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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 ULTRIX

test

test(1) 						      General Commands Manual							   test(1)

Name
       test - test conditional expression

Syntax
       test expr
       [ expr ]

Description
       The  command  evaluates the expression expr.  If the value of expr is true, the command returns a zero exit status; otherwise, it returns a
       nonzero exit status.  The command also returns a nonzero exit status if no arguments are specified.

Options
       The following primitives are used to construct expr:

       -r file		   Tests if the file exists and is readable.

       -w file		   Tests if the file exists and is writable.

       -f file		   Tests if the file exists and is not a directory.

       -d file		   Tests if the file exists and is a directory.

       -s file		   Tests if the file exists and has a size greater than zero.

       -t [ fildes ]	   Tests if the open file, whose file descriptor number is fildes (1 by default), is associated with a terminal device.

       -z s1		   Tests if the length of string s1 is zero.

       -n s1		   Tests if the length of the string s1 is nonzero.

       s1 = s2		   Tests if the strings s1 and s2 are equal.

       s1 != s2 	   Tests if the strings s1 and s2 are not equal.

       s1		   Tests if s1 is not the null string.

       n1 -eq n2	   Tests if number1 equals number2.

       n1 -ge n2	   Tests if number1 is greater than or equal to number2.

       n1 -gt n2	   Tests if number1 is greater than number2.

       n1 -le n2	   Tests if number1 is less than or equal to number2.

       n1 -lt n2	   Tests if number1 is less than number2.

       n1 -ne n2	   Tests if number1 is not equal to number2.

       These primitives can be combined with the following operators:

       !expr		   Negates evaluation of expression.

       expr -a expr	   Tests logical and of two expressions.

       expr -o expr	   Tests logical or of two expressions.

       ( expr... )	   Groups expressions.

       The -a operator takes precedence over the -o operator.  Note that all the operators and flags are separate  arguments  to  Note	also  that
       parentheses are meaningful to the Shell and must be escaped.

See Also
       find(1), sh(1), test(1sh5)

																	   test(1)
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