Plan 9 - man page for rewrite (plan9 section 6)
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rewrite - mail rewrite rules
Mail(1) uses rewrite rules to convert mail destinations into commands used to dispose of
the mail. Each line of the file is a rule. Blank lines and lines beginning with # are
Each rewriting rule consists of (up to) 4 strings:
A regular expression in the style of regexp(6). The pattern is applied to mail
destination addresses. The pattern match is case-insensitive and must match the
type The type of rule; see below.
arg1 An ed(1) style replacement string, with \n standing for the text matched by the nth
arg2 Another ed(1) style replacement string.
In each of these fields the substring \s is replaced by the login id of the sender and the
substring \l is replaced by the name of the local machine.
When delivering a message, mail starts with the first rule and continues down the list
until a pattern matches the destination address. It then performs one of the following
actions depending on the type of the rule:
>> Append the mail to the file indicated by expanding arg1, provided that file appears
to be a valid mailbox.
| Pipe the mail through the command formed from concatenating the expanded arg1 and
alias Replace the address by the address(es) specified by expanding arg1 and recur.
Replace the address by the address(es) output by the command formed by expanding
arg1 and recur.
Mail expands the addresses recursively until each address has matched a >> or | rule or
until the recursion depth indicates a rewriting loop (currently 32).
If mail(1) is called with more than one address and several addresses match | rules and
result in the same expanded arg1, the message is delivered to all those addresses by a
single command, composed by concatenating the common expanded arg1 and each expanded arg2.
This mail bundling is performed to reduce the number of times the same message is trans-
mitted across a network. For example, with the following rewrite rule
([^!]*.att.com)!(.*) | "/mail/lib/qmail '\s' 'net!\1'" "'\2'"
if user presotto runs the command
% mail research.att.com!ken research.att.com!rob
there will follow only one execution of the command
/mail/lib/qmail presotto net!research.att.com ken rob
Here /mail/lib/qmail is an rc(1) script used for locally queuing remote mail.
In the event of an error, the disposition of the mail depends on the name of the command
executing the rewrite. If the command is called mail and is run by $user, the command
will print an error and deposit the message in /mail/box/$user/dead.letter. If the com-
mand is called rmail, usually because it was invoked to deliver mail arriving over the
network, the message will be returned to the sender. The returned message will appear to
have been sent by user postmaster.
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