mh-format - Man Page

Format strings are built around escape sequences. There are three types of escape sequence: 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 in the message being processed.  For example, `%{date}' refers to the “Date:” field of the message. All component escapes have a string value.  Such values are usually compressed by converting any control characters (tab and newline included) to spaces, then eliding any leading or multiple spaces.  Some commands, however, may interpret some component escapes differently; be sure to refer to each command's manual entry for details. Some commands (such as ap(8) and mhl(1)) 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 integer value. A function escape may take an argument. The argument follows the function escape (and any separating whitespace is discarded) as in the following example:

%(function argument)

In addition to literal numbers or strings, the argument to a function escape can be another function, or a component, or a control escape.  When the argument is a function or a component, the argument is specified without a leading `%'. When the argument is a control escape, it is specified with a leading `%'.

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, which may be nested without ambiguity, 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 components and functions whose value is an integer, the condition is true if it is non-zero, and false if zero. For components and functions whose value is a string, the condition is true it is a non-empty string, and false if an empty string.

The `%?' control escape is optional, and can be used multiple times in a conditional block.  The `%|' control escape is also optional, but may only be used once.

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

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

Most arguments not of type expr are required. When escapes are nested (via expr arguments), evaluation is done from innermost to outermost.  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:”.

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 substituted as the argument: the register 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 escape 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.

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

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 it 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 there is one; if not, it will be equivalent to:

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

The following functions require a date component as an argument:

The following 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.

(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.)

The friendly{comp}) call will return any double-quoted “personal name” (that is, anything before <>), then it will return that.  If there's no personal name but there is a “note” (comments string after an email address), it will return that.  If there is neither of those it will just return the bare email address.

When a function or component escape is interpreted and the result will be printed immediately, 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. Using a negative value for the field width causes right-justification within the field, with padding on the left up to the field width. Padding is with spaces except for a left-padded putnumf when the width starts with zero. 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 the str register without any changes such as duplicate space removal or control character conversion.  Similarly, the (zputlit) function outputs the exact contents of the str register, but requires that those contents not occupy any output width.  It can therefore be used for outputting terminal escape sequences.

There are a limited number of function escapes to output terminal escape sequences.  These sequences are retrieved from the terminfo(5) database according to the current terminal setting.  The (bold), (underline), and (standout) escapes set bold mode, underline mode, and standout mode respectively. (hascolor) can be used to determine if the current terminal supports color. (fgcolor) and (bgcolor) set the foreground and background colors respectively.  Both of these escapes take one literal argument, the color name, which can be one of: black, red, green, yellow, blue, magenta, cyan, white.  (resetterm) resets all terminal attributes to their default setting.  These terminal escapes should be used in conjunction with (zputlit) (preferred) or (putlit), as the normal (putstr) function will strip out control characters.

The available output width is kept in an internal register; any output exceeding this width will be truncated.  The one exception to this is that (zputlit) functions will still be executed if a terminal reset code is being placed at the end of a line.

Some functions have different behavior depending on the command they are 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) does not add addresses to the duplicate-suppression cache.

Sometimes, the writer of a format function is confused because output is duplicated.  The general rule to remember is simple: If a function or component escape begins with a `%', 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: )

Unfortunately, if the Reply-to: header is not present, the output line 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 (so the rule about not starting with % applies).  But the component escape in our else statement (everything after the `%|') is not an argument to anything; it begins 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).  The example format string above has another bug: 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 function and component escapes are still in force and, 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 special 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 fmttest(1) utility invaluable when debugging problems with format strings.

With all the above in mind, here is a breakdown of the default format string for scan. 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 is present it is printed, followed by a space; otherwise a `*' is printed. 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, 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. Nonetheless (as noted above) the various scan format strings, inherited from older MH versions, are generally hard-coded to 4 digits for the message number. Thereafter, formatting problems 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 a function escape which returns an integer, it is 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.