Bourne tutorial
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+----------------------------------------------+ |Expression Output Exit Status | +----------------------------------------------+ |expr '' | '' 0 1
|expr '' | 0 0 1 | |expr '' | 1 1 0
|expr '' | A A 0 | |expr 0 | '' 0 1
|expr 0 | 1 1 0 | |expr 0 | A A 0
|expr 1 | anything 1 0 | |expr A | anything A 0
|expr '' & anything 0 1 | |expr 0 & anything 0 1
|expr 1 & '' 0 1 | |expr 1 & 0 0 1
|expr 1 & A 1 0 | |expr A & '' 0 1
|expr A & 0 0 1 | |expr A & 1 A 0
|expr A & B A 0 | +----------------------------------------------+ The string operator The last operator is the string operation. It is also the most complicated. The syntax is string : regular-expression The regular expression is assumed to have a "^" in front of it, so it always is aligned with the first character of the string. The output is the numbers of characters matched in the regular expression. That is, expr abc : abc outputs "3," while expr abc : abd outputs "0." As you can see, if it doesn't match, a zero is returned, the equivalent of "false." If a match occurs, a true value (non-zero) is returns, and you know how many characters match. This can be used to count characters, letters or numbers: # prints the number characters in variable a expr "$a" : '.*' # prints the number of lower case letters expr "$a" : '[a-z]*' # prints the number of lower case letters expr "$a" : '[0-9]*' Remember, the regular expression starts at the first character, so the last example will only count numbers at the beginning of the variable. If variable "a" has the value of "123abc" the expression would return a value of three. Expr returns the number of characters matched. Therefore, expr can be used to find the location of the first letter in a string: expr "$x" : '[^a-zA-Z]*[a-zA-Z]' Bourne Shell Tutorial http://www.grymoire.com/Unix/Sh.html 55 of 66 11/21/2011 12:03 PM Parenthesis can be used in the regular expression, like the substitute command in sed. If a match is found, the substring within the parenthesis is returned. Therefore, if the variable "a" has the value "123abc," the two commands below output the same string, which is "abc." # if $a = 123abc, then output 'abc' expr "$a" : '[0-9]*([a-z]*)' # same as above echo $a | sed 's/^[0-9]*([a-z]*)/1/' This feature is often used to extract part of a command line option. You should know that the parenthesis must have a back-slash before them, and expr must see them. That is, the backslash is not a signal to the shell that the following character is not a meta-character. The reason it must be excaped is to match the same syntax as sed, etc. Suppose you had a shell script with the following option myscript -x123 Now suppose you wanted to extract the number from the option. You could use a sed script like the one above. You could also use the expr command: x=`expr "$1" : '-x(.*)'` The expr command can also be used to test if a variable is a number: echo "Type in a number" read ans
number=`expr "$ans" : "([0-9]*)"` if [ "$number" != "$ans" ]; then echo "Not a number" elif [ "$number" -eq 0 ]; then echo "Nothing was typed" else
echo "$number is a fine number" fi You can combine tests. For instance, the following will truncate a string to four characters: expr "$x" : '(....)' | "$x" The results of each expression is another expression, so you can combine expressions. The following prints 32: expr 2 + 3 * 10 The expr command can even be used as a simple version of basename. The following will output the last part of a filename: expr "$x" : '.*/(.*)' '|' "$x" However, there is a bug in this code. If the variable "x" is equal to "/," then this evaluates to expr / : '.*/(.*)' '|' / If you remember, I pointed out this problem earlier. The shell assumes the slash is an operator, and not an Bourne Shell Tutorial http://www.grymoire.com/Unix/Sh.html 56 of 66 11/21/2011 12:03 PM expression. It complains about the syntax error. The solution is to place slashes before the variable: expr //$x : '.*/(.*)' Precedence of the Operators You can place parenthesis around expressions to construct more complex expressions. Make sure you quote them, as the shell treats parenthesis as special characters.. expr ( 2 + 3 ) * 10 The parenthesis overrides the default precedence, and the results is 50, to 32. These parenthesis are different than the ones used in regular expressions to perform a substitute operation. Expr should see the parenthesis without backslashes. The backslashes are for the shell. You could use quotes instead of backslashes. The natural precedence is grouped into six different levels, which are: +-------------------+ |Highest Precedence | +-------------------+ |
expr : expr | +-------------------+ | expr * expr | |
expr / expr | | expr % expr | +-------------------+ | expr + expr | |
expr - expr | +-------------------+ | expr = expr | | expr > expr | | expr >= expr | | expr < expr | | expr <= expr | |
expr != expr | +-------------------+ | expr expr | +-------------------+ | expr expr | +-------------------+ |Lowest Precedence | +-------------------+ Berkeley Extensions The Berkeley version of expr has three special functions: match substr
length The match operator acts like the colon. The substring operator acts like the awk function. The length operator returns the length of a string. I suggest you do not use them, for portability reasons. You do get this functionality if you place the directory "/usr/ucb" before either "/usr/bin" on Solaris, or "/usr/5bin" on SunOS. It doesn't matter if you use the build-in version, or the external version. Both versions support the Berkeley Bourne Shell Tutorial http://www.grymoire.com/Unix/Sh.html 57 of 66
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