaluates to null or
								 zero; otherwise, returns zero.
								 Returns the result of a decimal integer
								 comparison if both arguments are inte-
								 gers; otherwise, returns the result of a
								 string comparison using the locale-spe-
								 cific collation sequence. The result of
								 each comparison is 1 if the specified
								 relationship is true, or 0 if the rela-
								 tionship is false.
						expr1 = expr2	 Equal.
						expr1 > expr2	 Greater than.
						expr1 >= expr2	 Greater than or equal.
						expr1 < expr2	 Less than.
						expr1 <= expr2	 Less than or equal.
						expr1 != expr2	 Not equal.
						expr1 + expr2	 Addition of decimal integer-valued argu-
								 ments.
						expr1 - expr2	 Subtraction of decimal integer-valued
								 arguments.
						expr1 * expr2	 Multiplication of decimal integer-valued
								 arguments.
						expr1 / expr2	 Integer division of decimal integer-val-
								 ued arguments, producing an integer
								 result.
						expr1 % expr2	 Remainder of integer division of decimal
								 integer-valued arguments.
						expr1 : expr2	 Matching expression; see below.
						( expr )	 Grouping symbols. Any expression can be
								 placed within parentheses. Parentheses
								 can be nested to a depth of
								 {EXPR_NEST_MAX}.
						integer 	 An argument consisting only of an
								 (optional) unary minus followed by dig-
								 its.
						string		 A string argument; see below.

   Matching Expression
       The ':' matching operator shall compare the string resulting from the evaluation of expr1 with the  regular  expression	pattern  resulting
       from  the evaluation of expr2. Regular expression syntax shall be that defined in the Base Definitions volume of IEEE Std 1003.1-2001, Sec-
       tion 9.3, Basic Regular Expressions, except that all patterns are anchored to the beginning of the string (that is, only sequences starting
       at  the first character of a string are matched by the regular expression) and, therefore, it is unspecified whether '^' is a special char-
       acter in that context. Usually, the matching operator shall return a string representing the number of characters matched (  '0'  on  fail-
       ure).  Alternatively,  if the pattern contains at least one regular expression subexpression "[(...)]" , the string corresponding to "1"
       shall be returned.

   String Operand
       A string argument is an argument that cannot be identified as an integer argument or as one of the expression operator symbols shown in the
       OPERANDS section.

       The use of string arguments length, substr, index, or match produces unspecified results.

EXIT STATUS

       The following exit values shall be returned:

	0     The expression evaluates to neither null nor zero.

	1     The expression evaluates to null or zero.

	2     Invalid expression.

       >2     An error occurred.

CONSEQUENCES OF ERRORS

       Default.

       The following sections are informative.

APPLICATION USAGE

       After argument processing by the shell, expr is not required to be able to tell the difference between an operator and an operand except by
       the value. If "$a" is '=' , the command:

	      expr $a = '='

       looks like:

	      expr = = =

       as the arguments are passed to expr (and they all may be taken as the '=' operator). The following works reliably:

	      expr X$a = X=

       Also note that this volume of IEEE Std 1003.1-2001 permits implementations to extend utilities. The expr utility permits the integer  argu-
       ments  to be preceded with a unary minus. This means that an integer argument could look like an option. Therefore, the conforming applica-
       tion must employ the "--" construct of Guideline 10 of the Base Definitions volume of IEEE Std 1003.1-2001, Section  12.2,  Utility  Syntax
       Guidelines  to  protect	its  operands  if  there is any chance the first operand might be a negative integer (or any string with a leading
       minus).

EXAMPLES

       The expr utility has a rather difficult syntax:

	* Many of the operators are also shell control operators or reserved words, so they have to be escaped on the command line.

	* Each part of the expression is composed of separate arguments, so liberal usage of <blank>s is required. For example:

							  Invalid	     Valid
							  expr 1+2	     expr 1 + 2
							  expr "1 + 2"	     expr 1 + 2
							  expr 1 + (2 * 3)   expr 1 + ( 2 * 3 )

       In many cases, the arithmetic and string features provided as part of the shell command language are easier to use than	their  equivalents
       in expr. Newly written scripts should avoid expr in favor of the new features within the shell; see Parameters and Variables and Arithmetic
       Expansion .

       The following command:

	      a=$(expr $a + 1)

       adds 1 to the variable a.

       The following command, for "$a" equal to either /usr/abc/file or just file:

	      expr $a : '.*/(.*)' | $a

       returns the last segment of a pathname (that is, file). Applications should avoid the character '/' used alone as  an  argument;  expr  may
       interpret it as the division operator.

       The following command:

	      expr "//$a" : '.*/(.*)'

       is  a better representation of the previous example. The addition of the "//" characters eliminates any ambiguity about the division opera-
       tor and simplifies the whole expression. Also note that pathnames may contain characters contained in the IFS variable and should be quoted
       to avoid having "$a" expand into multiple arguments.

       The following command:

	      expr "$VAR" : '.*'

       returns the number of characters in VAR.

RATIONALE

       In  an  early  proposal,  EREs were used in the matching expression syntax.  This was changed to BREs to avoid breaking historical applica-
       tions.

       The use of a leading circumflex in the BRE is unspecified because many historical implementations have treated it as a  special	character,
       despite their system documentation. For example:

	      expr foo : ^foo	  expr ^foo : ^foo

       return 3 and 0, respectively, on those systems; their documentation would imply the reverse. Thus, the anchoring condition is left unspeci-
       fied to avoid breaking historical scripts relying on this undocumented feature.

FUTURE DIRECTIONS

       None.

SEE ALSO

       Parameters and Variables , Arithmetic Expansion

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and  the  original  IEEE
       and  The  Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003								   EXPR(P)