MH-FORMAT(5)

NAME
mh-format - format file for nmh message system

DESCRIPTION
Several nmh commands utilize either a format string or a format file during their execution. For example, scan uses a format string which directs it how to generate the scan listing for each message; repl uses a format file which directs it how to generate the reply to a message, and so on.

There are a few alternate scan listing formats available in nmh/etc/scan.time, nmh/etc/scan.size, and nmh/etc/scan.timely. Look in nmh/etc for other scan and repl format files which may have been written at your site.

It suffices to have your local nmh expert actually write new format commands or modify existing ones. This manual section explains how to do that. Note: familiarity with the C printf routine is assumed.

A format string consists of ordinary text, and special multi-character escape sequences which begin with `%'. When specifying a format string, the usual C backslash characters are honored: `\b', `\f', `\n', `\r', and `\t'. Continuation lines in format files end with `\' followed by the newline character.

  SYNTAX
Format strings are built around escape sequences. There are three types of escape sequences: header components, built-in functions, and flow control. Comments may be inserted in most places where a function argument is not expected. A comment begins with `%;' and ends with a (non-escaped) newline.

A component escape is specified as `%{component}', and exists for each header found in the message being processed. For example `%{date}' refers to the “Date:” field of the appropriate message. All component escapes have a string value. Normally, component values are compressed by converting any control characters (tab and newline included) to spaces, then eliding any leading or multiple spaces. However, commands may give different interpretations to some component escapes; be sure to refer to each command's manual entry for complete details. Some commands (such as ap and mhl) use a special component `%{text}' to refer to the text being processed; see their respective man pages for details and examples.

A function escape is specified as `%(function)'. All functions are built-in, and most have a string or numeric value. A function escape may have an argument. The argument follows the function escape: separating whitespace is discarded: `%(function argument)'.

In addition to literal numbers or strings, the argument to a function escape can be another function, a component, or a control escape. When the argument is a function or a component, they are listed without a leading `%'. When control escapes are used as function arguments, they written as normally, with a leading `%';

  Control escapes

A control escape is one of: `%<', `%?', `%|', or `%>'. These are combined into the conditional execution construct:

    %< condition format-text
    %? condition format-text
        ...
    %| format-text
    %>

Extra white space is shown here only for clarity. These constructs may be nested without ambiguity. They form a general if-elseif-else-endif block where only one of the format-texts is interpreted. In other words, `%<' is like the "if", `%?' is like the "elseif", `%|' is like "else", and `%>' is like "endif".

A `%<' or `%?' control escape causes its condition to be evaluated. This condition is a component or function. For integer valued functions or components, the condition is true if the function return or component value is non-zero, and false if zero. For string valued functions or components, the condition is true if the function return or component value is a non-empty string, and false for an empty string.

The `%?' control escape is optional, and may there may be more than one `%?' control escape in a conditional block. The `%|' control escape is also optional, but may be included at most once.

  Function escapes
Functions expecting an argument generally require an argument of a particular type. In addition to the number and string types, these include:

    Argument	Description	Example Syntax
    literal	A literal number	%(func 1234)
    	or string		%(func text string)
    comp	Any component		%(func{in-reply-to})
    date	A date component	%(func{date})
    addr	An address component	%(func{from})
    expr	Nothing	%(func)
    	or a subexpression	%(func(func2))
    	or control escape	%(func %<{reply-to}%|%{from}%>)

The types date and addr have the same syntax as comp, but require that the header component be a date string, or address string, respectively.

Most arguments not of type expr are required. When escapes are nested (via expr arguments), evaluation is done from inner-most to outer-most. As noted above, for the expr argument type, functions and components are written without a leading `%'. Control escape arguments must use a leading `%', preceded by a space.

For example,

    %<(mymbox{from}) To: %{to}%>

writes the value of the header component “From:” to the internal register named str; then (mymbox) reads str and writes its result to the internal register named num; then the control escape evaluates num. If num is non-zero, the string “To:” is printed followed by the value of the header component “To:”.

  Evaluation
The evaluation of format strings is performed by a small virtual machine. The machine is capable of evaluating nested expressions as described above, and in addition has an integer register num, and a text string register str. When a function escape that accepts an optional argument is processed, and the argument is not present, the current value of either num or str is used as the argument: which register is used depends on the function, as listed below.

Component escapes write the value of their message header in str. Function escapes write their return value in num for functions returning integer or boolean values, and in str for functions returning string values. (The boolean type is a subset of integers with usual values 0=false and 1=true.) Control escapes return a boolean value, setting num to 1 if the last explicit condition evaluated by a `%<' or `%?' control succeeded, and 0 otherwise.

All component escapes, and those function escapes which return an integer or string value, evaluate to their value as well as setting str or num. Outermost escape expressions in these forms will print their value, but outermost escapes which return a boolean value do not result in printed output.

  Functions
The function escapes may be roughly grouped into a few categories.

    Function	Argument   Result	Description
    msg		integer	message number
    cur		integer	message is current (0 or 1)
    unseen		integer	message is unseen (0 or 1)
    size		integer	size of message
    strlen		integer	length of str
    width		integer	output buffer size in bytes
    charleft		integer	bytes left in output buffer
    timenow		integer	seconds since the UNIX epoch
    me		string	the user's mailbox (username)
    myhost		string	the user's local hostname
    myname		string	the user's name
    localmbox		string	the complete local mailbox
    eq	literal	boolean	num == arg
    ne	literal	boolean	num != arg
    gt	literal	boolean	num > arg
    match	literal	boolean	str contains arg
    amatch	literal	boolean	str starts with arg
    plus	literal	integer	arg plus num
    minus	literal	integer	arg minus num
    divide	literal	integer	num divided by arg
    modulo	literal	integer	num modulo arg
    num	literal	integer	Set num to arg.
    num		integer	Set num to zero.
    lit 	literal	string	Set str to arg.
    lit		string	Clear str.
    getenv 	literal	string	Set str to environment value of arg
    profile	literal	string	Set str to profile component arg
    			value
    nonzero	expr	boolean	num is non-zero
    zero	expr	boolean	num is zero
    null	expr	boolean	str is empty
    nonnull	expr	boolean	str is non-empty
    void	expr		Set str or num
    comp	comp	string	Set str to component text
    compval	comp	integer	Set num to “atoi(comp)”
    decode	expr	string	decode str as RFC-2047 (MIME-encoded)
    			component
    unquote	expr	string	remove RFC-2822 quotes from str
    trim	expr		trim trailing white-space from str
    putstr	expr		print str
    putstrf	expr		print str in a fixed width
    putnum	expr		print num
    putnumf	expr		print num in a fixed width
    putlit	expr		print str without space compression
    nodate	string	integer	Argument not a date string (0 or 1)
    formataddr	expr		append arg to str as a
    			(comma separated) address list
    concataddr	expr		append arg to str as a
    			(comma separated) address list,
    			including duplicates,
    			see Special Handling
    putaddr	literal		print str address list with
    			arg as optional label;
    			get line width from num

The (me) function returns the username of the current user. The (myhost) function returns the localname entry in mts.conf, or the local hostname if localname is not configured. The (myname) function will return the value of the SIGNATURE environment variable if set, otherwise will return the passwd GECOS field (truncated at the first comma if it contains one) for the current user. The (localmbox) function will return the complete form of the local mailbox, suitable for use in a “From” header. It will return the “Local-Mailbox” profile entry if it is set; if it is not, it will be equivalent to:

    %(myname) <%(me)@%(myhost)>

The following functions require a date component as an argument:

    Function	Argument	Return	Description
    sec	date	integer	seconds of the minute
    min	date	integer	minutes of the hour
    hour	date	integer	hours of the day (0-23)
    wday	date	integer	day of the week (Sun=0)
    day	date	string	day of the week (abbrev.)
    weekday	date	string	day of the week
    sday	date	integer	day of the week known?
    			(1=explicit,0=implicit,-1=unknown)
    mday	date	integer	day of the month
    yday	date	integer	day of the year
    mon	date	integer	month of the year
    month	date	string	month of the year (abbrev.)
    lmonth	date	string	month of the year
    year	date	integer	year (may be > 100)
    zone	date	integer	timezone in hours
    tzone	date	string	timezone string
    szone	date	integer	timezone explicit?
    			(1=explicit,0=implicit,-1=unknown)
    date2local	date		coerce date to local timezone
    date2gmt	date		coerce date to GMT
    dst	date	integer	daylight savings in effect? (0 or 1)
    clock	date	integer	seconds since the UNIX epoch
    rclock	date	integer	seconds prior to current time
    tws	date	string	official 822 rendering
    pretty	date	string	user-friendly rendering

These functions require an address component as an argument. The return value of functions noted with `*' is computed from the first address present in the header component.

    Function	Argument	Return	Description
    proper	addr	string	official 822 rendering
    friendly	addr	string	user-friendly rendering
    addr	addr	string	mbox@host or host!mbox rendering*
    pers	addr	string	the personal name*
    note	addr	string	commentary text*
    mbox	addr	string	the local mailbox*
    mymbox	addr	integer	List has the user's address? (0 or 1)
    host	addr	string	the host domain*
    nohost	addr	integer	no host was present (0 or 1)*
    type	addr	integer	host type* (0=local,1=network,
    			-1=uucp,2=unknown)
    path	addr	string	any leading host route*
    ingrp	addr	integer	address was inside a group (0 or 1)*
    gname	addr	string	name of group*

(A clarification on (mymbox{comp}) is in order. This function checks each of the addresses in the header component “comp” against the user's mailbox name and any “Alternate-Mailboxes”. It returns true if any address matches, however, it also returns true if the “comp” header is not present in the message. If needed, the (null) function can be used to explicitly test for this case.)

  Formatting
When a function or component escape is interpreted and the result will be immediately printed, an optional field width can be specified to print the field in exactly a given number of characters. For example, a numeric escape like %4(size) will print at most 4 digits of the message size; overflow will be indicated by a `?' in the first position (like `?234'). A string escape like %4(me) will print the first 4 characters and truncate at the end. Short fields are padded at the right with the fill character (normally, a blank). If the field width argument begins with a leading zero, then the fill character is set to a zero.

The functions (putnumf) and (putstrf) print their result in exactly the number of characters specified by their leading field width argument. For example, %06(putnumf(size)) will print the message size in a field six characters wide filled with leading zeros; %14(putstrf{from}) will print the “From:” header component in fourteen characters with trailing spaces added as needed. For putstrf, using a negative value for the field width causes right-justification of the string within the field, with padding on the left up to the field width. The functions (putnum) and (putstr) are somewhat special: they print their result in the minimum number of characters required, and ignore any leading field width argument. The (putlit) function outputs the exact contents of str register without any changes such as duplicate space removal or control character conversion.

The available output width is kept in an internal register; any output past this width will be truncated.

  Special Handling
A few functions have different behavior depending on what command they are being invoked from.

In repl the (formataddr) function stores all email addresses encountered into an internal cache and will use this cache to suppress duplicate addresses. If you need to create an address list that includes previously-seen addresses you may use the (concataddr) function, which is identical to (formataddr) in all other respects. Note that (concataddr) will NOT add addresses to the duplicate-suppression cache.

  Other Hints and Tips
Sometimes to format function writers it is confusing as to why output is duplicated. The general rule to remember is simple: If a function or component escape is used where it starts with a %, then it will generate text in the output file. Otherwise, it will not.

A good example is a simple attempt to generate a To: header based on the From: and Reply-To: headers:

    %(formataddr %<{reply-to}%|%{from})%(putaddr To: )

Unfortuantely if the Reply-to: header is NOT present, the output line that is generated will be something like:

    My From User <from@example.com>To: My From User <from@example.com>

What went wrong? When performing the test for the if clause (%<), the component is not output because it is considered an argument to the if statement (hence the rule about the lack of % applies). But the component escape in our else statement (everything after the `%|') is NOT an argument to anything; the syntax is that it is written with a %, and thus the value of that component is output. This also has the side effect of setting the str register, which is later picked up by the (formataddr) function and then output by (putaddr). This format string has another bug as well; there should always be a valid width value in the num register when (putaddr) is called, otherwise bad formatting can take place.

The solution is to use the (void) function; this will prevent the function or component from outputting any text. With this in place (and using (width) to set the num register for the width, a better implementation would look like:

    %(formataddr %<{reply-to}%|%(void{from})%(void(width))%(putaddr To: )

It should be noted here that the side-effects of functions and component escapes still are in force: as a result each component test in the if-elseif-else-endif clause sets the str register.

As an additional note, the (formataddr) and (concataddr) functions have some behavior when it comes to the str register. The starting point of the register is saved and is used to build up entries in the address list.

You will find the ap and fmtdump utilities invaluable in debugging problems with format strings.

  Examples
With all this in mind, here's the default format string for scan. It's been divided into several pieces for readability. The first part is:

    %4(msg)%<(cur)+%| %>%<{replied}-%?{encrypted}E%| %>

which says that the message number should be printed in four digits. If the message is the current message then a `+' else a space should be printed; if a “Replied:” field is present then a `-' else if an “Encrypted:” field is present then an `E' otherwise a space should be printed. Next:

    %02(mon{date})/%02(mday{date})

the month and date are printed in two digits (zero filled) separated by a slash. Next,

    %<{date} %|*%>

If a “Date:” field was present, then a space is printed, otherwise a `*'. Next,

    %<(mymbox{from})%<{to}To:%14(decode(friendly{to}))%>%>

if the message is from me, and there is a “To:” header, print “To:” followed by a “user-friendly” rendering of the first address in the “To:” field; any MIME-encoded characters are decoded into the actual characters. Continuing,

    %<(zero)%17(decode(friendly{from}))%>

if either of the above two tests failed, then the “From:” address is printed in a mime-decoded, “user-friendly” format. And finally,

    %(decode{subject})%<{body}<<%{body}>>%>

the mime-decoded subject and initial body (if any) are printed.

For a more complicated example, next consider a possible replcomps format file.

    %(lit)%(formataddr %<{reply-to}

This clears str and formats the “Reply-To:” header if present. If not present, the else-if clause is executed.

    %?{from}%?{sender}%?{return-path}%>)\

This formats the “From:”, “Sender:” and “Return-Path:” headers, stopping as soon as one of them is present. Next:

    %<(nonnull)%(void(width))%(putaddr To: )\n%>\

If the formataddr result is non-null, it is printed as an address (with line folding if needed) in a field width wide with a leading label of “To:”.

    %(lit)%(formataddr{to})%(formataddr{cc})%(formataddr(me))\

str is cleared, and the “To:” and “Cc:” headers, along with the user's address (depending on what was specified with the “-cc” switch to repl) are formatted.

    %<(nonnull)%(void(width))%(putaddr cc: )\n%>\

If the result is non-null, it is printed as above with a leading label of “cc:”.

    %<{fcc}Fcc: %{fcc}\n%>\

If a -fcc folder switch was given to repl (see repl(1) for more details about %{fcc}), an “Fcc:” header is output.

    %<{subject}Subject: Re: %{subject}\n%>\

If a subject component was present, a suitable reply subject is output.

    %<{message-id}In-Reply-To: %{message-id}\n%>\
    %<{message-id}References: %<{references} %{references}%>\
    %{message-id}\n%>
    --------

If a message-id component was present, an “In-Reply-To:” header is output including the message-id, followed by a “References:” header with references, if present, and the message-id. As with all plain-text, the row of dashes are output as-is.

This last part is a good example for a little more elaboration. Here's that part again in pseudo-code:

    if (comp_exists(message-id))  then
    	print (“In-reply-to: ”)
    	print (message-id.value)
    	print (“\n”)
    endif
    if (comp_exists(message-id)) then
    	print (“References: ”)
    	if (comp_exists(references)) then
    	      print(references.value);
    	endif
    	print (message-id.value)
    	print (“\n”)
    endif

One more example: Currently, nmh supports very large message numbers, and it is not uncommon for a folder to have far more than 10000 messages. Nontheless (as noted above) the various scan format strings are inherited from older MH versions, and are generally hard-coded to 4 digits of message number before formatting problems start to occur. The nmh format strings can be modified to behave more sensibly with larger message numbers:

    %(void(msg))%<(gt 9999)%(msg)%|%4(msg)%>

The current message number is placed in num. (Note that (msg) is an int function, not a component.) The (gt) conditional is used to test whether the message number has 5 or more digits. If so, it is printed at full width: otherwise at 4 digits.

SEE ALSO
scan(1), repl(1), ap(8), dp(8)

CONTEXT
None