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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.
Following is the general format of the expr command.
expr arg1 operator arg2 ...
Multiple operators may be specified on the command line.
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|
|$ expr string : ing||# Last 3 match but comparison|
|0||# must start at beginning of arg1|
|$ expr string : strg||# Arg2 must match arg1 in its|
|$ 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|
|$ expr string : '..\(..\)'||# Skips first 2 chars of|
|ri||# arg1, returns next 2|
|$ expr string : '\(...\)'||# Returns first 3 chars of|
|$ expr string : '.*\(...\)'||# Returns last 3 chars of|
|$ expr string : '..\(.*\)..'||Returns center of arg1|
|ri||# first and last 2 chars|
|$ 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|
|arg1 / arg2||Displays the integer value of arg1 divided by arg2. For example,|
|$ expr 15 / 4|
|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|
|1arg1 + arg2||Display the value of arg1 added to arg2. For example,|
|$ expr $NUM + 1|
|arg1 - arg2||Display the value of arg2 subtracted from arg1. For example,|
|$ expr $NUM - 1|
Comparison Operators These operators are used to compare numbers or strings. A true condition displays a 1 and a false condition displays a 0. The < and > symbols must be escaped or quoted to prevent interpretation by the shell.
The return code from expr equals 0 if the expression is true. If the expression is false the return code is 1. Thus if expr displays a 0 the return code is 1. If a 1 is displayed the return code is 0.
|arg1 = arg2||Displays a 1 if arg1 and arg2 are equal. For example,|
|$ expr $NUM = $PREV||# NUM and PREV are not equal so|
|0||# 0 is displayed.|
|arg1 \> arg2||Displays a 1 if arg1 is greater than arg2.|
|$ expr dog \> cat||# Since d has a higher ASCII|
|1||# value than c a 1 is displayed.|
|arg1 \>= arg2||Displays a 1 if arg1 is greater than or equal to arg2.|
|arg1 \< arg2||Displays a 1 if arg1 is less than arg2.|
|arg1 \<= arg2||Displays a 1 if arg1 is less than or equal to arg2.|
|arg1 !> arg2||Displays a 1 if arg1 is not equal to arg2.|
|arg1 \& arg2||Returns the value of arg1 if arg1 and arg2 are not 0 or NUL; otherwise, 0 is returned. Arg1 can be another expression. For instance,|
|$ VAR=""||# Set VAR to NUL|
|$ PRT=""||# Set PRT to NUL|
|$ expr $VAR \& $PRT||# Return 0, arg1 and arg2|
|0||# are NUL|
|$ VAR="hplj"||# Set VAR to hplj printer|
|$ PRT="hpij"||# Set PRT to hpij printer|
|$ expr $VAR \& $PRT||# VAR is set, return its|
|arg1 \| arg2||Returns the value of arg1 if it is not 0 or NUL; otherwise, the value of arg2 is returned. Arg1 may consist of another expression (arg1 operand arg2). The following example reveals the syntax and use of this operand.|
|$ VAR=""||# Set variable VAR to NUL|
|$ expr $VAR \| undefined||# If VAR is NUL return|
|undefined||# the undefined string|
DIAGNOSTICS AND BUGS
The following error messages may be returned by expr:
|syntax error||If the operator or operand are not correct|
|nonnumeric argument||If a string is used in an arithmetic expression|
You must be careful in passing arguments to expr. It does not know the difference between operands and operators except by their values. Therefore, you may have to add a character to the operands for proper interpretation. For example,
A="+" B="+" expr $A = $B
would cause a problem, because expr interprets the + signs as operators instead of operands. To rectify this problem you can add some valid character to the operands:
A="x+" B="x+" expr $A = $B
Refer to the let commands described in Module 73.
The expr command writes its results to the standard output.
The following exit statuses are returned by expr:
|0||If the expression is not 0 or NUL|
|1||If the expression is 0 or NUL|
|2||If the expression is invalid|
You can use the expr command to perform arithmetic or string manipulation and making comparisons. It has been an old standby in the writing of shell scripts to perform numeric and string manipulation functions. With the arrival of the Korn shell and its many internal features, the expr command has lost ground in the number of uses it offers. The Korn shell's variable substitution and let command replaces many of the expr's functions. Although it can still be used to perform such needs, it is extremely slow when compared to the internal Korn shell commands.
In this activity you use the expr command to display the numbers from 1 to 10. This will demonstrate the speed of the expr command. Begin at the shell prompt.
NUM=1 while [ "$NUM" -le 10 ] do echo $NUM NUM=`expr $NUM + 1` done
|The csh code follows.|
set NUM=1 while ($NUM < 10) echo $NUM @NUM=$NUM+1 end
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