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PATHINFO.TXT
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1993-01-07
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6KB
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101 lines
INFORMATION ON THE DOS ENVIRONMENT PATH STRING
----------------------------------------------
All versions of PC and MS DOS above 1.x maintain an area in
memory called the environment. The environment contains a series of
text strings that are used by DOS and applications programs for
various purposes. You can see what strings are currently in the
enviroment by typing SET and pressing <RETURN> from the DOS level. As
a minimum, the environment will contain the string COMSPEC= followed
by a DOS directory path and (usually) COMMAND.COM. Most hard disk
equipped machines will show the following COMSPEC string:
COMSPEC=C:\COMMAND.COM
Other strings that frequently appear in the environment are the
PROMPT= and PATH= strings. Control of the environment is effected
through the DOS SET command, which may be entered at the DOS level or
from a batch (like AUTOEXEC.BAT) file. For example, if a user desired
that DOS used a copy of COMMAND.COM that was in a directory named
C:\DOS, he/she might place the command SET COMSPEC=C:\DOS\COMMAND.COM
in the AUTOEXEC.BAT file. After booting, DOS would load COMMAND.COM
from C:\DOS each time it needed to reload the command interpreter.
All other copies of COMMAND.COM in the system would be superfluous and
ignored by DOS.
One very useful string that can be placed in the DOS environment
by the user is the PATH= string. When the name of an executable file
is issued from the DOS level, DOS will first look in the current
subdirectory for the file. If it finds it, it loads and executes it
and all is fine. If it cannot find the file in the current directory,
DOS will search the environment for the PATH= designator. If one
exists, DOS will start searching the system directories that are in the
PATH= string for the executable file. For example, suppose that
the AUTOEXEC.BAT contains the command SET PATH=C:\DOS;D:\UTIL;E:\JUNK.
Note that directory path names are separated by semicolons and can
(and should) include the drive designator. Now further suppose that
the user is in a directory named D:\SOMENAME and desires to run the
program MYPROG.EXE but MYPROG.EXE is not resident in D:\SOMENAME.
When the user enters MYPROG, DOS, unable find it in the current
directory, starts searching the directories in the PATH= string
starting with C:\DOS. If MYPROG.EXE was in D:\UTIL, DOS would load
and execute the program and D:\UTIL would be the active directory when
the program received control from DOS. Obviously, if DOS cannot find
MYPROG.EXE in any of directories in the PATH= string, the message BAD
COMMAND OR FILENAME will be displayed.
The exact same sequence is followed if a running program calls
DOS to execute a program but with one possible important difference.
For technical reasons, some programs (like ZipMaster) need to load a
secondary copy of COMMAND.COM and then pass the name of the program to
be executed to COMMAND.COM. This is accomplished by searching the
environment string for the COMSPEC= string to locate COMMAND.COM,
changing to that directory, and telling DOS to load COMMAND.COM. The
command passed to DOS might look somethin like this:
COMMAND.COM MYPROG.EXE
This tells DOS to run COMMAND.COM and for COMMAND.COM to load and
execute MYPROG.EXE. Even if MYPROG.EXE were located in the active
directory when the applications program passed this command to DOS,
COMMAND.COM might not find MYPROG.EXE because the applications program
switched to the directory where COMMAND.COM was located in order to
start things rolling. Therefore, unless MYPROG.EXE was in a directory
contained in the PATH= string or in the same directory as COMMAND.COM,
DOS would not find it.
Each Identifier in the DOS path string MUST be unique. For
example, consider the path identifier C:\UTIL\DOS\MYDIR. Placing
this string in the DOS path string will NOT automatically place the
the dirctories C:\UTIL and C:\UTIL\DOS in the DOS path. Each must
be uniquely identified by the correct path name. For each of these
directories to be identified in the DOS path, the following would
be entered:
C:\UTIL;C:\UTIL\DOS;C:\UTIL\DOS\MYDIR
ZipMaster depends upon the presence of an archive system's
executable files (PKZIP.EXE, PKUNZIP.EXE, etc.). It calls these
programs by using the command sequence just described and therefore
cannot function properly unless these programs are in a directory
specified in the PATH= string. The simplest thing to do is to place
your archive system files and LIST.COM in one subdirectory and place
its DOS path name in the environment. For example, suppose you are
using PKWare and you place PKZIP, PKUNXIP, and LIST in a directory
named C:\ARCUTILS. Placing the following line in your AUTOEXEC.BAT
file and rebooting will ensure that ZipMaster will function properly:
PATH=C:\ARCUTILS
Obviously, the PATH= string can contain other directory path names and
you may already have a PATH= command in your AUTOEXEC.BAT file. If
so, just add the directory containing the archive system files to the
string. For maximum speed, make it the first directory name in the
string.
When ZipMaster loads it will check your system environment
to see if a PATH= string exists. If so, it will search each
directory in the string to see if any of the supported archive
system files can be found in the directories defined in the PATH.