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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 BSD

xargs

XARGS(1)						      General Commands Manual							  XARGS(1)

NAME

       xargs - construct argument list(s) and execute utility

SYNOPSIS

       xargs [ -t ][[ -x ] -n number ][ -s size ][ utility [ arguments...  ]]

DESCRIPTION

       The  xargs utility reads space, tab, newline and end-of-file delimited arguments from the standard input and executes the specified utility
       with them as arguments.

       The utility and any arguments specified on the command line are given to the utility upon each invocation, followed by some number  of  the
       arguments read from standard input.  The utility is repeatedly executed until standard input is exhausted.

       Spaces,	tabs  and  newlines  may  be embedded in arguments using single (`` ' '') or double (``"'') quotes or backslashes (``'').  Single
       quotes escape all non-single quote characters, excluding newlines, up to the matching single quote.  Double quotes  escape  all	non-double
       quote characters, excluding newlines, up to the matching double quote.  Any single character, including newlines, may be escaped by a back-
       slash.

       The options are as follows:

       -n number Set the maximum number of arguments taken from standard input for each invocation of the utility.  An invocation of utility  will
		 use less than number standard input arguments if the number of bytes accumulated (see the s option) exceeds the specified size or
		 there are fewer than number arguments remaining for the last invocation of utility.  The current  default  value  for	number	is
		 5000.

       -s size	 Set  the  maximum number of bytes for the command line length provided to utility.  The sum of the length of the utility name and
		 the arguments passed to utility (including /dev/null terminators) will be less than or equal to this number.  The current default
		 value for size is ARG_MAX - 2048.

       -t	 Echo the command to be executed to standard error immediately before it is executed.

       -x	 Force	xargs  to  terminate  immediately if a command line containing number arguments will not fit in the specified (or default)
		 command line length.

       If no utility is specified, echo(1) is used.

       Undefined behavior may occur if utility reads from the standard input.

       The xargs utility exits immediately (without processing any further input) if a	command  line  cannot  be  assembled,  utility	cannot	be
       invoked, an invocation of the utility is terminated by a signal or an invocation of the utility exits with a value of 255.

       The xargs utility exits with a value of 0 if no error occurs.  If utility cannot be invoked, xargs exits with a value of 127.  If any other
       error occurs, xargs exits with a value of 1.

SEE ALSO

       echo(1), find(1)

STANDARDS

       The xargs utility is expected to be IEEE Std 1003.2("POSIX") compliant.

								   June 6, 1993 							  XARGS(1)
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