[ILUG] Opposite of Cat >>

[Brian Foster]

  | Date: Mon, 10 Jun 2002 14:22:18 +0100
  | From: Stephen Shirley <diamond at skynet.ie>
  | 
  | On Mon, Jun 10, 2002 at 12:57:06PM +0100, Brian Foster wrote:
  |  > this is trivial to un-install --- just delete all lines
  |  > between (and including) the two magic marker lines.
  |  > numerous tools can do this, sed(1) is perhaps a good choice.
  | 
  | I'm just wondering, could you give a snippet of sed code to demonstrate
  | this?  I've used sed for line editing lots of times, but anything more
  | complex, and the man page maketh no sense to me.  I've even tried looking
  | for example sed scripts on the sed homepage, but no joy.  Therefore, for
  | this sort of thing, i'd use sed, cause i know how -)

 b.t.w., I'm told O'Reilly's sed & awk book (or is it awk & sed?)
 is good ....  I cannot vouch for that book myself, as I learned
 sed (and awk) the hard way.

 the basic sed command in this case is simply:

    sed -e 'addr1,addr2 d' ...

 i.e., starting with the line identified by <addr1>, delete each
 line until (and including) the line identified by <addr2>.  for
 the solution proposed, addr1 is a bog-simple /^...$/ RE matching
 the begin magic marker, and addr2 is a similar RE for the end.
 below is a Bourne-ish example, tested with GNU sed 3.02, and
 which (AFAIK) will work with any version of any sed anywhere.

=====(cut here and below)=====(sed multi-line delete example)=====(bash script)=====
#!/bin/bash

    # the following two sh variables are not strictly necessary.
    # they are used only to keep the sed(1)-command readable.
    # $begin is an RE matching exactly the first magic marker line;
    # and $end is an RE matching exactly the second magic marker.
    #
readonly begin='^#BEGIN "example" !DO _NOT_ DELETE THIS LINE!$'
readonly   end='^#END   "example" !DO _NOT_ DELETE THIS LINE!$'

    # this is really just a one-liner sed/shell script:
    #
    #    sed -e '/Begin/,/End/d' ...
    #
    # which deletes all lines between each and every Begin-End
    # pair (inclusive).  if there is an End-less Begin, then
    # all lines from that End-less Begin to EOF are deleted!
    #
sed -e "/$begin/,/$end/d" <<\EOF
here are some lines
to be kept
#BEGIN "example" !DO _NOT_ DELETE THIS LINE!
these lines
are to be removed
#END   "example" !DO _NOT_ DELETE THIS LINE!
and these lines
at the end
should also be kept
EOF
=====(cut here and above)=====(sed multi-line delete example)=====(bash script)=====

cheers!
	-blf-
--
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