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MHFIXMSG(1mh) MHFIXMSG(1mh)

NAME
mhfixmsg - nmh's MIME-email rewriter with various transformations

DESCRIPTION
mhfixmsg rewrites MIME messages, applying specific transformations such as decoding of MIME-
encoded message parts and repairing invalid MIME headers.

MIME messages are specified in RFC 2045 to RFC 2049 (see mhbuild(1)). The mhlist command is
invaluable for viewing the content structure of MIME messages. mhfixmsg passes non-MIME mes‐
sages through without any transformations. If no transformations apply to a MIME message,
the original message or file is not modified or removed. Thus, mhfixmsg can safely be run
multiple times on a message.

The -decodetext switch enables a transformation to decode each base64 and quoted-printable
text message part to the selected 8-bit, 7-bit, or binary encoding. If 7-bit is selected for
a base64 part but it will only fit 8-bit, as defined by RFC 2045, then it will be decoded to
8-bit quoted-printable. Similarly, with 8-bit, if the decoded text would be binary, then the
part is not decoded (and a message will be displayed if -verbose is enabled). Note that -de‐‐
codetext binary can produce messages that are not RFC 2045 compliant.

When the -decodetext switch is enabled, each carriage return character that precedes a line‐
feed character is removed from text parts encoded in ASCII, ISO-8859-x, UTF-8, or Win‐
dows-12xx.

The -decodetypes switch specifies the message parts, by type and optionally subtype, to which
-decodetext applies. Its argument is a comma-separated list of type/subtype elements. If an
element does not contain a subtype, then -decodetext applies to all subtypes of the type.
The default is -decodetypes text; it can be overridden, e.g., with -decodetypes text/plain to
restrict -decodetext to just text/plain parts.

By default, carriage return characters are preserved or inserted at the end of each line of
text content. The -crlflinebreaks switch selects this behavior and is enabled by default.
The -nocrlflinebreaks switch causes carriage return characters to be stripped from, and not
inserted in, text content when it is decoded and encoded. Note that its use can cause the
generation of MIME messages that do not conform to RFC 2046, §4.1.1, paragraph 1.

The -textcharset switch specifies that all text/plain parts of the message(s) should be con‐
verted to charset. Charset conversions require that nmh be built with iconv(3); see the mh‐‐
param(1) man page for how determine whether your nmh installation includes that. To convert
text parts other than text/plain, an external program can be used, via the -reformat switch.

The -reformat switch enables a transformation for text parts in the message. For each text
part that is not text/plain and that does not have a corresponding text/plain in a multi‐
part/alternative part, mhfixmsg looks for a mhfixmsg-format-text/subtype profile entry that
matches the subtype of the part. If one is found and can be used to successfully convert the
part to text/plain, mhfixmsg inserts that text/plain part at the beginning of the containing
multipart/alternative part, if present. If not, it creates a multipart/alternative part.

With the -reformat switch, multipart/related parts are handled differently than multipart/al‐
ternative. If the multipart/related has only a single part that is not text/plain and can be
converted to text/plain, a text/plain part is added and the type of the part is changed to
multipart/alternative. If the multipart/related has more than one part but does not have a
text/plain part, mhfixmsg tries to add one.

The -replacetextplain switch broadens the applicability of -reformat, by always replacing a
corresponding text/plain part, if one exists. If -verbose is enabled, the replacement will
be shown as two steps: a removal of the text/plain part, followed by the usual insertion of
a new part.

-reformat requires a profile entry for each text part subtype to be reformatted. The mh‐
fixmsg-format-text/subtype profile entries are based on external conversion programs, and are
used in the same way that mhshow uses its mhshow-show-text/subtype entries. When nmh is in‐
stalled, it searches for a conversion program for text/html content, and if one is found, in‐
serts a mhfixmsg-format-text/html entry in /etc/nmh/mhn.defaults. An entry of the same name
in the user's profile takes precedence. The user can add entries for other text subtypes to
their profile.

The -fixboundary switch enables a transformation to repair the boundary portion of the Con‐
tent-Type header field of the message to match the boundaries of the outermost multipart part
of the message, if it does not. That condition is indicated by a “bogus multipart content in
message” error message from mhlist and other nmh programs that parse MIME messages.

The -fixcte switch enables a transformation to change the Content-Transfer-Encoding from an
invalid value to 8-bit in message parts with a Content-Type of multipart and message, as re‐
quired by RFC 2045, §6.4. That condition is indicated by a “must be encoded in 7bit, 8bit,
or binary” error message from mhlist and other nmh programs that parse MIME messages.

The -fixtype switch ensures that each part of the message has the correct MIME type shown in
its Content-Type header. It may be repeated. It is typically used to replace “applica‐
tion/octet-stream” with a more descriptive MIME type. It may not be used for multipart and
message types.

mhfixmsg applies two transformations unconditionally. The first removes an extraneous trail‐
ing semicolon from the parameter lists of MIME header field values. The second replaces RFC
2047 encoding with RFC 2231 encoding of name and filename parameters in Content-Type and Con‐
tent-Disposition header field values, respectively.

The -verbose switch directs mhfixmsg to output informational message for each transformation
applied.

The return status of mhfixmsg is 0 if all of the requested transformations are performed, or
non-zero otherwise. (mhfixmsg will not decode to binary content with the default -decodetext
setting, but a request to do so is not considered a failure, and is noted with -verbose.) If
a problem is detected with any one of multiple messages such that the return status is non-
zero, then none of the messages will be modified.

The -file file switch directs mhfixmsg to use the specified file as the source message,
rather than a message from a folder. Only one file argument may be provided. The -file
switch is implied if file is an absolute pathname. If the file is “-”, then mhfixmsg accepts
the source message on the standard input stream. If the -outfile switch is not enabled when
using the standard input stream, mhfixmsg will not produce a transformed output message.

mhfixmsg, by default, transforms the message in place. If the -outfile switch is enabled,
then mhfixmsg does not modify the input message or file, but instead places its output in the
specified file. An outfile name of “-” specifies the standard output stream.

Combined with the -verbose switch, the -outfile switch can be used to show what transforma‐
tions mhfixmsg would apply without actually applying them, e.g.,

mhfixmsg -outfile /dev/null -verbose

As always, this usage obeys any mhfixmsg switches in the user's profile.

-outfile can be combined with rcvstore to add a single transformed message to a different
folder, e.g.,

mhfixmsg -outfile - | \
/usr/lib/mh/rcvstore +folder

Summary of Applicability
The transformations apply to the parts of a message depending on content type and/or encoding
as follows:

-decodetext base64 and quoted-printable encoded text parts
-decodetypes limits parts to which -decodetext applies
-crlflinebreaks text parts
-textcharset text/plain parts
-reformat text parts that are not text/plain
-fixboundary outermost multipart part
-fixcte multipart or message part
-fixtype all except multipart and message parts

Backup of Original Message/File
If it applies any transformations to a message or file, and the -outfile switch is not used,
mhfixmsg backs up the original the same way as rmm. That is, it uses the rmmproc profile
component, if present. If not present, mhfixmsg moves the original message to a backup file.
The -rmmproc switch may be used to override this profile component. The -normmproc switch
disables the use of any rmmproc profile component and negates all prior -rmmproc switches.

Integration with inc
mhfixmsg can be used as an add-hook, as described in /usr/share/doc/nmh/README-HOOKS. Note
that add-hooks are called from all nmh programs that add a message to a folder, not just inc.
Alternatively, a simple shell alias or function can be used to call mhfixmsg immediately af‐
ter a successful invocation of inc. One approach could be based on:

msgs=`inc -format '%(msg)'` && [ -n "$msgs" ] && scan $msgs && mhfixmsg -nochange‐
cur $msgs

Another approach would rely on adding a sequence to Unseen-Sequence, which inc sets with the
newly incorporated messages. Those could then be supplied to mhfixmsg. An example is shown
below.

Integration with procmail
By way of example, here is an excerpt from a procmailrc file that filters messages through
mhfixmsg before storing them in the user's nmh-workers folder. It also stores the incoming
message in the Backups folder in a filename generated by mkstemp, which is a non-POSIX util‐
ity to generate a temporary file. Alternatively, mhfixmsg could be called on the message af‐
ter it is stored.

PATH = /usr/bin/mh:$PATH
LANG = en_US.utf8
MAILDIR = `mhparam path`
#### The Backups directory is relative to MAILDIR.
MKSTEMP = 'mkstemp -directory Backups -prefix mhfixmsg'
MHFIXMSG = 'mhfixmsg -noverbose -file - -outfile -'
STORE = /usr/lib/mh/rcvstore

:0 w: nmh-workers/procmail.$LOCKEXT
* ^TOnmh-workers AT nongnu.org
| tee `$MKSTEMP` | $MHFIXMSG | $STORE +nmh-workers

EXAMPLES
Basic usage
To run mhfixmsg on the current message in the current folder, with default transformations to
fix MIME boundaries and Content-Transfer-Encoding, to decode text and application/ics content
parts to 8 bit, and to add a corresponding text/plain part where lacking:

mhfixmsg -verbose

Specified folder and messages
To run mhfixmsg on specified messages, without its informational output:

mhfixmsg +inbox last:4

View without modification
By default, mhfixmsg transforms the message in place. To view the MIME structure that would
result from running mhfixmsg on the current message, without modifying the message:

mhfixmsg -outfile - | mhlist -file -

Search message without modification
To search the current message, which possibly contains base64 or quoted printable encoded
text parts, without modifying it, use the -outfile switch:

mhfixmsg -outfile - | grep pattern

-outfile can be abbreviated in usual MH fashion, e.g., to -o. The search will be on the en‐
tire message, not just text parts.

Translate text/plain parts to UTF-8
To translate all text/plain parts in the current message to UTF-8, in addition to all of the
default transformations:

mhfixmsg -textcharset utf-8

Fix all messages in a folder
To run mhfixmsg on all of the messages in a folder:

mhfixmsg +folder all

Alternatively, mhfixmsg can be run on each message separately, e.g., using a Bourne shell
loop:

for msg in `pick +folder`; do mhfixmsg +folder $msg; done

The two appearances of the +folder switch in that command protect against concurrent context
changes by other nmh command invocations.

Run on newly incorporated messages
To run mhfixmsg on messages as they are incorporated:

inc && mhfixmsg -nochangecur unseen

This assumes that the Unseen-Sequence profile entry is set to unseen, as shown in mh-pro‐
file(5).

FILES
mhfixmsg looks for mhn.defaults in multiple locations: absolute pathnames are accessed di‐
rectly, tilde expansion is done on usernames, and files are searched for in the user's Mail
directory as specified in their profile. If not found there, the directory “/etc/nmh” is
checked.

$HOME/.mh_profile The user profile
/etc/nmh/mhn.defaults Default mhfixmsg conversion entries

PROFILE COMPONENTS
Path: To determine the user's nmh directory
Current-Folder: To find the default current folder
rmmproc: Program to delete original messages or files

SEE ALSO
iconv(3), inc(1), mh-mkstemp(1), mh-profile(5), mhbuild(1), mhlist(1), mhparam(1), mhshow(1),
procmail(1), procmailrc(5), rcvstore(1), rmm(1)

DEFAULTS
`+folder' defaults to the current folder
`msgs' defaults to cur
`-decodetext 8bit'
`-decodetypes text,application/ics'
`-crlflinebreaks'
`-notextcharset'
`-reformat'
`-noreplacetextplain'
`-fixboundary'
`-fixcte'
`-changecur'
`-noverbose'

CONTEXT
If a folder is given, it will become the current folder. The last message selected from a
folder will become the current message, unless the -nochangecur switch is enabled. If the
-file switch or an absolute pathname is used, the context will not be modified.

nmh-1.7.1 2016-11-08 MHFIXMSG(1mh)