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The article introduces header editing with sed. Here's another sed example.

As the article shows, your system may make the first line of each incoming message a "From " separator. The email address and date on that line can be useful information, but it can be lost when the message is forwarded. Here's a procmail recipe with two sed commands to fix that:

# Copy "From user date" into a header field:
:0 f
| sed -e 1p -e '1s/^From/X-Envelope-From:/'

The procmail f flag means "use this recipe as a filter"; that is, "filter the message through this shell command, and don't consider the message delivered after this recipe completes." The first sed expression, -e 1p, tells sed "on line 1, execute print." This maintains the existing "From " separator by outputting a copy of the unedited line. Next, we edit the original line (not the copy we just outputted) so it becomes a header field. The second editing expression tells sed "on line 1, substitute the string From with the string X-Envelope-From:". (You can define your own legal header fields by starting the name with X-. So, to add random header fields to messages -- for tracking purposes, for instance -- precede the name with X- unless you're sure it's a valid field name.) So, our recipe will convert an incoming message from this:

From a@b Thu Jun 26 05:03:59 2003
Received: from mail.foo.xyz ([2.75.12.109])
...

to this:

From a@b Thu Jun 26 05:03:59 2003
X-Envelope-From: a@b Thu Jun 26 05:03:59 2003
Received: from mail.foo.xyz ([2.75.12.109])
...

sed Script Files

You can also put sed editing commands into a script file, and give its filename in your .procmailrc file. (Be sure, though, that the script file is on a filesystem that's available from the mail server or whatever host procmail is running on, though! For example, if the script file is on a filesystem available only over NFS, and the NFS server is down, the mail server may not be able to access the script.) Here's the revised recipe:

# Copy "From user date" into a header field:
:0 f
| sed -f /somedir/env-sender-field.sed

and the env-sender-field.sed script file would look like this:

1 {
    p
    s/^From/X-Envelope-From:/
}

That script-file syntax is especially handy when you want to apply a series of commands to the message header, which is matched by the address range 1,/^$/. You only need to write the address range once; all commands that apply to the header go between the curly-braces:

1,/^$/ {
    s/^Subject:/Subject: [spam] /
}

Trouble with Multi-line Header Fields

sed doesn't understand header fields that are continued onto another line. Matching multi-line fields is possible, but it's not very straightforward. To work around this, you can pipe messages through formail -c before piping them to sed.

You might want to use a scripting language with more features than sed. One advantage of sed, though, is that it's small and fast. Large program interpreters -- for Perl or Python, for example -- may take much longer to start and run, which could add a significant load (and delay) on a busy mail server host.

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