mh-format(5) 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                       DescriptionExample 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 <[email protected]>To: My From User <[email protected]>

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.

CONTEXT

None

SEE ALSO

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