--- name: "mhshow" section: "1mh" platform: "linux" package: "nmh" version: "1.8-3" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/mhshow/1mh" --- # mhshow(1mh) display nmh MIME messages ## 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)](/mh-format/5.md) 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)](/mh-format/5.md) 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)](/inc/1.md)). 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-\/\: \ or mhshow-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-\ and mhshow-show-\ (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-\/\ If this isn't found, **mhshow** will search for an entry of the form: mhshow-show-\ 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)](/iconv/3.md), 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)](/iconv/3.md), then all text/plain parts of the message(s) will be displayed using the character set of the current locale. See [*mhparam*(1)](/mhparam/1.md) for how to determine whether your **nmh** installation includes [*iconv*(3)](/iconv/3.md) 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)](/locale/1.md) 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-\ 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)](/less/1.md) 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)](/mhstore/1.md) 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-/ mhshow-show- ``` 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)](/mh-profile/5.md) 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)](/mh-format/5.md) 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)](/mh-format/5.md) 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- string Value of from Content-Type header cdispo- string Value of 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-~^Template for environment to render character sets ^mhshow-show-*~^Template for displaying contents ^moreproc:~^Default program to display text/plain content ``` ## SEE ALSO [*iconv*(3)](/iconv/3.md), [*mhbuild*(1)](/mhbuild/1.md), [*mhl*(1)](/mhl/1.md), [*mhlist*(1)](/mhlist/1.md), [*mhparam*(1)](/mhparam/1.md), [*mhstore*(1)](/mhstore/1.md), [*sendfiles*(1)](/sendfiles/1.md) ## 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.