Linux Pocket Guide, 2nd Edition

Name

expr — stdin  stdout  - file  -- opt  --help  --version

Synopsis

expr expression

The expr command does simple math (and other expression evaluation) on the command line:

$ expr 7 + 3
10
$ expr '(' 7 + 3 ')' '*' 14   Special shell characters are quoted
140
$ expr length ABCDEFG
7
$ expr 15 '>' 16
0                           Meaning false

Each argument must be separated by whitespace. Notice that we had to quote or escape any characters that have special meaning to the shell. Parentheses (escaped) may be used for grouping. Operators for expr include:

Operator	Numeric operation	String operation
+	Addition
-	Subtraction
*	Multiplication
/	Integer division
%	Remainder (modulo)
<	Less than	Earlier in dictionary.
<=	Less than or equal	Earlier in dictionary, or equal.
>	Greater than	Later in dictionary.
>=	Greater than or equal	Later in dictionary, or equal.
=	Equality	Equality.
!=	Inequality	Inequality.
|	Boolean “or”	Boolean “or”.
&	Boolean “and”	Boolean “and”.
s : regexp		Does the regular expression regexp match string s?
substr s p n		Print n characters of string s, beginning at position p. (p =1 is the first character.)
index s chars		Return the index of the first position in string s containing a character from string chars. Return 0 if not found. Same behavior as the C function index( ).

For Boolean expressions, the number 0 and the empty string are considered false; any other value is true. For Boolean results, 0 is false and 1 is true. ...