Environment Usage in MS-DOS.
----------------

All pathnames can have forward or back slashes.  (This is also true
of file names explicitly used in perl.  The system call handlers in
MS-DOS like / fine; it's command.com and other programs that are picky.)

Environment variables are interpreted when they are needed.
(Exception: creation of $TMP from $TMPDIR is done at startup.)  Thus the
perl script can change them.  For example, if a huge "pipe" file is
to be created, and there's room on the hard disk (c:) but not the
usual $TMP, which is a RAM disk, one could write:

	ENV{'TMP'} = "c:/tmp";
	open (HANDLE, "program|");

The state of the environment passed to perl.exe determines argument
parsing, though, since this is done before compilation and interpretation
of the perl script.

PATH	    Semicolon-separated path list.  The current directory is
	    search first unless $MKSARGS is defined and MKS support is
	    compiled in, in which case the current directory is searched
	    only in response to an explicit "."or null path component.

MKSARGS     Set to 1 enable all MKSisms.  The MKS tools can assume that the
	    other pieces of the toolkit are lying around, but perl.exe
	    can't.  (It may be the only MKS-compatible program on a
	    system.)  If have have the MKS tools, you should set this
	    variable.  Perl will recoginize and generate MKS compatible
	    arguments in any case, but without the switch will default
	    SLASHC to "-c" instead of "-ce", will fail to run MKS
	    glob, and will run perl's glob instead of the Korn shell
	    to expand expressions like <*.c>.  Note that MKS support can
	    also be compiled out.

ROOTDIR	    Full drive and path where MKS toolkit is installed.  Example,
	    "c:/mks" or "d:/".  Typically is already set in MKS environment.
	    Used only if $MKSARGS is set and $GLOB is not.  See GLOB.
	    Ignored if MKS support compiled out.

TMP	    First choice for temporary files, e.g., "h:\\tmp".  If not set,
	    uses $TMPDIR (see below), if that's not set, the current directory
	    is used.  Swapping also goes here unless $EXESWAP is defined.
	    Temp files are pseudo-pipes, the swap file, and the -e file.

TMPDIR	    If $TMPDIR is set and $TMP is not, the following is done internally:
		( $ENV{'TMP'} = $ENV{'TMPDIR'} ) =~ s,/,\\,g;
	    (Backslashes are reversed as a gesture to decendents of
	    the perl process.)  Creation of $ENV{'TMP'} from $TMPDIR is
	    done at perl.exe startup.  Note that the MKS tools use $TMPDIR as
	    a first choice; as a gesture of compatibility here, though,
	    here it is a second choice.

EXESWAP     First choice for swap out file location.  A RAM disk is
	    a nice choice.  $TMP is used if this isn't set.  (See also
	    $TMPDIR).  The swap file created the first time swapping is
	    invoked and is left open until perl exits or does an exec.
	    Set to ".off" (note illegal DOS name) to inhibit swapping--
	    useful for speedy running of small subprocesses.  This feature
	    (inhibition) can be turned on and off:
		$ENV{'EXESWAP'} = 'e:/tmp';	# directory for swap file
		system "ls";	# perl opens swap file and swaps to run ls
		$ENV{'EXESWAP'} = '.off';	# temporarily disable swap
		system "who";	# no swapping, who run in available memory
		$ENV{'EXESWAP'} = 'yes';	# anything other that '.off'.
		system "ps";	# swapping to original file

GLOB	    First choice for MKS globbing program: full path, name,
	    and extension.  Example: "d:/mks/etc/glob.exe".  The perl
	    globbing program (used for <*.c> expansion) is found, as
	    before, via the $PATH.  Used only in an MKS environment, and
	    then only when perl is run from a non-MKS program.  Ignored
	    if MKS support compiled out.

SHELL	    Full path name and extension of the shell used used for
	    subprocesses when wildcard expansion is required, e.g.,
	    "c:/mks/bin/sh.exe".  If undefined, COMSPEC is used.
	    Presumably this could be the MKS korn shell, but it can
	    be another shell (e.g., 4DOS) and thus $SHELL is inspected
	    even if MKS support is compiled out.

COMPSEC	    Full path name of DOS command interpreter, e.g., "c:\\command.com"
	    Used only if $SHELL is not defined.  If not found, "\\command.com"
	    is used.  (It is bad practice to allow $COMSPEC to default or to
	    have it have anything other than a full drive and path name.  You
	    don't want your programs looking for command.com on alternate
	    drives.)

METACHAR    List of characters that are metacharacters to the $SHELL or
	    $COMPSPEC.  Used to determine if command can be run directly or
	    if a subshell must be invoked.  If undefined, |<> is used
	    for COMSPEC and *"?<>|()&][$#`'\ for SHELL.

SLASHC	    The shell option for invoking a command.  Defaults:
	    /c for $COMSPEC, DOS version 4.x or better [sic]
	    sc where s is the switch character, for $COMSPEC, DOS < 4.0
	    -ce for $SHELL if $MKSARGS is set and MKS support is compiled in.
	    (-e needed to get status back).  -c for other $SHELLs.
	    (This is a guess.)

PERLLIB	    Directory containing perl library.  Defaults to
	    /usr/local/lib/perl.  Should be applicable even in Unix.

Globbing Conventions
------------------------------------

Unix and MKS do globbing differently than MS-DOS.  Under Unix and MKS,
"abc*" matches abc.c,  Under DOS, though, the * won't match '.'
The MKS tools use the former interpretation; the glob program distributed
with perl and the globbing built in to perl.exe use the latter.  (Don't
install the distributed glob.exe if you are using the MKS tools.)

You can also write your own globber and call it ___setargv (__setargv in C)
to replace the Microsoft library stuff.

Interpretation of Incoming Arguments
------------------------------------

There are three cases:
    1) Perl.exe was invoked MKS-style.  The arguments are pulled from the
       environment as per MKS dodsexec(5).  Several kilobytes of pre-globbed
       arguments can be passed this way.  Works even if $MKSARGS is not set,
       but not if MKS support is compiled out.

    2) Perl.exe was was invoked in non-MKS style but the MKS tools are intact.
       This will happen if there's if perl.exe is run by an incompatible
       program, such as Microsoft NMAKE, even though the toolkit is intact.
       The toolkit is presumed present iff $MKSARGS is set.  The MKS glob
       program is looked for in $GLOB, $ROOTDIR/etc/glob.exe, and
       /etc/glob.exe in that order.  (User really should set $ROOTDIR or
       $GLOB to avoid problems with "which drive?")  This step is not
       done if MKS support is compiled out.

    3) Perl.exe was invoked and $MKSARGS was not set or MKS support was
       compiled out.  Perl globs the arguments itself, using the
       ___setargv code in the Microsoft library.  That code deals with
       double (but not single) quotes and expands wildcards in the
       usual DOS manner, i.e., as in DIR and COPY.

Globbing <wildcards> Expressions
------------------------------------

Expressions like <*.c> are expanded by a subshell in Unix.  Under DOS,
there are the following cases:

1) $MKSARGS is defined and MKS support is compiled in.  The $SHELL is used,
   running echo piped to tr exactly as with the Bourne shell on Unix perl.

2) The supplied glob.exe is run.  This program simply takes the supplied
   command line, DOS-globs it, and writes the result, "\r\n" separated,
   to the perl process via a "pipe."

Running Subprocesses
------------------------------------

Perl.exe will by default swap itself out almost entirely when it runs a
subprocess other than MKS $GLOB.  (See EXESWAP).  The swap file is
opened when the first subprocess is run and is left open until perl
exits or does an exec.

The command line to be run is scanned for $METACHARacters as described
above.  If none are found, the subcommand is invoked directly.  If
metacharacter(s) are found, a $SHELL or $COMSPEC is invoked to run the command.

Use of a single | in a open() command does not constitute a metacharcter:
this is a directive to perl to open a pipe.  The following, too, has
no SHELL metacharacters since the subprocess is simply pwd:
	chop($direct = `pwd`);

Beware of MS-DOS "internal" commands; i.e., those that are built into
command.com.  Examples are DIR and COPY.  COMMAND.COM users can use
these directly if the command has $METACHARacters; if not, you must
invoke an explicit command.com:

	system "dir >my.fil";		   # > is a metacharacter
	system "$ENV{'COMSPEC'} /c ver";   # no metacharacters, must be explicit

Users of other $SHELLs must use the second format for anything to be
passed to command.com.

Be aware that no wild card expansion is going to be done by command.com
usless you're using one of the built-in commands that does it (e.g., COPY).
You can use <> expansion to get around this.  Even those who have fancy
$SHELLs should take note of this, since having perl run the $SHELL and
then the command uses less memory than if perl runs the $SHELL which runs
the command.

	$files = <*.c>;
	`the_com $files`;
intead of
	`the_com *.c`;

Typical MKS setup in profile.ksh
------------------------------------

# If you're using the MKS stuff, you probably don't have to do anything
# other than set MKSARGS.

export MKSARGS=1
# ROOTDIR set by init process or etc.rc or here.
export TMPDIR=e:/tmp
# EXESWAP left to default to $TMPDIR
# GLOB left to default to $ROOTDIR/etc/glob.exe
# SHELL set by init process or here.
# COMSPEC not used by perl.exe but probably defined for other uses.
# METACHAR not defined, left to default.
export PERLLIB=c:/lib/perl

Typical command.com setup in autoexec.bat
------------------------------------

Rem You probably don't need to do anything except set $TMP, which you
Rem may be doing anyway.

Rem MKSARGS not set
Rem ROOTDIR not used
set TMP=d:\tmp
Rem EXESWAP left to default to $TMP
Rem GLOB not used
Rem SHELL not set
Rem COMPEC set by config.sys SHELL command or by MS-DOS startup.
Rem METACHAR not defined, left to default.
set PERLLIB=c:/lib/perl

Typical setup in autoexec.bat for user of some other shell
------------------------------------
Rem MKSARGS not set
Rem ROOTDIR not used
set TMP=d:\tmp
Rem EXESWAP left to default to $TMP
Rem GLOB not used
set SHELL=c:\bin\whatever.exe
Rem COMPEC set by config.sys SHELL command or by MS-DOS startup.
set METACHAR=?*<>|	# or whatever, shell dependent
set PERLLIB=c:/lib/perl