4.20 Conditional Statements

Conditional expressions, or conditionals for short, test some conditions and alter the control flow depending on the result. There are two kinds of conditional statements: if-else branches and switch statements.

The syntax of an if-else branching construct is:

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

Here, condition is an expression that governs control flow within the statement. Both then-body and else-body are lists of mailfromd statements. If condition is true, then-body is executed, if it is false, else-body is executed. The ‘else’ part of the statement is optional. The condition is considered false if it evaluates to zero, otherwise it is considered true. For example:

if $f = ""
  accept
else
  reject
fi

This will accept the message if the value of the Sendmail macro $f is an empty string, and reject it otherwise. Both then-body and else-body can be compound statements including other if statements. Nesting level of conditional statements is not limited.

To facilitate writing complex conditional statements, the elif keyword can be used to introduce alternative conditions, for example:

if $f = ""
  accept
elif $f = "root"
  echo "Mail from root!"
else
  reject
fi

Another type of branching instruction is switch statement:

switch condition
do
case x1 [or x2 …]:
  stmt1
case y1 [or y2 …]:
  stmt2
  .
  .
  .
[default:
  stmt]
done

Here, x1, x2, y1, y2 are literal expressions; stmt1, stmt2 and stmt are arbitrary mailfromd statements (possibly compound); condition is the controlling expression. The vertical dotted row represent another eventual ‘case’ branches.

This statement is executed as follows: the condition expression is evaluated and if its value equals x1 or x2 (or any other x from the first case), then stmt1 is executed. Otherwise, if condition evaluates to y1 or y2 (or any other y from the second case), then stmt2 is executed. Other case branches are tried in turn. If none of them matches, stmt (called the default branch) is executed.

There can be as many case branches as you wish. The default branch is optional. There can be at most one default branch.

An example of switch statement follows:

switch x
do
case 1 or 3:
  add "X-Branch" "1"
  accept
case 2 or 4 or 6:
  add "X-Branch" "2"
default:
  reject
done

If the value of mailfromd variable x is 2 or 3, it will accept the message immediately, and add a ‘X-Branch: 1’ header to it. If x equals 2 or 4 or 6, this code will add ‘X-Branch: 2’ header to the message and will continue processing it. Otherwise, it will reject the message.

The controlling condition of a switch statement may evaluate to numeric or string type. The type of the condition governs the type of comparisons used in case branches: for numeric types, numeric equality will be used, whereas for string types, string equality is used.