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Chapter 31 Defined Macros |
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Chapter 31 Defined Macros |
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Occasionally, it is necessary to test a macro to see whether a value has been assigned to it. To perform such a test, a special prefix and two operators are used. The general form is
if
else
endif
$?x text1 $| text2 $. 
if
x
is defined if
x
is not defined
This expression yields one of two possible values:
text1
if the macro named
x
has a value,
text2
if it doesn't. The entire above expression, starting with the
$?
and ending with the
$.
, yields a single value, which may contain multiple tokens.
The following, for example, includes the configuration-file version in the SMTP greeting message but does so only if that version (in
$v
; see
Section 31.10.38
) is defined:
O SmtpGreetingMessage=$j Sendmail ($v/$?Z$Z$|generic$.) ready at $b
Here the parenthetical version information is expressed one way if
Z
has a value (like
1.4
):
($v/$Z)
but is expressed differently if
Z
lacks a value:
($v/generic)
The
else
part (
$|
) of this conditional expression is optional. If it is omitted, the result is the same as if the
text2
were omitted:
$?xtext1$|$. $?xtext1$.
Both of the preceding yield the same result. If
x
has a value, then
text1
becomes the value of the entire expression. If
x
lacks a value, then the entire expression lacks a value (produces no tokens).
Note that it is
not
advisable to use the
$?
conditional expression in rules. It may not have the intended effect, because macro conditionals are expanded when the configuration file is read.
V8 sendmail allows conditionals to nest. To illustrate, consider the following expression:
$?x $?y both $| xonly $. $| $?y yonly $| none $. $.
This is just like the example in the previous section:
$?x text1 $| text2 $.
except that
text1
and
text2
are both conditionals:
text1 = $?y both $| xonly $. text2 = $?y yonly $| none $.
The grouping when conditionals nest is from the outside in. In the following example, parentheses have been inserted to show the groupings (they are not a part of either expression):
($?x(text1)$|(text2)$.)($?x($?y both $| xonly $.)$|($?y yonly $| none $.)$.)
Interpretation is from left to right. The logic of the second line above is therefore this: If both
$x
and
$y
have values, the result is
both
. If
$x
has a value but
$y
lacks one, the result is
xonly
. If
$x
lacks a value but
$y
has one, the result is
yonly
. And if both lack values, the result is
none
.
The
sendmail
program does not enforce or check for balance in nested conditionals. Each
$?
should have a corresponding
$.
to balance it. If they do not balance,
sendmail
will not detect the problem. Instead, it may interpret the expression in a way that you did not intend.
The depth to which conditionals may be nested is limited only by our ability to easily comprehend the result. More than two deep is not recommended, and more than three deep is vigorously discouraged.
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| 31.5 Macro Expansion: $ and $& |
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31.7 Categories of Macros |