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Previous: 45.27 Turn Off echo for "Secret" Answers Chapter 45
Shell Programming for the Initiated
Next: 45.29 Testing Characters in a String with expr
 

45.28 Quick Reference: expr

expr
expr is a very handy tool in shell programming, since it provides the ability to evaluate a wide range of arithmetic, logical, and relational expressions. It evaluates its arguments as expressions and prints the result. expr is a standard UNIX utility; the GNU version is on the CD-ROM.

Here's the syntax. The [brackets] mean "optional"; don't type the brackets:

expr arg1 operator arg2 [ operator arg3 ... ]

Arguments and operators must be separated by spaces. In many cases, an argument is an integer, typed literally or represented by a shell variable. There are three types of operators: arithmetic, relational, and logical.

Exit status ( 44.7 ) values for expr are 0 if the expression evaluates non-zero and non-null, 1 if the expression evaluates to 0 or null, or 2 if the expression is invalid.

Arithmetic operators

Use these to produce mathematical expressions whose results are printed.

+

Add arg2 to arg1 .

-

Subtract arg2 from arg1 .

*

Multiply the arguments.

/

Divide arg1 by arg2 .

%

Take the remainder when arg1 is divided by arg2 .

Addition and subtraction are evaluated last, unless they are grouped inside parentheses. The symbols * , ( , and ) have meaning to the shell, so they must be escaped (preceded by a backslash or enclosed in quotes).

Relational operators

Use these to compare two arguments. Arguments can also be words, in which case comparisons assume a < z and A < Z. If the comparison statement is true, expr writes 1 to standard output ( 13.1 ) ; if false, it writes 0. The symbols > and < must be escaped.

=

Are the arguments equal?

!=

Are the arguments different?

>

Is arg1 greater than arg2 ?

>=

Is arg1 greater than or equal to arg2 ?

<

Is arg1 less than arg2 ?

<=

Is arg1 less than or equal to arg2 ?

Logical operators

Use these to compare two arguments. Depending on the values, the result written to standard output can be arg1 (or some portion of it), arg2 , or 0. The symbols | and & must be escaped.

|

Logical OR; if arg1 has a non-zero (and non-null) value, the output is arg1 ; otherwise, the output is arg2 .

&

Logical AND; if both arg1 and arg2 have a non-zero (and non-null) value, the output is arg1 ; otherwise, the output is 0.

:

Sort of like grep ( 27.1 ) ; arg2 is a pattern to search for in arg1 . arg2 must be a regular expression in this case. If the arg2 pattern is enclosed in \( \) , the output is the portion of arg1 that matches; otherwise, the output is simply the number of characters that match. A pattern match always applies to the beginning of the argument (the ^ symbol is assumed by default).

45.28.1 Examples

Division happens first; output is 10:

$ 

expr 5 + 10 / 2

Addition happens first; output is 7 (truncated from 7.5):

$ 

expr \( 5 + 10 \) / 2

Add 1 to variable i ; this is how variables are incremented in Bourne shell scripts:



i=`expr "$i" + 1`

Output 1 (true) if variable a is the string "hello":

$ 

expr "$a" = hello

Output 1 (true) if variable b plus 5 equals 10 or more:

$ 

expr "$b" + 5 \>= 10

In the examples below, variable p is the string "version.100". This command returns the number of characters in p :



$ 

expr "$p" : '.*'

   
Output is 11

Match all characters and print them:

$ 

expr "$p" : '\(.*\)'

   
Output is "version.100"

Output the number of lowercase letters matched:

$ 

expr "$p" : '[a-z]*'

   
Output is 7

Match a string of lowercase letters:

$ 

expr "$p" : '\([a-z]*\)'

   
Output is \"version"

Truncate $x if it contains five or more characters; if not, just output $x . (Logical OR uses the second argument when the first one is 0 or null; i.e., when the match fails.)

$ 

expr "$x" : '\(.....\)'  "$x"

- DG from O'Reilly & Associates' UNIX in a Nutshell (SVR4/Solaris)


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