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doc/README.variables

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+GENERAL ENCHANCEMENTS TO EXTENSION LOGIC : 
+
+QUOTING: 
+
+exten => s,5,BackGround,blabla
+
+The parameter (blabla) can be quoted ("blabla"). In this case, a 
+comma does not terminate the field. 
+
+Also, characters special to variable substitution, expression evaluation, etc
+(see below), can be quoted. For example, to literally use a $ on the 
+string "$1231", quote it with a preceeding \. Special characters that must
+be quoted to be used, are [ ] $ " \. (to write \ itself, use \\). 
+
+VARIABLES: 
+
+Parameter strings can include variables. Variable names are arbitrary strings. 
+They are stored in the respective channel structure. 
+
+To set a variable to a particular value, do : 
+
+;exten => 1,2,SetVar,varname=value
+
+You can substitute the value of a variable everywhere using ${variablename}.
+For example, to stringwise append $lala to $blabla and store result in $koko, 
+do: 
+
+;exten => 1,2,SetVar,koko=${blabla}${lala}
+
+There are also the following special variables: 
+
+${CALLERID}	Caller ID
+${EXTEN}	Current extension
+${CONTEXT}      Current context
+${PRIORITY}	Current priority
+
+There are two reference modes - reference by value and reference by name. 
+To refer to a variable with its name (as an argument to a function that 
+requires a variable), just write the name. To refer to the variable's value, 
+enclose it inside ${}. For example, SetVar takes as the first argument 
+(before the =) a variable name, so: 
+
+;exten => 1,2,SetVar,koko=lala
+;exten => 1,3,SetVar,${koko}=blabla
+
+stores to the variable "koko" the value "lala" and to variable "lala" the 
+value "blabla". 
+
+In fact, everything contained ${here} is just replaced with the value of 
+the variable "here". 
+
+EXPRESSIONS: 
+
+Everything contained inside a bracket pair prefixed by a $ (like $[this]) is 
+considered as an expression and it is evaluated. Evaluation works similar to 
+(but is done on a later stage than) variable substitution: the expression 
+(including the square brackets) is replaced by the result of the expression 
+evaluation. The arguments and operands of the expression MUST BE separated 
+with spaces (take care NOT to leave ANY spaces between opening and closing 
+square brackets and the first and last arguments). 
+
+For example, after the sequence: 
+
+exten => 1,1,SetVar,"lala=$[1 + 2]";
+exten => 1,2,SetVar,"koko=$[2 * ${lala}]";
+
+the value of variable koko is "6".
+
+Operators are listed below in order of increasing precedence.  Operators
+with equal precedence are grouped within { } symbols.
+
+     expr1 | expr2
+             Return the evaluation of expr1 if it is neither an empty string
+             nor zero; otherwise, returns the evaluation of expr2.
+
+     expr1 & expr2
+             Return the evaluation of expr1 if neither expression evaluates to
+             an empty string or zero; otherwise, returns zero.
+
+     expr1 {=, >, >=, <, <=, !=} expr2
+             Return 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
+             Return the results of addition or subtraction of integer-valued
+             arguments.
+
+     expr1 {*, /, %} expr2
+             Return 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 `^'.
+
+             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.
+
+The parser must be parsed with bison (bison is REQUIRED - yacc cannot 
+produce pure parsers, which are reentrant) 
+
+CONDITIONALS
+
+There is one conditional operator - the conditional goto : 
+
+;exten => 1,2,gotoif,condition?label1:label2
+
+If condition is true go to label1, else go to label2. Labels are interpreted
+exactly as in the normal goto command.
+
+"condition" is just a string. If the string is empty or "0", the condition
+is considered to be false, if it's anything else, the condition is true. 
+This is designed to be used together with the expression syntax described 
+above, eg : 
+
+exten => 1,2,gotoif,$[${CALLERID} = 123456]?2|1:3|1
+
+
+Example of use : 
+
+exten => s,2,SetVar,"vara=1"
+exten => s,3,SetVar,"varb=$[${vara} + 2]"
+exten => s,4,SetVar,"varc=$[${varb} * 2]"
+exten => s,5,GotoIf,"$[${varc} = 6]?99|1:s|6";
+