[Chapter 45] 45.28 Quick Reference: 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)


45.27 Turn Off echo for "Secret" Answers		45.29 Testing Characters in a String with expr