Software Vault: The Gold Collection/Software Vault - The Gold Collection (American Databankers) (1993).ISO/cdr11/mpc93jun.zip/ENVIRON.TXT

Text File | 1993-04-19 | 16KB | 364 lines

                          The DOS Environment
                            By Ken Johnson,

     The DOS environment is a special area of memory that DOS uses as
     a "cubbyhole" to store information needed at a later time.
     Knowing what it is and how it is used is becoming important, as
     software programs and batch files get more sophisticated and make
     more use of the environment.  This month we'll look at the
     environment and environment variables.

     The DOS environment contains string (text) variables that DOS,
     some application programs, and batch files can check and use.
     Environment variables that DOS uses include PROMPT, PATH,
     COMSPEC, and DIRCMD, which we'll discuss below.  Application
     programs can create and use a variety of environment variables.
     Three examples on my computers are TEMP (defines the Windows 3.0
     directory for temporary files), GMK4 (defines the Grammatik IV
     subdirectory), and 123CPBW (indicates whether Lotus 123 release
     3.x is running on a monochrome monitor).

     The SET command is used to create and delete environment
     variables.  Environment variables are "global", since once
     created the can be used by any program or batch file (though they
     can't be used on the command line).  Usually SET commands are
     included in the AUTOEXEC.BAT file so that environment variables
     will be created each time you boot up your computer.  Any
     application program that uses environment variables will usually
     add the SET statement to your AUTOEXEC.BAT file when it's
     installed.

     The form of the SET command is:

     SET variable=value

     where "variable" is the variable name and "value" is the text
     string it represents.  Some examples are:

     SET DIRCMD=/OE
     SET TEMP=C:\WINDOWS\TEMP
     SET COMSPEC=C:\DOS\COMMAND.COM
     SET 123CPBW=YES

     The variable name is always converted to upper case, but the
     value remains as it was entered.  Also be aware the names are
     space sensitive, so "SET USER=JOE" is not the same as "SET USER =
     JOE".  To remove an environment variable, enter SET variable=
     with no value specified.

     By entering "SET" alone on the command line, you'll see a listing
     of what's in the environment.  This will include your current
     PATH and PROMPT, the name and path of your command interpreter
     (i.e., COMMAND.COM) in the COMSPEC variable, and any other
     environment variables that have been created.

     Now let's look at some environmental variables and how they are
     used.


     The COMSPEC Variable

     The COMSPEC environment variable points to the location (drive
     and directory) of your command interpreter, usually COMMAND.COM.
     This is important because COMMAND.COM loads into memory in two
     parts, a permanent portion and a transient portion.  The
     transient part of COMMAND.COM can be removed from RAM if an
     application program needs that memory.  When the program ends,
     DOS needs to reload COMMAND.COM to continue.

     The COMSPEC variable tells DOS where to find COMMAND.COM if it
     needs to be reloaded.  If DOS can't find COMMAND.COM, you'll get
     a "Cannot load COMMAND, system halted" message and have to
     reboot.  Normally the COMSPEC variable is set at the beginning of
     your AUTOEXEC.BAT file, but the preferred alternative is to use
     the SHELL= statement in CONFIG.SYS to point to COMMAND.COM.
     We'll discuss the SHELL statement below.


     The PATH command/variable

     The PATH is a listing of the drives and directories that DOS
     should search when you enter the name of an executable file on
     the command line.  DOS will always look in the current (default)
     directory first, then search the directories listed in the PATH
     in the order specified.

     The PATH is specified as:

     PATH=[d:]\directory;[d:]\directory; . . .

     where "d:" is an option drive identifier and "directory" is the
     directory name; you separate the directory names with a
     semicolon.  The total length of the PATH statement must be less
     than 128 characters, since the DOS command line is limited to 127
     characters.  Notice that though PATH is an environment variable,
     you don't have to enter it with the SET command.  Both "SET PATH=
     . . ." and "PATH= . . ." will work.

     Since the path is searched in the order specified, make sure to
     put the directories containing your most used programs at the
     beginning.  For example, PATH=C:\DOS;C:\;C:\UTILITY shows that
     DOS will search for a program in this order:

     1.   The default directory (always searched first, before
          the path is ever accessed)
     2.   The DOS subdirectory on the C: drive
     3.   The root directory of the C: drive (C:\)
     4.   The UTILITY subdirectory on the C: drive

     Another key to the PATH is to keep it as short as possible; in
     other words, don't put ALL your directories on the PATH.  There
     are two reasons for this.  First, since the PATH is an
     environment variable it takes up environment space that may be
     needed by other variables.  In DOS 3.x for example, a 120
     character PATH statement uses three-fourths of the default
     environmental space.  Secondly, every time you make a typing
     mistake DOS will search the entire path before returning the "Bad
     command or file name" message.  Longer paths mean more time to
     wait each time you make a mistake.

     Since the PATH is part of the environment, you can view the path
     with the other environment variables by entering "SET" at the DOS
     prompt.  You also can enter "PATH" or "PATH=" to see just the
     currently defined PATH.  To remove the PATH from the environment
     (thus forcing DOS only to search the current directory), enter
     either "PATH ;" or "SET PATH=" at the DOS prompt.


     The PROMPT command/variable

     You've seen the default DOS prompt hundreds of times:  the
     infamous "C>".  The PROMPT variable determines what prompt is
     displayed, so you can use it to change "C>" to something a little
     more user friendly.  The prompt is specified as:

          PROMPT text

     where "text" is any string of text or special prompt characters.
     These special characters will display system information as part
     of the prompt:

     $P  --  the default directory
     $T  --  the current time (HH:MM:SS.xx)
     $D  --  the current date (MM-DD-YY)
     $V  --  the DOS version
     $N  --  the default drive
     $_  --  Carriage Return plus Line Feed
     $E  --  the ESCape character
     $H  --  backspace (erases the previous character)
     $G  --  the > character
     $L  --  the < character
     $B  --  the | character
     $Q  --  the = character

     As you can see, the standard DOS prompt is $N$G, the default
     drive and the ">" sign.  If you enter the PROMPT command with
     nothing after it, DOS resets the prompt to this default.  As with
     the PATH, you do not have to use the SET statement with PROMPT.
     Some sample prompts:

     PROMPT $P$G
     will display the current drive and directory with the
     greater than sign.  This is the most common PROMPT setting, since
     it shows you exactly where in the directory structure you are
     located.  For example, if you are in the DOCS directory of the
     WP51 directory on your C: drive, your prompt will show:

     C:\WP51\DOCS>_

     PROMPT Time is $T$H$H$H$H$H$H$_$P$_Command--$G
     will give a multi level prompt (the six $H are used to erase
     the seconds and hundredths from the time):

     Time is 10:35
     C:\WP51\DOCS
     Command-->_

     PROMPT
     will reset the prompt to the default ($N$G):

     C:>

     There is one minor problem when having DOS display the current
     directory ($P).  Each time you press Enter, DOS will read the
     disk to find the correct directory for the prompt.  If your
     default drive is a floppy drive and you take the disk out, DOS
     will try to read the empty drive and you'll get an error message:

     Not ready reading drive A:
     Abort, Retry, Fail?

     At this point you can put the disk back in and press R.  Or you
     can simply press F, in which case your prompt will become:

     Current drive is no longer valid>_

     Just change back to your hard drive and your old prompt will
     reappear.


     The DIRCMD variable

     In DOS 5.0, the DIRectory command now supports several new
     switches to control what files are displayed and in what order.
     For example, the /OGEN switch will display files sorted by
     extension and name, with directories listed first.  Rather than
     have to remember (and type in) certain switches with every DIR
     command, you can use the DIRCMD variable to indicate which
     switches to be used by default.  For example,

     SET DIRCMD=/OGEN

     will give you a directory sorted by extension and name (with
     directories first) every time you use the DIR command.


     The TEMP variable

     The TEMP variable is used most often to tell a program where to
     store temporary files.  These are "working" files created as the
     program runs.  Under normal operation, the program will delete
     these temporary files when  finished with them.  One such program
     is Windows 3.0, which uses a \TEMP subdirectory to store it's
     temporary files.  When you install Windows it will add the
     appropriate SET TEMP statement to your AUTOEXEC.BAT file.
     Assuming Windows is on your C: drive, this probably will be SET
     TEMP=C:\WINDOWS\TEMP.

     One trick to speed up Windows is to use a RAM disk for the TEMP
     subdirectory.  A RAM disk is an area of memory that is treated as
     a physical disk drive.  You can set up a RAM disk by using a
     driver such as RAMDRIVE.SYS, which comes with Windows.  Once
     you've installed the RAM disk, change the SET TEMP statement in
     your AUTOEXEC.BAT file to point the RAM disk's drive designator.
     For example, if the RAM drive were drive E:, specify SET
     TEMP=E:\.

     Since it is much faster to write to RAM than to a physical disk
     drive, using a RAM disk for Windows' TEMP subdirectory will
     improve Windows' performance.  Another benefit of using a RAM
     disk for Windows' temporary files is that you'll be sure they are
     deleted when no longer needed.  Windows normally deletes all
     temporary files, unless it ends abnormally.  If you've every
     gotten an "Unrecoverable Application Error" and had to reboot
     your system, Windows may have left some temporary files in the
     \TEMP subdirectory.  By using a RAM disk, the temporary files
     will be wiped out whenever you reboot or turn off your system.


     Using Environment Variables in Batch Files

     One powerful feature of environment variables is the ability to
     use them in batch files in DOS 3.2 and above.  You surround the
     name of the variable with percent signs (for example, %PATH%);
     when the batch file executes, the variable name expands to it's
     value.  Remember that the variable's value is case sensitive and
     space sensitive.  For example, part of your batch file might
     include:

     [==> SHARON, please use smaller font so that lines are not split
     <==]

     SET DONE=false
     :LOOP
       .
       .  (other commands here, one of which is SET DONE=true)
       .
     IF %DONE%==false GOTO LOOP

     Since the path is an environment variable, it is often used in
     batch files.  For example, you can create two simple batch files
     that will add a directory to your current path in the first
     position, then restore the original path.  These are NEWPATH.BAT
     and OLDPATH.BAT:

     [==> Note: please use smaller font so that lines are not split
     <==]

     ECHO OFF
     REM NEWPATH.BAT
     IF %1!==! GOTO NODIR
     SET OLD=%PATH%
     PATH=%1;%OLD%
     ECHO Your path is now %PATH%
     GOTO END
     :NODIR
     ECHO Please enter the directory to add to the path
     :END


     ECHO OFF
     REM OLDPATH.BAT
     IF %OLD%!==! GOTO END
     PATH=%OLD%
     SET OLD=
     :END

     The NEWPATH batch file first checks to make sure that a new
     directory was entered on the command line (which becomes
     replaceable parameter %1).  It then stores the original path in a
     new environment variable called OLD.  The PATH is redefined, with
     the new directory appended on the front of the old path
     (%1;%OLD%).  The batch file will then display the new path with
     the ECHO command.

     When you are ready to restore the original path, simply execute
     the OLDPATH batch file.  It uses the OLD environment variable as
     the new path, then deletes the OLD environmental variable.
     Notice that the batch file first checks whether there is an OLD
     environment variable, and simply branches to the end if it
     doesn't exist.  Without an OLD environment variable the
     "PATH=%OLD%" statement would resolve as "PATH=", which would
     display the current path.


     Expanding the Size of the Environment with SHELL

     Since it is an area of RAM, it is possible to fill the memory
     allocated to the environment.  This is particularly true if you
     have a long PATH and create several environment variables.  In
     the NEWPATH batch file example above, you are significantly
     increasing the size of your environment because it now contains
     both the old path and the new path.  If the environment does
     fill, you'll receive an "Out of Environment Space" message.  This
     means you must increase the size of the environment.

     The default environment is 160 bytes in DOS 2.x, 3.x, and 4.x,
     and 256 bytes in DOS 5.0.  In DOS 3.1 and above you can specify
     the desired environment size with the SHELL command in your
     CONFIG.SYS file.  In DOS 2.x and 3.0, you can only increase the
     environment by modifying (also called "patching") the COMMAND.COM
     program -- not a job for the faint hearted!

     The SHELL statement in CONFIG.SYS tells DOS name of the command
     interpreter (usually COMMAND.COM), its location, and the size of
     the DOS environment.  The SHELL statement usually looks something
     like this:

     SHELL=C:\DOS\COMMAND.COM C:\DOS /P /E:512

     As you might guess, the "/E:nnn" switch sets the size of the
     environment, 512 bytes in this example.  In DOS 3.2 and above,
     simply specify the size in bytes.  If you are using DOS 3.1, the
     environment is specified in 16-byte blocks, so indicate the
     number of blocks on the /E switch.  To get a 512 byte
     environment, include /E:32 on the SHELL command (16 bytes times
     32 blocks = 512 bytes).

     Let's look at the other parts of the SHELL statement.
     "C:\DOS\COMMAND.COM" shows the name and location of the command
     interpreter.  "C:\DOS" points to the directory where COMMAND.COM
     is located, so that it can be reloaded into memory if necessary.
     This will create the COMSPEC variable in the environment, meaning
     you don't have to explicitly SET it in your AUTOEXEC.BAT file.
     Finally, the "/P" switch indicates COMMAND.COM should remain
     permanently in memory.

     The DOS environment is one of those (perhaps few!) things with
     computers that you may never need to worry about.  But making
     good use of the environment by setting a custom PROMPT, and
     efficient PATH, and a fast TEMP subdirectory can help you be a
     more productive PC user.  Just keep an eye on your environment
     size!