Module 48 expr

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Module 48
expr

DESCRIPTION

The external expr command evaluates and compares expressions. An expression is composed of operands and operators. The operands are strings or integers; the operators are special characters used to inform expr how to interpret and compare the operands.

Operands and operators must be separated by blanks. Characters having special meaning to the shell must be escaped using the backslash (\) or quoted by single or double quotes. The parenthesis used for regular expression must always be preceded by backslashes. Strings containing blanks must be quoted to be considered one argument. You may precede integers with a minus sign ( “) to represent a negative number.

The expr command can be used to perform many functions but it should be used as a last resort command. The ksh and csh shell provide much faster and usually easier ways of performing calculations and comparisons.

COMMAND FORMAT

Following is the general format of the expr command.

 expr arg1 operator arg2 ... 

Multiple operators may be specified on the command line.

Arguments

The following list describes the arguments that may be passed to the expr command. The list is in decreasing precedence of the operators.

String Matching Operators

arg1 : arg2

Matching operator. Compares the characters in arg1 with the characters in arg2 . Returns the number of characters in arg1 matched by characters in arg2 . Arg1 should be a string and arg2 should be a regular expression recognized by the ed command, with the exception of the ^ character, because all expressions start at the beginning of arg2 . The following examples illustrate the use of the matching operator:

	$ expr string : str	# First 3 characters match
	3
	$ expr string : ing	# Last 3 match but comparison
		# must start at beginning of arg1
	$ expr string : strg	# Arg2 must match arg1 in its
		# entirety
	$ expr string : .*	# .* is a regular expression
	6	# that matches any number of
		# any characters
	$ expr string : .*i	# .*i matches any set of
	4	# characters ending with i
	You may find times when you need to return the matching portion of the string instead of the number of matching characters. You can use the \(...\) notation to perform this function. For example:
	$ expr string : \(.*\)	# Returns all chars in arg1
	string
	$ expr string : ..\(..\)	# Skips first 2 chars of
	ri	# arg1, returns next 2
	$ expr string : \(...\)	# Returns first 3 chars of
	str	# arg1
	$ expr string : .*\(...\)	# Returns last 3 chars of
	ing	# arg1
	$ expr string : ..\(.*\)..	Returns center of arg1
	ri	# first and last 2 chars
		# removed
	$ expr string : \(stx\)	# Returns nothing
	$ expr string : st\(.*\)	# Return all of arg1 after
	ring	# skipping st

Integer Arithmetic Operators      The following operators perform integer arithmetic. The returned value of a division operation is truncated to the largest integer. The multiplication, division, and modulus functions have equal precedence. The addition and subtraction operators have equal precedence but are lower in precedence than the previous three arithmetic operators. The * for multiplication must be escaped with a \ (backslash) or quoted to avoid shell interpretation.

arg1 \* arg2	Displays the value of arg1 multiplied by arg2 . For example,
	$ expr 80 \* 24
	1920
arg1 / arg2	Displays the integer value of arg1 divided by arg2. For example,
	$ expr 15 / 4
	3
	Notice the result of 15 / 4 is 3.75 but the value is truncated to the next lower number.
arg1 % arg2	Displays the remainder (modulus) of arg1 divided by arg2 . For instance,
	$ expr 15 % 4
	3
1 arg1 + arg2	Display the value of arg1 added to arg2 . For example,
	$ NUM=0
	$ expr $NUM + 1
	1
arg1 “ arg2	Display the value of arg2 subtracted from arg1 . For example,
	$ expr $NUM - 1

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