Copyright 1991-2016 by Kevin G. Barkes

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DCL DIALOGUE

Originally published December, 1991

Even More Stupid DCL Tricks

By Kevin G. Barkes

It's the end of the year, my desk looks like the remains of
a stationery store explosion, and I'm a week behind
deadline. So, gentle readers, it's time once again to delve
into the wonderfully wacky world of stupid DCL tricks, or
Stuff I Would Have Already Known If I Had Read The Release
Notes Before Calling The CSC.

INSTANT FILE NAME

While file i/o in DCL should be avoided due to performance
issues, it's sometimes necessary. You may need to write the
output of a command to a file for later reference, or
generate a temporary command file which is executed and
deleted.

The creation of temporary files under VMS can be a tricky
thing. Multiple users may use the same command file, and if
the procedure blindly creates a temporary file in a common
directory with a "hard-coded" file name, unpleasantries can
result.

I use a little one-line .COM file called GENNAME.COM
(Program 1) to do my temp file naming for me.

The call to the F$GETJPI lexical function returns the
ten-digit process identification number of the user process
which executes the procedure. For example, something like:

0000005219

The F$CVTIME lexical provides us with the current date and
time in the form of:

1991-10-25 14:11:18.06

Of course, the space, colons and period are invalid
characters when used in a VMS file name. The hyphens, while
legal, can be a bit confusing, and my preference is to
eliminate them.

We use DCL's string reduction capability to remove the
errant characters. The line is a bit hard to read because
the hyphen character is used for several purposes.

The hyphens following F$CVTIME() and the "." are DCL
continuation characters, which tell DCL the command
continues on the next lines. The hyphens contained in quotes
are characters we wish to remove from the string. Finally,
the remaining hyphens (those surrounded by spaces) act as
DCL string reduction operators.

So, the date and time string returned by F$CVTIME() is
reduced to:

1991102514111806

".TMP" is added to the string using the + concatenation
operator, placing in the global symbol GENNAME the file name

00000052191991102514111806.TMP

composed of the process id and the date and time at
one-hundredth of a second resolution. This provides a pretty
much completely unique file name. There are some rather
bizarre circumstances under which it wouldn't be totally
unique, but thinking about them gives me a headache.

Parenthetically, I was discussing string concatenation and
reduction during a recent DCL class to whom I'd just
explained the marvels of DEC command qualifer syntax. They
were somewhat numbed by such usage as /NODISUSER, /NOPAUSE,
and /NOSUSPEND. When I asked them what was the opposite of
concatenation, one responded "Noconcatenation?"

******************************

SUPER SCOPING

The SET SYMBOL command originally surfaced in one of the
later "point" releases of VMS Version 4. It permitted the
masking of symbols to DCL at the local or global level so
that they did not interfere with expected command procedure
execution.

For example, say you defined the symbol DELETE to equal
DELETE/CONFIRM/LOG in your LOGIN.COM file. You executed a
command procedure that created several temporary files and
later tried to delete them. At each deletion attempt, you'd
be prompted to enter a "Y" or "N" to confirm deletion.

By using the SET SYMBOL/SCOPE=NOGLOBAL command, you could
turn off DCL's ability to access global symbols, thus
eliminating the DELETE problem. Unfortunately, it also
eliminated the ability to create any new global symbols.

Since the last time I looked closely at the DCL Dictionary,
DEC added /GENERAL and /VERB qualifiers to SET SYMBOL's
/SCOPE qualifier. There are some peculiarities involved with
their use (you should read the doc on this one), but
basically the concept is simple.

When you issue the command

$ SET SYMBOL/SCOPE=NOGLOBAL/VERB

DCL will not attempt to translate the first word on the
command line to see if it's a symbol. It just tries to
execute the word as a command. So, in our example, instead
of translating DELETE to DELETE/CONFIRM/LOG, it just
executes the plain vanilla DELETE command.

What makes the /VERB qualifier particularly useful is that
it does not affect the way DCL translates other symbols
which might exist on the command line. This makes it much
more versatile than the plain /SCOPE=NOGLOBAL qualifier,
which totally shuts down all global symbol translations and
assignments.

SET SYMBOL also has a /GENERAL qualifier which causes DCL to
translate ONLY the first word on the command line and to
inhibit translation of the remaining symbols which may exist
on the line; in other words, the reverse of /VERB.

Maybe they should have called it /NOVERB. Or /NOUN?

***************************

Glen Hoffing of GE Aerospace in Camden, NJ provides a
command procedure (Program 2) which makes file
"housekeeping" a bit easier.

PURGE_MIDDLE.COM deletes all versions of a file except the
oldest and the newest, and accepts full wildcarding. "When I
make changes to one or more modules in a software
application," Glen explains, "it generally takes me several
code/compile/debug iterations before I get it right. When I
am done, I want to save the oldest (baseline) version of the
source, and the newest, and delete all the intermediate
versions. This command file allows me to do this
automatically."

As with all procedures which use the DELETE command, be
certain to test it in a directory with expendable scratch
files. Otherwise, enjoy.

And happy holidays to you all.

***************************

Kevin G. Barkes is an independent consultant and occasional
reader of DEC documentation. He publishes the KGB Report
newsletter, operates the www.kgbreport.com website, lurks on
comp.os.vms and can be reached at kgbarkes@gmail.com.

**************************

PROGRAM 1:

$! GENNAME.COM
$ GENNAME == F$GETJPI("","PID") -
  + F$CVTIME() -
  - "-" - "-" - " " - ":" - ":" - "." -
  + ".TMP"

**************************

PROGRAM 2:

$! PURGE_MIDDLE.COM
$ OLD_FN = "" !Initialize previous file name
$ OLD_FT = "" !Initialize previous file type
$ LOOP:       !Loop through each file
$ FIL = F$SEARCH(P1+";*",1) !Get file name
$  IF (FIL .NES. "")  !If file name not blank...
$   THEN
$   FN  = F$PARSE(FIL,,,"NAME")    !Get file name
$   FT  = F$PARSE(FIL,,,"TYPE")    !Get file type
$   VNO = F$PARSE(FIL,,,"VERSION") !Get file version
$!  Eliminate the ";" from the version number:
$   VNO = F$EXTRACT(1,F$LENGTH(VNO)-1,VNO)
$    IF (FN .NES. OLD_FN .OR. FT .NES. OLD_FT)
$     THEN !If this is an old file name
$!          If previous name found
$      IF (OLD_FN .NES. "" .OR. OLD_FT .NES. "")
$       THEN !if more than 1 version
$        IF (VNO_LOW .NES. VNO_HIGH)
$         THEN ! Purge middle versions of files
$         DEL 'OLD_FN''OLD_FT';*-
          /EXCL=('OLD_FN''OLD_FT';'VNO_HIGH','-
          'OLD_FN''OLD_FT';'VNO_LOW')
$         WRITE SYS$OUTPUT - ! Notify user of purge
          "...Purging file ''OLD_FN'''OLD_FT'",-
          " (saving versions ''VNO_LOW' and ''VNO_HIGH')"
$        ENDIF
$       ENDIF
$       OLD_FN = FN !save file name as previous file
$       OLD_FT = FT !save file type as previous type
$       VNO_HIGH = VNO !1st ver of new file is high ver
$      ENDIF
$      VNO_LOW = VNO !last version found is low ver
$      GOTO LOOP     !loop-find next file
$     ENDIF
$!    End of loop: process last file
$!    If no files found:
$      IF (OLD_FN .EQS. "" .AND. OLD_FT .EQS. "")
$!     Notify the user:
$       THEN
$       WRITE SYS$OUTPUT "Error! No file names match",-
        " this specification."
$        ELSE !else purge last file found
$!       If more than one version:
$         IF (VNO_LOW .NES. VNO_HIGH)
$          THEN
$!         Purge middle versions of files:
$          DEL 'FN''FT';*-
           /EXCL=('FN''FT';'VNO_HIGH','FN''FT';'VNO_LOW')
$          WRITE SYS$OUTPUT - !Notify user of purge
           "...Purging file ''FN'''FT'",-
           " (saving versions ''VNO_LOW' and ''VNO_HIGH')"
$         ENDIF
$         WRITE SYS$OUTPUT "File purge complete."
$        ENDIF