Sendmail, 3rd Edition

Macro Conditionals: $?, $|, and $.

Occasionally, it is necessary to test a sendmail 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 expression, starting with the $? and ending with the $., yields a single value, which can 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 $Z; see $Z) is defined:

O SmtpGreetingMessage=$j Sendmail ($v/$?Z$Z$|generic$.) ready at $b
                                                ↑note

Here, the parenthetical version information is expressed one way if $Z has a value (such as 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, text1 becomes the value of the entire expression. If x lacks a value, the entire expression lacks a value (produces no tokens).

Note that it is not advisable to use the $? conditional expression in rules. Such a use can have other than the intended effect because macro conditionals are expanded when the configuration file is read.

Conditionals ...