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Top Forums Shell Programming and Scripting Concatenation of a large number of files Post 302472237 by Corona688 on Tuesday 16th of November 2010 03:04:41 PM
Old 11-16-2010
Quote:
Originally Posted by ctsgnb
ls command should be coded to be able to handle huge number of argument.
It's an operating system limitation. They can't. Ergo, you should program in ways that don't cram potentially unlimited numbers of arguments into a commandline. In some shells, you can get away with cramming billions of arguments into a builtin (and ONLY a builtin), but you really can't depend on that.

Bad:cat * > output
Good:ls | xargs cat > output xargs understands maximum arguments and can split them intelligently across several cat calls.

The same goes for things like for LINE in `cat foo` ; do ... ; done when foo exceeds 4K in size you may discover the maximum size of a shell variable on your system. Instead: while read LINE ; do ... ; done < foo

Essentially the idea's the same in all cases: Never force the shell to hold an entire anything in memory. (That actually applies to most programming languages, but shell makes it deceivingly easy to do so...) You'll hit walls at inconvenient times and it's never efficient. Process in bits.

Last edited by Corona688; 11-16-2010 at 04:16 PM..
 

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SHELL-QUOTE(1p) 					User Contributed Perl Documentation					   SHELL-QUOTE(1p)

NAME
shell-quote - quote arguments for safe use, unmodified in a shell command SYNOPSIS
shell-quote [switch]... arg... DESCRIPTION
shell-quote lets you pass arbitrary strings through the shell so that they won't be changed by the shell. This lets you process commands or files with embedded white space or shell globbing characters safely. Here are a few examples. EXAMPLES
ssh preserving args When running a remote command with ssh, ssh doesn't preserve the separate arguments it receives. It just joins them with spaces and passes them to "$SHELL -c". This doesn't work as intended: ssh host touch 'hi there' # fails It creates 2 files, hi and there. Instead, do this: cmd=`shell-quote touch 'hi there'` ssh host "$cmd" This gives you just 1 file, hi there. process find output It's not ordinarily possible to process an arbitrary list of files output by find with a shell script. Anything you put in $IFS to split up the output could legitimately be in a file's name. Here's how you can do it using shell-quote: eval set -- `find -type f -print0 | xargs -0 shell-quote --` debug shell scripts shell-quote is better than echo for debugging shell scripts. debug() { [ -z "$debug" ] || shell-quote "debug:" "$@" } With echo you can't tell the difference between "debug 'foo bar'" and "debug foo bar", but with shell-quote you can. save a command for later shell-quote can be used to build up a shell command to run later. Say you want the user to be able to give you switches for a command you're going to run. If you don't want the switches to be re-evaluated by the shell (which is usually a good idea, else there are things the user can't pass through), you can do something like this: user_switches= while [ $# != 0 ] do case x$1 in x--pass-through) [ $# -gt 1 ] || die "need an argument for $1" user_switches="$user_switches "`shell-quote -- "$2"` shift;; # process other switches esac shift done # later eval "shell-quote some-command $user_switches my args" OPTIONS
--debug Turn debugging on. --help Show the usage message and die. --version Show the version number and exit. AVAILABILITY
The code is licensed under the GNU GPL. Check http://www.argon.org/~roderick/ or CPAN for updated versions. AUTHOR
Roderick Schertler <roderick@argon.org> perl v5.8.4 2005-05-03 SHELL-QUOTE(1p)
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