expr

Evaluate expression.
expr evaluates expression and writes the result to standard output.

Syntax
      expr expression

Options
     Operators are listed below in order of increasing precedence.  Operators
     with equal precedence are grouped within { } symbols.

     expr1 | expr2
       Returns the evaluation of expr1 if it is neither an empty string
       nor zero; otherwise, returns the evaluation of expr2.

     expr1 & expr2
       Returns the evaluation of expr1 if neither expression evaluates
       to an empty string or zero; otherwise, returns zero.

     expr1 {=, >, >=, <, <=, !=} expr2
       Returns the results of integer comparison if both arguments are
       integers; otherwise, returns the results of string comparison
       using the locale-specific collation sequence.  The result of each
       comparison is 1 if the specified relation is true, or 0 if the
       relation is false.

     expr1 {+, -} expr2
       Returns the results of addition or subtraction of integer-valued
       arguments.

     expr1 {*, /, %} expr2
       Returns the results of multiplication, integer division, or
       remainder of integer-valued arguments.

     expr1 : expr2
       The `:' operator matches expr1 against expr2, which must be a
       regular expression.  The regular expression is anchored to the
       beginning of  the string with an implicit `^'.  expr expects
       "basic" regular expressions, see re_format(7) for more informa-
       tion on regular expressions.

       If the match succeeds and the pattern contains at least one regu-
       lar expression subexpression `\(...\)', the string correspond-
       ing to `\1' is returned; otherwise the matching operator
       returns the number of characters matched.  If the match fails and
       the pattern contains a regular expression subexpression the null
       string is returned; otherwise 0.

     Parentheses are used for grouping in the usual manner.

Examples

# A partial match will return the number of characters that match:
$ expr ss64 : ss6
3

# The condition in string 2 must entirely match string 1
$ expr ss64 : ss7
0

# Adding numbers
$ expr 5 + 2
7

# When multiplying the * has to be escaped
$ expr 5 \* 3
15

# Incrementing a variable (arithmetic expansion)
$ demo=1
$ demo=`expr $demo + 1`
$ echo $demo
2
$ demo=`expr $demo + 1`
$ echo $demo
3

Return the filename portion of a pathname stored in variable a. The // characters act to eliminate ambiguity with the division operator.
 $ expr //$a : '.*/\(.*\)'

Return the number of characters in variable $demo.
$ demo=ss64
$ expr $demo : '.*'

Exit Status
expr exits with one of the following values:
0 the expression is neither an empty string nor 0.
1 the expression is an empty string or 0.
2 the expression is invalid..

"Silence is the perfect expression of scorn" ~ George Bernard Shaw (Back to Methuselah, 1921)

Related:

expr man page - Apple.com
awk - Find and Replace text within file(s)
eval - Evaluate several commands/arguments
for - Loop, expand words, and execute commands
test - Evaluate a conditional expression


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