O'Reilly logo

Sendmail, 3rd Edition by Bryan Costales

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

Comments

Comments provide you with the documentation necessary to maintain the configuration file. Because comments slow down sendmail by only a negligible amount, and only at startup, it is better to overcomment than to undercomment.

Blank lines and lines that begin with a # character are considered comments and are ignored. A blank line is one that contains no characters at all (except for its terminating newline). Indentation characters (spaces and tabs) are invisible and can turn an apparently blank line into an empty-looking line, which is not ignored:

# text			 a comment
tabtext a continuation line 
			 a blank line
tab an "empty-looking line"

Except for two special cases, pre-V8 comments occupy the entire line. The two special cases are the R and S configuration commands. The R command is composed of three tab-separated fields, the third field being a comment that does not require a leading # character:

Rlhs      rhs       comment

The pre-V8.7 S command looks only for a number following it and ignores everything else, so it can also be followed by a comment:

S3 this is a comment

Prior to V8, no other commands allow comments to follow on the same line:

CWlocalhost mailhost  # This won't work prior to V8

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required