Shell Programming and Scripting

Greetings.

I am happily using constructs like the following; I have set -x to show fine distinctions.

Code:

$ exec 4> afile
+ exec
+ 4> afile

$ print -u4 This is the first line in afile

As you can see from the -x expansion, the shell performed the exec command and redirected file descriptor [4] to afile. And yes, the print to fd[4] works just fine.

Now what if I want to use a variable FD, as I would in C or Perl?

Code:

$ fd2=6
$ exec ${fd2}> another
+ exec 6
+ 1> another
ksh: exec: 6: not found

Not the difference: Instead of the blank exec command, it tried to execute a file named 6 and redirected stdout (fd[1]) to "another". This did create the file named "another" but it also closed the shell, as any erroneous exec would do.

So here's my question: Is there a way to open file descriptor via a variable?

By the way, back to fd[4]: Now that I have opened it, I can do this:

Code:

$ fd=4
$ print -u${fd} This is a second line in afile

and it expands to

Code:

+ print -u4 This is a second line in afile

as expected.

And, of course, whatever syntax can be used to open a FD via a variable needs to work to close it i.e. exec ${fd}>&-

Ideas, anyone?

Thanks.

-- Rasputin Paskudniak (Signature file pending..)

Maybe you can give a try using the eval command so that the variable's value is substituted before the command is run

Actually, cts, I had tried a syntax for using eval before I posted. I omitted it because my post was long enough. If I recall, it was something like:

Code:

$ fd=4
$ eval 'exec ${fd}> another'

I'm on a windows box right now so I can't be certain but I think the above eval command resulted in a closed shell just as surely as the naked exec command had done. So to repeat my "smiley" (if it may thus be called):

If you can come up with a variation that does NOT close the shell, it will be progress.

BTW, I just tried that variable form in a Cygwin BASH window. It also failed - could not find 4 - but at least it did not close the shell. Ah, but it also causes all stdout output to be redirected to the target file, making the shell kinda useless. This is both with and without the eval.

More ideas are SOOO welcome!

Rasputin Paskudniak

Greetings again.

Very fortunately, I was able to bring my question to DGK himself and obtained permission to quote him. Here is his response, which answers several questions and even addresses my intended follow-up.
The really great thing about this is that it relieves me of the responsibility of finding an available file descriptor to use for my ancillary I/O; in obtaining the next available file descriptor from a pool, it behaves very much like the OPEN calls in C and Perl.

With gratitude to Dr. K, who has thus spared y'all from that lengthy followup,
-- Rasputin Paskudniak

Removed.