Provided by: nmh_1.8~RC2-2_amd64 
NAME
mhshow - display nmh MIME messages
SYNOPSIS
mhshow [-help] [-version] [+folder] [msgs] [-file file] [-part number] ... [-type
content] ... [-prefer content] ... [-noprefer] [-concat | -noconcat] [-textonly |
-notextonly] [-inlineonly | -noinlineonly] [-header | -noheader] [-form formfile]
[-markform formfile]
DESCRIPTION
The mhshow command displays contents of a MIME (multi-media) message, or collection of
messages.
mhshow manipulates multi-media messages as specified in RFC 2045 to RFC 2049. Currently
mhshow only supports encodings in message bodies, and does not support the encoding of
message headers as specified in RFC 2047.
By default, mhshow will display only the text parts of a message that are not marked as
attachments. This behavior can be changed by the -notextonly and -noinlineonly switches.
In addition, by using the -part, -type, and -prefer switches, you may limit and reorder
the set of parts to be displayed, based on part number and/or content type. The inclusion
of any -part or -type switches will override the default settings of -textonly and
-inlineonly.
The -header switch controls whether mhshow will print a message separator header before
each message that it displays. The header format can be controlled using -headerform, to
specify a file containing mh-format(5) instructions. A copy of the built-in default
headerform can be found in /etc/nmh/mhshow.header, for reference. In addition to the
normal set of mh-format(5) instructions, a "%{folder}" escape provides a string
representing the current folder.
By default, mhshow will concatenate all content under one pager. If you want each part to
be displayed separately, you can override the default behavior with -noconcat.
The -file file switch directs mhshow to use the specified file as the source message,
rather than a message from a folder. If you specify this file as “-”, then mhshow will
accept the source message on the standard input. Note that the file, or input from
standard input, should be a validly formatted message, just like any other nmh message.
It should not be in mail drop format (to convert a file in mail drop format to a folder of
nmh messages, see inc(1)).
The -part switch can be given (one or more times) to restrict the set of subparts that
will be displayed. (Obviously with no -part switches, all parts will be considered.) If
a -part switch specifies a specific subpart (i.e., a "leaf" in the tree of MIME parts),
then that part will always be displayed. If a -part switch references a
multipart/alternative part, then (in the absence of a -type switch) only the default
subpart of that multipart will be displayed.
A part specification consists of a series of numbers separated by dots. For example, in a
multipart content containing three parts, these would be named as 1, 2, and 3,
respectively. If part 2 was also a multipart content containing two parts, these would be
named as 2.1 and 2.2, respectively. Note that the -part switch is effective only for
messages containing a multipart content. If a message has some other kind of content, or
if the part is itself another multipart content, the -part switch will not prevent the
content from being acted upon.
The -type switch can also be used to restrict (or, when used in conjunction with -part, to
further restrict) the display of parts according to content type. One or more -type
switches part will only select the first match from a multipart/alternative, even if there
is more than one subpart that matches (one of) the given content type(s).
Using either -part or -type switches alone will cause either switch to select the part(s)
they match. Using them together will select only the part(s) matched by both (sets of)
switches. In other words, the result is the intersection, and not the union, of their
separate match results.
A content specification consists of a content type and a subtype. The initial list of
“standard” content types and subtypes can be found in RFC 2046.
A list of commonly used contents is briefly reproduced here:
Type Subtypes
---- --------
text plain, enriched
multipart mixed, alternative, digest, parallel
message rfc822, external-body
application octet-stream, postscript
image jpeg, gif, png
audio basic
video mpeg
A legal MIME message must contain a subtype specification.
To specify a content, regardless of its subtype, just use the name of the content, e.g.,
“audio”. To specify a specific subtype, separate the two with a slash, e.g.,
“audio/basic”. Note that regardless of the values given to the -type switch, a multipart
content (of any subtype listed above) is always acted upon. Further note that if the
-type switch is used, and it is desirable to act on a message/external-body content, then
the -type switch must be used twice: once for message/external-body and once for the
content externally referenced.
In the absence of -prefer, mhshow will select the "best" displayable subpart from
multipart/alternative content. The -prefer switch can be used (one or more times, in
order of ascending preference) to let MH know which content types from a
multipart/alternative MIME part are preferred by the user, in order to override the
default selection for display. For example, mail is often sent containing both plaintext
and HTML-formatted versions of the same content, and the HTML version is usually indicated
to be the "best" format for viewing. Using “-prefer text/plain” will cause the plaintext
version to be displayed if possible, but still allow display of the HTML part if there is
no plaintext subpart available. Using “-prefer text/plain -prefer image/png” would add a
preference for PNG images, which might or might not ever appear in the same
multipart/alternative section with text/plain. Implementation note: RFC 2046 requires
that the subparts of a multipart/alternative be ordered according to "faithfulness to the
original content", and MH by default selects the subpart ranked most "faithful" by that
ordering. The -prefer switch reorders the alternative parts (only internally, never
changing the message file) to move the user's preferred part(s) to the "most faithful"
position. Thus, when viewed by mhlist, the ordering of multipart/alternative parts will
appear to change when invoked with or without various -prefer switches. Since the last of
multiple -prefer options "wins", a -prefer on the command line will override any in a
profile entry.
The -noprefer switch will cancel any previous -prefer switches.
Unseen Sequence
If the profile entry “Unseen-Sequence” is present and non-empty, then mhshow will remove
each of the messages shown from each sequence named by the profile entry.
Showing the Contents
The headers of each message are displayed with the mhlproc (usually mhl), using the
standard format file, mhl.headers. You may specify an alternative format file with the
-form formfile switch. If the format file mhl.null is specified, then the display of the
message headers is suppressed.
Next, the contents are extracted from the message and are stored in a temporary file.
Usually, the name of the temporary file is the word “mhshow” followed by a string of
characters. Occasionally, the method used to display a content (described next), requires
that the file end in a specific suffix. For example, the soffice command (part of the
StarOffice package) can be used to display Microsoft Word content, but it uses the suffix
to determine how to display the file. If no suffix is present, the file is not correctly
loaded. Similarly, older versions of the gs command append a “.ps” suffix to the filename
if one was missing. As a result, these cannot be used to read the default temporary file.
To get around this, your profile can contain lines of the form:
mhshow-suffix-<type>/<subtype>: <suffix>
or
mhshow-suffix-<type>: <suffix>
to specify a suffix which can be automatically added to the temporary file created for a
specific content type. For example, the following lines might appear in your profile:
mhshow-suffix-text: .txt
mhshow-suffix-application/msword: .doc
mhshow-suffix-application/PostScript: .ps
to automatically append a suffix to the temporary files.
The matching with the content type identifier is case-insensitive, both in mhshow-
suffix-<type> and mhshow-show-<type> (below) profile entries.
The method used to display the different contents in the messages bodies will be
determined by a “display string”. To find the display string, mhshow will first search
your profile for an entry of the form:
mhshow-show-<type>/<subtype>
If this isn't found, mhshow will search for an entry of the form:
mhshow-show-<type>
to determine the display string.
If a display string is found, any escapes (given below) will be expanded. The result will
be executed under “/bin/sh”, with the standard input set to the content.
The display string may contain the following escapes:
%a Insert parameters from Content-Type field
%{parameter} Insert the parameter value from the Content-Type field
%f Insert filename containing content
%F %f, and stdin is terminal not content
%l display listing prior to displaying content
%s Insert content subtype
%d Insert content description
%% Insert the character %
mhshow will execute at most one display string at any given time, and wait for the current
display string to finish execution before executing the next display string.
The {parameter} escape is typically used in a command line argument that should only be
present if it has a non-null value. It is highly recommended that the entire escape be
wrapped in double quotes. Shell parameter expansion can construct the argument only when
it is non-null, e.g.,
mhshow-show-text/html: charset="%{charset}";
w3m ${charset:+-I $charset} -T text/html %F
That example also shows the use of indentation to signify continuation: the two text lines
combine to form a single entry. Note that when dealing with text that has been converted
internally by iconv(3), the “charset” parameter will reflect the target character set of
the text, rather than the original character set in the message.
Note that if the content being displayed is multipart, but not one of the subtypes listed
above, then the f- and F-escapes expand to multiple filenames, one for each subordinate
content. Furthermore, stdin is not redirected from the terminal to the content.
If a display string is not found, mhshow behaves as if these profile entries were supplied
and supported:
mhshow-show-text/plain: %lmoreproc %F
mhshow-show-message/rfc822: %lshow -file %F
Note that “moreproc” is not supported in user profile display strings.
If a subtype of type text doesn't have a profile entry, it will be treated as text/plain.
mhshow has default methods for handling multipart messages of subtype mixed, alternative,
parallel, and digest. Any unknown subtype of type multipart (without a profile entry),
will be treated as multipart/mixed.
If none of these apply, then mhshow will check to see if the message has an
application/octet-stream content with parameter “type=tar”. If so, mhshow will use an
appropriate command. If not, mhshow will complain.
Example entries might be:
mhshow-show-audio/basic: raw2audio 2>/dev/null | play
mhshow-show-image: xv %f
mhshow-show-application/PostScript: lpr -Pps
If an f- or F-escape is not quoted with single quotes, its expansion will be wrapped with
single quotes.
Finally, mhshow will process each message serially -- it won't start showing the next
message until all the commands executed to display the current message have terminated.
Showing Alternate Character Sets
If mhshow was built with iconv(3), then all text/plain parts of the message(s) will be
displayed using the character set of the current locale. See mhparam(1) for how to
determine whether your nmh installation includes iconv(3) support. To convert text parts
other than text/plain, or if mhshow was not built with iconv, an external program can be
used, as described next.
Because a content of type text might be in a non-ASCII character set, when mhshow
encounters a “charset” parameter for this content, it checks if your terminal can display
this character set natively. mhshow checks this by examining the current character set
defined by the locale(1) environment variables. If the value of the locale character set
is equal to the value of the charset parameter, then mhshow assumes it can display this
content without any additional setup. If the locale is not set properly, mhshow will
assume a value of “US-ASCII”. If the character set cannot be displayed natively, then
mhshow will look for an entry of the form:
mhshow-charset-<charset>
which should contain a command creating an environment to render the character set. This
command string should containing a single “%s”, which will be filled-in with the command
to display the content.
Example entries might be:
mhshow-charset-iso-8859-1: xterm -fn '-*-*-medium-r-normal-*-*-120-*-*-c-*-iso8859-*'
-e %s
or
mhshow-charset-iso-8859-1: '%s'
The first example tells mhshow to start xterm and load the appropriate character set for
that message content. The second example tells mhshow that your pager (or other program
handling that content type) can handle that character set, and that no special processing
is needed beforehand.
Note that many pagers strip off the high-order bit, or have problems displaying text with
the high-order bit set. However, the pager less has support for single-octet character
sets. For example, messages encoded in the ISO-8859-1 character set can be viewed using
less, with these environment variable settings:
LESSCHARSET latin1
LESS -f
The first setting tells less to use the ISO-8859-1 definition to determine whether a
character is “normal”, “control“, or “binary”. The second setting tells less not to warn
you if it encounters a file that has non-ASCII characters. Then, simply set the moreproc
profile entry to less, and it will get called automatically. (To handle other single-
octet character sets, look at the less(1) manual entry for information about the
LESSCHARDEF environment variable.)
External Access
For contents of type message/external-body, mhshow supports these access-types:
• afs
• anon-ftp
• ftp
• local-file
• mail-server
• url
For the “anon-ftp” and “ftp” access types, mhshow will look for the “nmh-access-ftp”
profile entry, e.g.,
nmh-access-ftp: myftp.sh
to determine the pathname of a program to perform the FTP retrieval.
This program is invoked with these arguments:
domain name of FTP-site
username
password
remote directory
remote filename
local filename
“ascii” or “binary”
The program should terminate with an exit status of zero if the retrieval is successful,
and a non-zero exit status otherwise.
For the “url” access-type, mhshow will look for the “nmh-access-url” profile entry. See
mhstore(1) for more details.
User Environment
Because the display environment in which mhshow operates may vary for different machines,
mhshow will look for the environment variable MHSHOW. If present, this specifies the name
of an additional user profile which should be read. Hence, when a user logs in on a
particular display device, this environment variable should be set to refer to a file
containing definitions useful for the given display device. Normally, only entries that
deal with the methods to display different content type and subtypes
mhshow-show-<type>/<subtype>
mhshow-show-<type>
need be present in this additional profile. Finally, mhshow will attempt to consult
/etc/nmh/mhn.defaults
which is created automatically during nmh installation.
See "Profile Lookup" in mh-profile(5) for the profile search order, and for how duplicate
entries are treated.
Content-Type Marker
mhshow will display a marker containing information about the part being displayed next.
The default marker can be changed using the -markform switch to specify a file containing
mh-format(5) instructions to use when displaying the content marker. A copy of the
default markform can be found in /etc/nmh/mhshow.marker, for reference. In addition to
the normal set of mh-format(5) instructions, the following component escapes are
supported:
Escape Returns Description
part string MIME part number
content-type string MIME Content-Type of part
description string Content-Description header
disposition string Content disposition (attachment or inline)
ctype-<PARAM> string Value of <PARAM> from Content-Type header
cdispo-<PARAM> string Value of <PARAM> from
Content-Disposition header
%(size) integer The size of the decoded part, in bytes
%(unseen) boolean Returns true for suppressed parts
In this context, the %(unseen) function indicates whether mhshow has decided to not
display a particular part due to the -textonly or -inlineonly switches.
All MIME parameters and the “Content-Description” header will have RFC 2231 decoding
applied and be converted to the local character set.
FILES
mhshow looks for all format files and 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
$MHSHOW Additional profile entries
/etc/nmh/mhn.defaults System default MIME profile entries
/etc/nmh/mhl.headers The headers template
/etc/nmh/mhshow.marker Example content marker
/etc/nmh/mhshow.header Example message separator header
PROFILE COMPONENTS
Path: To determine the user's nmh directory
Current-Folder: To find the default current folder
Unseen-Sequence: To name sequences denoting unseen messages
mhlproc: Default program to display message headers
nmh-access-ftp: Program to retrieve contents via FTP
nmh-access-url: Program to retrieve contents via HTTP
mhshow-charset-<charsTe>mplate for environment to render character sets
mhshow-show-<type>* Template for displaying contents
moreproc: Default program to display text/plain content
SEE ALSO
iconv(3), mhbuild(1), mhl(1), mhlist(1), mhparam(1), mhstore(1), sendfiles(1)
DEFAULTS
`+folder' defaults to the current folder
`msgs' defaults to cur
`-concat'
`-textonly'
`-inlineonly'
`-form mhl.headers'
CONTEXT
If a folder is given, it will become the current folder. The last message selected will
become the current message.