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ISBN : 978-2-7460-9712-4
EAN : 9782746097124
(Editions ENI)


CentOS 2.1AS







mhn − display/list/store/cache MIME messages


mhn [+folder] [msgs] [−file file]
[−part number]... [−type content]...
[−show] [−noshow] [−list] [-nolist]
[−store] [−nostore] [−cache] [−nocache]
[−headers] [−noheaders] [−realsize] [−norealsize]
[−serialonly] [−noserialonly] [−form formfile]
[−pause] [−nopause] [−auto] [−noauto]
[−rcache policy] [−wcache policy] [−check] [−nocheck]
[−verbose] [−noverbose] [−version] [−help]

mhn −build file
[−ebcdicsafe] [−noebcdicsafe]
[−rfc934mode] [−norfc934mode]



The mhn command allows you to display, list, store, or cache the contents of a MIME (multi-media) messages.

mhn manipulates multi-media messages as specified in RFC−2045 thru RFC−2049. Currently mhn only supports encodings in message bodies, and does not support the encoding of message headers as specified in RFC−2047.

The switches ’−list’, ’−show’, ’−store’, and ’-cache’ direct the operation of mhn. Only one of these switches may be used at a time. These switches are used to operate on the content of each of the named messages. By using the ’−part’ and ’−type’ switches, you may limit the scope of the given operation to particular subparts (of a multipart content) and/or particular content types.

The switch ’−build’ is used to construct a MIME message. It is for backward compatibility and instructs mhn to execute the mhbuild command. It is preferred that you use the mhbuild command directly. See the mhbuild(1) man page for details.

The option ’−file file’ directs mhn to use the specified file as the source message, rather than a message from a folder. If you specify this file as “-”, then mhn 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)).

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 for only 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.

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:






plain, enriched


mixed, alternative, digest, parallel


rfc822, partial, external-body


octet-stream, postscript


jpeg, gif, png





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.

The ’−check’ switch tells mhn to check each content for an integrity checksum. If a content has such a checksum (specified as a Content-MD5 header field), then mhn will attempt to verify the integrity of the content.

The ’−list’ switch tells mhn to list the table of contents associated with the named messages.

The ’−headers’ switch indicates that a one-line banner should be displayed above the listing. The ’−realsize’ switch tells mhn to evaluate the “native” (decoded) format of each content prior to listing. This provides an accurate count at the expense of a small delay. If the ’−verbose’ switch is present, then the listing will show any “extra” information that is present in the message, such as comments in the Content-Type header.

The ’−show’ switch tells mhn to display the contents of the named messages.

The headers of each message are displayed with the mhlproc (usually mhl), using the standard format file mhl.headers. You may specify an alternate 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 the word "mhn" 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. Similarily, 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:

mhn-suffix-<type>/<subtype>: <suffix>


mhn-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:

mhn-suffix-text: .txt
mhn-suffix-application/msword: .doc
mhn-suffix-application/PostScript: .ps

to automatically append a suffix to the temporary files.

The method used to display the different contents in the messages bodies will be determined by a “display string”. To find the display string, mhn will first search your profile for an entry of the form:


to determine the display string. If this isn’t found, mhn will search for an entry of the form:


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:


Insert parameters from Content-Type field


exclusive execution


Insert filename containing content


%e, %f, and stdin is terminal not content


display listing prior to displaying content


%l, and ask for confirmation


Insert content subtype


Insert content description


Insert the character %

For those display strings containing the e- or F-escape, mhn will execute at most one of these at any given time. Although the F-escape expands to be the filename containing the content, the e-escape has no expansion as far as the shell is concerned.

When the p-escape prompts for confirmation, typing INTR (usually control-C) will tell mhn not to display that content. The p-escape can be disabled by specifying the switch ’−nopause’. Further, when mhn is display a content, typing QUIT (usually control-\) will tell mhn to wrap things up immediately.

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. Further, stdin is not redirected from the terminal to the content.

If a display string is not found, mhn has several default values:

mhn-show-text/plain: %pmoreproc ’%F’
mhn-show-message/rfc822: %pshow -file ’%F’

If a subtype of type text doesn’t have a profile entry, it will be treated as text/plain.

mhn 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 mhn will check to see if the message has an application/octet-stream content with parameter “type=tar”. If so, mhn will use an appropriate command. If not, mhn will complain.

Example entries might be:

mhn-show-audio/basic: raw2audio 2>/dev/null | play
mhn-show-image: xv ’%f’
mhn-show-application/PostScript: lpr -Pps

Note that when using the f- or F-escape, it’s a good idea to use single-quotes around the escape. This prevents misinterpretation by the shell of any funny characters that might be present in the filename.

Finally, mhn 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. In the case of a multipart content (of any subtype listed above), the content contains advice indicating if the parts should be displayed serially or in parallel. Because this may cause confusion, particularly on uni-window displays, the ’−serialonly’ switch can be given to tell mhn to never display parts in parallel.

Because a content of type text might be in a non-ASCII character set, when mhn encounters a “charset” parameter for this content, it checks if your terminal can display this character set natively. Mhn checks this by examining the the environment variable MM_CHARSET. If the value of this environment variable is equal to the value of the charset parameter, then mhn assumes it can display this content without any additional setup. If this environment variable is not set, mhn will assume a value of “US-ASCII”. If the character set cannot be displayed natively, then mhn will look for an entry of the form:


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:

mhn-charset-iso-8859-1: xterm -fn ’-*-*-medium-r-normal-*-*-120-*-*-c-*-iso8859-*’ -e %s


mhn-charset-iso-8859-1: ’%s’

The first example tells mhn to start xterm and load the appropriate character set for that message content. The second example tells mhn 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. The source to less is available on many ftp sites carrying free software. In order to view messages sent in the ISO-8859-1 character set using less, put these lines in your .login file:

setenv LESSCHARSET latin1
setenv LESS "-f"

The first line tells less to use the ISO-8859-1 definition for determining whether a character is “normal”, “control“, or “binary”. The second line 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.)

The ’−store’ switch tells mhn to store the contents of the named messages in “native” (decoded) format. Two things must be determined: the directory to store the content, and the filenames. Files are written in the directory given by the nmh-storage profile entry, e.g.,

nmh-storage: /tmp

If this entry isn’t present, the current working directory is used.

If the ’−auto’ switch is given, then mhn will check if the message contains information indicating the filename that should be used to store the content. This information should be specified as the attribute “name=filename” in the Content-Type header for the content you are storing. For security reasons, this filename will be ignored if it begins with the character ’/’, ’.’, ’|’, or ’!’, or if it contains the character ’%’. For the sake of security, this switch is not the default, and it is recommended that you do NOT put the ’−auto’ switch in your .mh_profile file.

If the ’−auto’ switch is not given (or is being ignored for security reasons) then mhn will look in the user’s profile for a “formatting string” to determine how the different contents should be stored. First, mhn will look for an entry of the form:


to determine the formatting string. If this isn’t found, mhn will look for an entry of the form:


to determine the formatting string.

If the formatting string starts with a “+” character, then content is stored in the named folder. A formatting string consisting solely of a “+” character is interpreted to be the current folder.

If the formatting string consists solely of a “-” character, then the content is sent to the standard output.

If the formatting string starts with a ’|’, then the display string will represent a command for mhn to execute which should ultimately store the content. The content will be passed to the standard input of the command. Before the command is executed, mhn will change to the appropriate directory, and any escapes (given below) in the display string will be expanded.

Otherwise the formatting string will represent a pathname in which to store the content. If the formatting string starts with a ’/’, then the content will be stored in the full path given, else the file name will be relative to the value of nmh-storage or the current working directory. Any escapes (given below) will be expanded, except for the a-escape.

A command or pathname formatting string may contain the following escapes. If the content isn’t part of a multipart (of any subtype listed above) content, the p-escapes are ignored.


Parameters from Content-type (only valid with command)


Insert message number


Insert part number with leading dot


Insert part number without leading dot


Insert content type


Insert content subtype


Insert character %

If no formatting string is found, mhn will check to see if the content is application/octet-stream with parameter “type=tar”. If so, mhn will choose an appropriate filename. If the content is not application/octet-stream, then mhn will check to see if the content is a message. If so, mhn will use the value “+”. As a last resort, mhn will use the value “%m%P.%s”.

Example profile entries might be:

mhn-store-text: %m%P.txt
mhn-store-text: +inbox
mhn-store-message/partial: +
mhn-store-audio/basic: | raw2audio -e ulaw -s 8000 -c 1 > %m%P.au
mhn-store-image/jpeg: %m%P.jpg
mhn-store-application/PostScript: %m%P.ps

When asked to store a content containing a partial message, mhn will try to locate all of the portions and combine them accordingly. The default is to store the combined parts as a new message in the current folder, although this can be changed using formatting strings as discussed above. Thus, if someone has sent you a message in several parts (such as the output from sendfiles), you can easily reassemble them all into a single message in the following fashion:

% mhn -list 5-8
msg part type/subtype size description
5 message/partial 47K part 1 of 4
6 message/partial 47K part 2 of 4
7 message/partial 47K part 3 of 4
8 message/partial 18K part 4 of 4
% mhn -store 5-8
reassembling partials 5,6,7,8 to folder inbox as message 9
% mhn -list -verbose 9
msg part type/subtype size description
9 application/octet-stream 118K
(extract with uncompress | tar xvpf -)

This will store exactly one message, containing the sum of the parts. It doesn’t matter whether the partials are specified in order, since mhn will sort the partials, so that they are combined in the correct order. But if mhn can not locate every partial necessary to reassemble the message, it will not store anything.

For contents of type message/external-body, mhn supports these access-types:


For the “anon-ftp” and “ftp” access types, mhn 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
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.

If this entry is not provided, then mhn will use a simple built-in FTP client to perform the retrieval.

When mhn encounters an external content containing a “Content-ID:” field, and if the content allows caching, then depending on the caching behavior of mhn, the content might be read from or written to a cache.

The caching behavior of mhn is controlled with the ’−rcache’ and ’−wcache’ switches, which define the policy for reading from, and writing to, the cache, respectively. One of four policies may be specified: “public”, indicating that mhn should make use of a publically-accessible content cache; “private”, indicating that mhn should make use of the user’s private content cache; “never”, indicating that mhn should never make use of caching; and, “ask”, indicating that mhn should ask the user.

There are two directories where contents may be cached: the profile entry nmh-cache names a directory containing world-readable contents, and, the profile entry nmh-private-cache names a directory containing private contents. The former should be an absolute (rooted) directory name. For example,

nmh-cache: /tmp

might be used if you didn’t care that the cache got wiped after each reboot of the system. The latter is interpreted relative to the user’s nmh directory, if not rooted, e.g.,

nmh-private-cache: .cache

(which is the default value).

When you encounter a content of type message/external-body with access type “mail-server”, mhn will ask you if may send a message to a mail-server requesting the content, e.g.,

% show 1
Retrieve content by asking mail-server@...

SEND file

? yes
mhn: request sent

Regardless of your decision, mhn can’t perform any other processing on the content.

However, if mhn is allowed to request the content, then when it arrives, there should be a top-level “Content-ID:” field which corresponds to the value in the original message/external-body content. You should now use the ’-cache’ switch to tell mhn to enter the arriving content into the content cache, e.g.,

% mhn -cache 2
caching message 2 as file ...

You can then re-process the original message/external-body content, and “the right thing should happen”, e.g.,

% show 1

Because the display environment in which mhn operates may vary for different machines, mhn will look for the environment variable $MHN. 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


need be present in this additional profile. Finally, mhn will attempt to consult one other additional user profile, e.g.,


which is created automatically during nmh installation. ^$HOME/.mh_profile~^The user profile ^$MHN~^Additional profile entries ^/etc/nmh/mhn.defaults~^System default MIME profile entries ^/etc/nmh/mhl.headers~^The headers template ^Path:~^To determine the user’s nmh directory ^Current−Folder:~^To find the default current folder ^mhlproc:~^Default program to display message headers ^nmh-access-ftp:~^Program to retrieve contents via FTP ^nmh-cache~^Public directory to store cached external contents ^nmh-private-cache~^Personal directory to store cached external contents ^mhn-charset-<charset>~^Template for environment to render character sets ^mhn-show-<type>*~^Template for displaying contents ^nmh-storage~^Directory to store contents ^mhn-store-<type>*~^Template for storing contents ^moreproc:~^Default program to display text/plain content mhbuild(1), mhl(1), sendfiles(1)
Proposed Standard for Message Encapsulation
Multipurpose Internet Mail Extensions (MIME) Part One:
Format of Internet Message Bodies
Multipurpose Internet Mail Extensions (MIME) Part Two:
Media Types
Multipurpose Internet Mail Extensions (MIME) Part Three:
Message Header Extensions for Non-ASCII Text
Multipurpose Internet Mail Extensions (MIME) Part Four:
Registration Procedures
Multipurpose Internet Mail Extensions (MIME) Part Five:
Conformance Criteria and Examples
. ’+folder’ defaults to the current folder ’msgs’ defaults to cur ’−noauto’ ’−nocache’ ’−nocheck’ ’−form mhl.headers’ ’−headers’ ’−pause’ ’−rcache ask’ ’−realsize’ ’−noserialonly’ ’−show’ ’−noverbose’ ’−wcache ask’ If a folder is given, it will become the current folder. The last message selected will become the current message. Partial messages contained within a multipart content are not reassembled with the ’−store’ switch.