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Full Discussion: sed internal working
Top Forums Shell Programming and Scripting sed internal working Post 302494121 by Perderabo on Saturday 5th of February 2011 09:49:27 AM
Old 02-05-2011
The most important factor to writing fast shell scripts is to minimize the number of external programs invoked. It is a big deal to create a new process, go find the file containing sed or awk, open it, read it in, transfer control it, wait for it to exit, reclaim the process' resources and deliver the return code to the shell. When I need to manipulate a string I would rather code a dozen internal shell operations that invoke a single copy of sed. This is especially true of string manipulation in a loop.

Once a decision hs been made to invoke an external program, some are faster than others, but this is a relatively minor consideration. sed can tackle fewer jobs than awk. It's a much smaller program than awk. awk always tries to crack a line into fields whether or not this is useful. So sed can outperform awk on the tasks that sed can easily do. tr is smaller still and can outperform sed on the even smaller setset of tasks that tr can handle. But I rarely spend a lot of time worrying about stuff like this. They are all external programs. I try to avoid as many as possible.

Of all of the shell scripts I have posted on this site, the most frequently used is datecalc. And datecalc invokes no external programs at all.
datecalc
 

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LEARN ABOUT LINUX

shtool-subst

SHTOOL-SUBST.TMP(1)					      GNU Portable Shell Tool					       SHTOOL-SUBST.TMP(1)

NAME

       shtool-subst - GNU shtool sed(1) substitution operations

SYNOPSIS

       shtool subst [-v|--verbose] [-t|--trace] [-n|--nop] [-w|--warning] [-q|--quiet] [-s|--stealth] [-i|--interactive] [-b|--backup ext]
       [-e|--exec cmd] [-f|--file cmd-file] [file] [file ...]

DESCRIPTION

       This command applies one or more sed(1) substitution operations to stdin or any number of files.

OPTIONS

       The following command line options are available.

       -v, --verbose
	   Display some processing information.

       -t, --trace
	   Enable the output of the essential shell commands which are executed.

       -n, --nop
	   No operation mode. Actual execution of the essential shell commands which would be executed is suppressed.

       -w, --warning
	   Show warning on substitution operation resulting in no content change on every file. The default is to show a warning on substitution
	   operations resulted in no content change on all files.

       -q, --quiet
	   Suppress warning on substitution operation resulting in no content change.

       -s, --stealth
	   Stealth operation. Preserve timestamp on file.

       -i, --interactive
	   Enter interactive mode where the user has to approve each operation.

       -b, --backup ext
	   Preserve backup of original file using file name extension ext.  Default is to overwrite the original file.

       -e, --exec cmd
	   Specify sed(1) command directly.

       -f, --file cmd-file
	   Read sed(1) command from file.

EXAMPLE

	#   shell script
	shtool subst -i -e 's;(c) ([0-9]*)-2000;(c) 1-2001;' *.[ch]

	#    RPM spec-file
	%install
	    shtool subst -v -n 
		-e 's;^(prefix=).*;1 $RPM_BUILD_ROOT%{_prefix};g' 
		-e 's;^(sysconfdir=).*;1 $RPM_BUILD_ROOT%{_prefix}/etc;g' 
		`find . -name Makefile -print`
	    make install

HISTORY

       The GNU shtool subst command was originally written by Ralf S.  Engelschall <rse@engelschall.com> in 2001 for GNU shtool.  It was prompted
       by the need to have a uniform and convenient patching frontend to sed(1) operations in the OpenPKG package specifications.

SEE ALSO

       shtool(1), sed(1).

18-Jul-2008							   shtool 2.0.8 					       SHTOOL-SUBST.TMP(1)
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