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Full Discussion: difference bewteen pipe, xargs, and exec
Top Forums Programming difference bewteen pipe, xargs, and exec Post 302563933 by Corona688 on Wednesday 12th of October 2011 12:28:55 PM
Old 10-12-2011
First off, exec is nothing like the first two. exec does a variety of things from executing a command without returning(the shell is replaced by the new command), as well as opening/closing files.

pipes transfer the output of one program into the input of another. Without the pipe or redirection, they'd be trying to read from your keyboard and trying to print to your screen, but you can feed them data from any source and send their output anywhere you like. Shell utilities are intended to be flexible that way.

Code:
# useless example
echo asdf | cat

'echo' prints to standard output, 'cat' reads from standard input, the pipe joins them together so 'asdf' is read by cat and printed.

But what about this:

Code:
# This won't print anything
echo something | echo

echo does not read from standard input, it takes commandline parameters and nothing but parameters. So how do we translate from pipe into that?

This is what xargs is for.
Code:
echo a b c d | xargs command

is equivalent to command a b c d. Whenever you need to translate from a pipe or file into commandline arguments, xargs serves.

Code:
# This prints 'something' into xargs' input, causing xargs to run 'echo something'.
echo something | xargs echo

---------- Post updated at 10:28 AM ---------- Previous update was at 10:26 AM ----------

If you post some code which is confusing you, I'll try to explain it.
This User Gave Thanks to Corona688 For This Post:
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LEARN ABOUT DEBIAN

algorithm::checkdigits::m11_001

CheckDigits::M11_001(3pm)				User Contributed Perl Documentation				 CheckDigits::M11_001(3pm)

NAME

       CheckDigits::M11_001 - compute check digits for ISBN, ISSN, VAT RN (PT), HKID (HK), Wagon number (BR), NHS (GB), VAT (SL)

SYNOPSIS

	 use Algorithm::CheckDigits;

	 $isbn = CheckDigits('isbn');

	 if ($isbn->is_valid('3-88229-192-3')) {
	       # do something
	 }

	 $cn = $isbn->complete('3-88229-192-');
	 # $cn = '3-88229-192-3'

	 $cd = $isbn->checkdigit('3-88229-192-3');
	 # $cd = '3'

	 $bn = $isbn->basenumber('3-88229-192-3');
	 # $bn = '3-88229-192-'

DESCRIPTION

   ALGORITHM
       1.  The sequence of digits is processed right to left.  Every digit is multiplied with their position in the sequence (i.e.  the digit left
	   to the check digit has the weight 2 then 3 etc.).

	   With a Hongkong ID (hkid) the leftmost char is replaced with its position in the alphabet and then multiplied with 8 (its weight).

       2.  The sum of all products is computed.

       3.  The sum of step 2 is taken modulo 11.

       4.  The checkdigit is the difference of the sum from step 3 to eleven under the following conditions:

	   isbn,issn
		   If the difference is 10, the check digit is 'X'.

		   If the difference is 11, the check digit is 0.

	   ustid_pt
		   If the difference is greater then 9, the check digit is '0'.

	   hkid    If the difference is 10, the check digit is 'A'.

		   If the difference is 11, the check digit is 0.

	   wagonnr_br
		   If the difference is 10, the check digit is 0.

		   If the difference is 11, the check digit is 1.

	   nhs_gb  If the difference is 10, the number would not be taken.

		   If the difference is 11, the check digit is 0.

	   vat_sl  This is a little bit unclear, don't trust on the method for this type.

   METHODS
       is_valid($number)
	   Returns true only if $number consists solely of numbers and the last digit is a valid check digit according to the algorithm given
	   above.

	   Returns false otherwise,

       complete($number)
	   The check digit for $number is computed and concatenated to the end of $number.

	   Returns the complete number with check digit or '' if $number does not consist solely of digits and spaces.

       basenumber($number)
	   Returns the basenumber of $number if $number has a valid check digit.

	   Return '' otherwise.

       checkdigit($number)
	   Returns the checkdigit of $number if $number has a valid check digit.

	   Return '' otherwise.

   EXPORT
       None by default.

AUTHOR

       Mathias Weidner, <mathias@weidner.in-bad-schmiedeberg.de>

SEE ALSO

       perl, CheckDigits, www.pruefziffernberechnung.de.

perl v5.10.0							    2008-05-17						 CheckDigits::M11_001(3pm)
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