3.3 Conditional Execution

Programs consisting of a single action are rarely useful. In most cases you will want to do some checking and decide whether to process the message depending on its result. For example, if you do not want to accept messages from the address ‘<badguy@some.net>’, you could write the following program:

prog envfrom
do
  if $f = "badguy@some.net"
    reject
  else
    accept
  fi
done

This example illustrates several important concepts. First or all, $f in the third line is a Sendmail macro reference. Sendmail macros are referenced the same way as in sendmail.cf, with the only difference that curly braces around macro names are optional, even if the name consists of several letters. The value of a macro reference is always a string.

The equality operator (‘=’) compares its left and right arguments and evaluates to true if the two strings are exactly the same, or to false otherwise. Apart from equality, you can use the regular relational operators: ‘!=’, ‘>’, ‘>=’, ‘<’ and ‘<=’. Notice that string comparison in mailfromd is always case sensitive. To do case-insensitive comparison, translate both operands to upper or lower case (See tolower, and see toupper).

The if statement decides what actions to execute depending on the value its condition evaluates to. Its usual form is:

if expression then-body [else else-body] fi

The then-body is executed if the expression evaluates to true (i.e. to any non-zero value). The optional else-body is executed if the expression yields false (i.e. zero). Both then-body and else-body can contain other if statements, their nesting depth is not limited. To facilitate writing complex conditional statements, the elif keyword can be used to introduce alternative conditions, for example:

prog envfrom
do
  if $f = "badguy@some.net"
    reject
  elif $f = "other@domain.com"
    tempfail 470 "Please try again later"
  else
    accept
  fi
done

See switch, for more elaborate forms of conditional branching.