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Full Discussion: Replace line with sed on OS X
Top Forums Shell Programming and Scripting Replace line with sed on OS X Post 302720701 by DGPickett on Wednesday 24th of October 2012 02:04:03 PM
Old 10-24-2012
Inserting a line is rewriting the file from that point, so just sed to a temp file and then cat the temp file content over the original. You can also drive ex, vi's foundation, from a script very like sed but with additional commands.
Code:
echo '2c
new line
.
w
q' | ex your_file

Really, to actually rewrite just from line 2, not the whole file, you might need a C/PERL routine that overwrites existing positions within the file, extending or shortening it at the end. I suspect most tools truncate and write data from temp files when you save. After all, they are not tolerant of other processes mmap()ing the file or seeking around and reading lower locations that might disappear momentarily. I will tusc a vi and see. Yep, vi close()s, opens with creat() and writes it all. So, back to "use sed with a temp file." If you dislike temp files and the file is smaller than max env size (a meg or so), You can store the file in the env:
Code:
z=`sed '...' your_file`
echo "$z" >your_file

I wrote a C tool once that overwrote $1 from stdin without truncating and trimmed the file length at the end if shorter using fcntl(), so I guess PERL can do that, too. But generally, update in place is a fantasy.
Last edited by DGPickett; 10-24-2012 at 03:30 PM..
 

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LEARN ABOUT DEBIAN

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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