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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(1L)																 XARGS(1L)

NAME
xargs - build and execute command lines from standard input SYNOPSIS
xargs [-0prtx] [-e[eof-str]] [-i[replace-str]] [-l[max-lines]] [-n max-args] [-s max-chars] [-P max-procs] [--null] [--eof[=eof-str]] [--replace[=replace-str]] [--max-lines[=max-lines]] [--interactive] [--max-chars=max-chars] [--verbose] [--exit] [--max-procs=max-procs] [--max-args=max-args] [--no-run-if-empty] [--version] [--help] [command [initial-arguments]] DESCRIPTION
This manual page documents the GNU version of xargs. xargs reads arguments from the standard input, delimited by blanks (which can be pro- tected with double or single quotes or a backslash) or newlines, and executes the command (default is /bin/echo) one or more times with any initial-arguments followed by arguments read from standard input. Blank lines on the standard input are ignored. xargs exits with the following status: 0 if it succeeds 123 if any invocation of the command exited with status 1-125 124 if the command exited with status 255 125 if the command is killed by a signal 126 if the command cannot be run 127 if the command is not found 1 if some other error occurred. OPTIONS --null, -0 Input filenames are terminated by a null character instead of by whitespace, and the quotes and backslash are not special (every character is taken literally). Disables the end of file string, which is treated like any other argument. Useful when arguments might contain white space, quote marks, or backslashes. The GNU find -print0 option produces input suitable for this mode. --eof[=eof-str], -e[eof-str] Set the end of file string to eof-str. If the end of file string occurs as a line of input, the rest of the input is ignored. If eof-str is omitted, there is no end of file string. If this option is not given, the end of file string defaults to "_". --help Print a summary of the options to xargs and exit. --replace[=replace-str], -i[replace-str] Replace occurences of replace-str in the initial arguments with names read from standard input. Also, unquoted blanks do not termi- nate arguments. If replace-str is omitted, it defaults to "{}" (like for `find -exec'). Implies -x and -l 1. --max-lines[=max-lines], -l[max-lines] Use at most max-lines nonblank input lines per command line; max-lines defaults to 1 if omitted. Trailing blanks cause an input line to be logically continued on the next input line. Implies -x. --max-args=max-args, -n max-args Use at most max-args arguments per command line. Fewer than max-args arguments will be used if the size (see the -s option) is exceeded, unless the -x option is given, in which case xargs will exit. --interactive, -p Prompt the user about whether to run each command line and read a line from the terminal. Only run the command line if the response starts with `y' or `Y'. Implies -t. --no-run-if-empty, -r If the standard input does not contain any nonblanks, do not run the command. Normally, the command is run once even if there is no input. --max-chars=max-chars, -s max-chars Use at most max-chars characters per command line, including the command and initial arguments and the terminating nulls at the ends of the argument strings. The default is as large as possible, up to 20k characters. --verbose, -t Print the command line on the standard error output before executing it. --version Print the version number of xargs and exit. --exit, -x Exit if the size (see the -s option) is exceeded. --max-procs=max-procs, -P max-procs Run up to max-procs processes at a time; the default is 1. If max-procs is 0, xargs will run as many processes as possible at a time. Use the -n option with -P; otherwise chances are that only one exec will be done. SEE ALSO
find(1L), locate(1L), locatedb(5L), updatedb(1) Finding Files (on-line in Info, or printed) XARGS(1L)
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