mhfixmsg - nmh's MIME-email rewriter with various transformations
mhfixmsg [-help] [-version] [+folder] [msgs | absolute pathname | -file file] [-decodetext 8bit|7bit|binary | -nodecodetext] [-decodetypes type/[subtype][,...]] [-decodeheaderfieldbodies utf-8 | -nodecodeheaderfieldbodies] [-crlflinebreaks | -nocrlflinebreaks] [-textcharset charset | -notextcharset] [-reformat | -noreformat] [-replacetextplain | -noreplacetextplain] [-fixboundary | -nofixboundary] [-fixcte | -nofixcte] [-checkbase64 | -nocheckbase64] [-fixtype mimetype] [-outfile outfile] [-rmmproc program] [-normmproc] [-changecur | -nochangecur] [-verbose | -noverbose]
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 messages 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 -decodetext binary can produce messages that are not compliant with RFC 5322, §2.1.1.
When the -decodetext switch is enabled, each carriage return character that precedes a linefeed character is removed from text parts encoded in ASCII, ISO-8859-x, UTF-8, or Windows-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 -decodetypes includes text; it can be overridden, e.g., with -decodetypes text/plain to restrict -decodetext to just text/plain parts.
The -decodeheaderfieldbodies switch enables decoding of header field bodies to the specified character set. The -nodecodeheaderfieldbodies inhibits this transformation. The transformation can produce a message that does not conform with RFC 2047, §1, paragraph 6, because the decoded header field body could contain unencoded non-ASCII characters. It is therefore not enabled by default. Decoding of most header field bodies, or to a character set that is different from that of the user's locale, requires that nmh be built with iconv(3); see mhparam(1) for how to determine whether your nmh installation includes that.
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 with RFC 2046, §4.1.1, paragraph 1.
The -textcharset switch specifies that all text/plain parts of the message(s) should be converted to charset. Charset conversions require that nmh be built with iconv(3); see mhparam(1) for how to 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 -textcharset switch can also be used, depending on the nmh installation as described below, to specify the Content-Type charset parameter for text/plain parts added with -reformat.
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 multipart/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/alternative. 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 mhfixmsg-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 installed, it searches for a conversion program for text/html content, and if one is found, inserts 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 character set (charset) of text/plain parts added by -reformat is determined by the external program that generates the content. Detection of the content charset depends on how the nmh installation was configured. If a program, such as file with a --mime-encoding option, was found that can specify the charset of a file, then that will be used for the Content-Type charset parameter. To determine if your nmh was so configured, run mhparam mimeencodingproc and see if a non-empty string is displayed.
If your nmh was not configured with a program to determine the charset of a file, then the value of the -textcharset switch is used. It is up to the user to ensure that the -textcharset value corresponds to the character set of the content generated by the external program.
The -fixboundary switch enables a transformation to repair the boundary portion of the Content-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 required 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 -checkbase64 switch enables a check of the encoding validity in base64-encoded MIME parts. The check looks for a non-encoded text footer appended to a base64-encoded part. Per RFC 2045 §6.8, the occurrence of a "=" character signifies the end of base-64 encoded content. If none is found, a heuristic is used: specifically, two consecutive invalid base64 characters signify the beginning of a plain text footer. If a text footer is found and this switch is enabled, mhfixmsg separates the base64-encoded and non-encoded content and places them in a pair of subparts to a newly constructed multipart/mixed part. That multipart/mixed part replaces the original base64-encoded part in the MIME structure of the message.
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 “application/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 trailing 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 Content-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 transformations mhfixmsg would apply without actually applying them, e.g.,
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.,
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 -decodeheaderfieldbodies all message parts -crlflinebreaks text parts -textcharset text/plain parts -reformat text parts that are not text/plain -fixboundary outermost multipart part -fixcte multipart or message part -checkbase64 base64 encoded parts -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 after a successful invocation of inc. One approach could be based on:
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 utility to generate a temporary file. Alternatively, mhfixmsg could be called on the message after 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 * ^TOnmhemail@example.com | tee `$MKSTEMP` | $MHFIXMSG | $STORE +nmh-workers
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:
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 entire 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 $msg; done
The two appearances of the +folder switch in that command protect against concurrent context changes by other nmh command invocations.
Run on newly incorporated messages¶
To run mhfixmsg on messages as they are incorporated:
inc && mhfixmsg -nochangecur unseen
This assumes that the Unseen-Sequence profile entry is set to unseen, as shown in mh-profile(5).
mhfixmsg looks for mhn.defaults in multiple locations: absolute pathnames are accessed directly, tilde expansion is done on usernames, and files are searched for in the user's Mail directory as specified in their profile. If not found there, the directory “/etc/nmh” is checked.
^$HOME/.mh_profile~^The user profile ^/etc/nmh/mhn.defaults~^Default mhfixmsg conversion entries
^Path:~^To determine the user's nmh directory ^Current-Folder:~^To find the default current folder ^rmmproc:~^Program to delete original messages or files
`+folder' defaults to the current folder `msgs' defaults to cur `-decodetext 8bit' `-decodetypes text,application/ics' `-nodecodeheaderfieldbodies' `-crlflinebreaks' `-notextcharset' `-reformat' `-noreplacetextplain' `-fixboundary' `-fixcte' `-checkbase64' `-changecur' `-noverbose'
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.