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Top Forums Shell Programming and Scripting KSH: Opening Files based on a file list Post 302443266 by agama on Sunday 8th of August 2010 12:24:44 AM
Old 08-08-2010
Quote:
Originally Posted by drumminfool91
I'm trying to understand xargs, though. What does that do for me here? I only ask b/c I've never used before.
It is possible to have written the command like this:

Code:
cat $(egrep -l "pattern" *)

except that the result of the egrep might be more arguments than can be handled on a single command line. By piping the output to xargs, xargs will execute the command that you give it (cat in this case) with n number of arguments such that n does not exceed the limit from a command line perspective. It may invoke the command multiple times to accomplish its task.

With this said, I realise that my original command would be flawed as the '*' given to egrep might expand into too many arguments, causing an error, and thus the xargs in this case would do no good. This would probably be better:

Code:
ls | xargs egrep -l "system.*log|log.*system" | xargs cat > result

The ls command generates a list of filenames which is given to egrep via the first xargs. The result of the searches is then given to cat via the second xargs.

When writing a script it is usually best to assume the worst case and use xargs in this situation. If you're willing to live with the errors, and are pretty sure that there will not be more files in a directory than the max allowable args on a command line, then the first form (without xargs) is easiest and straightforward.
 

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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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