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Full Discussion: Understanding sed
Top Forums Shell Programming and Scripting Understanding sed Post 302774419 by gary_w on Friday 1st of March 2013 10:20:39 PM
Old 03-01-2013
An attempt to further explain the regular expression, with an ulterior motive of setting up for a question.

Given:
Code:
sed 's/^\([^:]*\):[^:]:/\1::/'

search for a pattern in the string matching:
Code:
^    = start of the line
\(   = Start of first remembered pattern
[^:] = followed by any character that is not a :
*    = followed by any number of the previous character class
       (characters that are not colons)
\)   = end of first remembered pattern
:    = followed by a colon
[^:] = followed by any character that is not a colon
:    = followed by a colon

<describes the first 2 fields, along with their seperators>

Replace with:
Code:
\1   = the first remembered pattern (the first field)
::   = followed by 2 literal colons

In other words, replace the first 2 colon separated fields
with the first field and 2 colons (deletes the 2nd field).

Question: if this sed command was in a script, could it be commented like I did above in the code? Can a sed regex be multi-line with comments?

One could also do:
Code:
s/:[^:]*:/::/

 

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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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