formail

TLDR

Extract headers from email

$ formail -x [Subject:] < [email.txt]

Add header to email

$ formail -a "[X-Custom: value]" < [email.txt]

Split mbox into messages

$ formail -s [command] < [mbox]

Extract From line

$ formail -x From: < [email.txt]

Force addition of header

$ formail -A "[X-Custom: value]" < [email.txt]

Generate auto-reply

$ formail -r < [email.txt]

SYNOPSIS

formail [options] [output] < input

formail is a mail processing utility from the procmail suite. It extracts headers, adds or modifies headers, splits mbox files, and helps generate automated replies.
The tool reads from stdin and writes to stdout, designed for use in pipelines and procmail recipes. It handles RFC 822 mail format intricacies.

PARAMETERS

-x header

Extract specific header.

-X header

Extract header with continuation lines.

-a header

Add header if not present.

-A header

Add header always.

-i header

Replace header.

-u header

Make header unique.

-r

Generate reply headers.

-s command

Split and process with command.

-b

Don't escape body "From " lines.

-n count

Output only first n messages.

EXAMPLES

$ # Split mbox and save each message
formail -s procmail < mbox

# Extract sender address
formail -x From: < message | head -1

# Add header for filtering
formail -a "X-Filtered: yes" < message

CAVEATS

Part of procmail suite which is no longer actively developed. MIME handling is limited. Complex headers may need careful extraction. Consider modern alternatives for new projects.

HISTORY

formail was developed by Stephen van den Berg as part of the procmail mail processing suite in the 1990s. While procmail is no longer actively maintained, formail remains useful for mail processing scripts.