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Full Discussion: exec command and field descriptors..
Top Forums UNIX for Dummies Questions & Answers exec command and field descriptors.. Post 58924 by Perderabo on Saturday 4th of December 2004 04:02:16 PM
Old 12-04-2004
The trouble is that exec can do several different things...it really needs to be several commands. The code you gave won't work. Change that to
exec $fd < inputfile
and it probably will. But that is not what's meant by an fd.

A fd is always an integer. In shell scripts, it will be a very low integer. By convention:
0 = standard input
1 = standard output
2 = standard error output

The idea is that you write your program to output to fd 1 without knowing what fd one is. Then at execution time you can do stuff like:
echo this > first.file
echo that > second.file

It would be terrible if echo always sent stuff to "first.file". You would need to do:
echo that
cp first.file second.file
or something like that.

By default 0 1 2 are all connected to /dev/tty so you can type input to a program and see the results in your window.

Here is an experiment I just did:
$ expr 1 + 2
3
$ expr 1 + 2 > expr.out
$ cat expr.out
3
$ expr cat + dog > expr.out
expr: non-numeric argument
$

With the last expr command, I have an error. Since the error goes to 2 which is still /dev/tty, I see it immediately, even though the standard which is 1 goes to a file. That why we have both 1 and 2. You can send 1 into a file while 2 is still displayed to you.

Don't want to see error messages? Bad idea usually, but you can do:
expr cat + dog > expr.out 2>/dev/null

And now error messages are thrown away.

expr cat + dog > expr.out
really means
expr cat + dog 1> expr.out
but if you leave the integer off, 1 is assumed for > while 0 is assumed for <
 

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LEARN ABOUT DEBIAN

griffith

griffith(1)															       griffith(1)

NAME

       griffith - film collection manager

SYNOPSIS

       griffith

DESCRIPTION

       Griffith  is a film collection manager.	Adding items to the movie collection is as quick and easy as typing the film title and selecting a
       supported source. Griffith will then try to fetch all the related information from the Web.

OPTIONS

       -h --help
	      Show usage information.

       -D --debug
	      Run in debug mode.

       -C --clean
	      find and delete orphan files in posters directory.

       --check-dep
	      check dependencies - if all needed Python modules are installed, it prints nothting.

       --sqlecho
	      Prints all SQL queries to standard output.

       --home DIR
	      Set Griffith's home directory. Use this option if you want to change the default ~/.griffith

       Printing movie list:

       -c <expr> --cast=<expr>
	      cast field must contain <expr>

       -d <expr> --director=<expr>
	      director field must contain <expr>

       -o <expr> --original_title=<expr>
	      original title field must contain <expr>

       -t <expr> --title=<expr>
	      title field must contain <expr>

       -y <number> --year=<number>
	      movie must be released in year <number>

       -s <columns> --sort=<columns>
	      set sort columns to <columns>. Example: --sort=o_title,number,director

SEE ALSO

       Homepage: <http://www.griffith.cc/>

BUGS

       If you want to help or report any bugs founded please visit homepage <http://www.griffith.cc/> or <https://bugs.launchpad.net/griffith>

AUTHORS

       Griffith was written by Vasco Nunes and Piotr OA1/4arowski.  See the AUTHORS and THANKS files for a list of other contributors.

       Copyright (C) 2005-2008 Vasco Nunes <vasco.m.nunes@gmail.com>, Piotr OA1/4arowski <ozarow+griffith@gmail.com>,

       This man page was written by Piotr Ozarowski

								   Dec 20, 2006 						       griffith(1)
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