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Full Discussion: Awk,sed : change every 2nd field ":" to "|"
Top Forums Shell Programming and Scripting Awk,sed : change every 2nd field ":" to "|" Post 302825667 by Scrutinizer on Tuesday 25th of June 2013 12:42:51 AM
Old 06-25-2013
The sed command uses a substitute command ( s/.../.../ ) . The first part of the expression contains a basic regular expression (regex). The escaped parentheses \( and \) are used to group parts of matched text that can be back referenced by \1 in the second part of the s-command.

So in this case only the second colon is outside the grouped part and does not get back referenced, so effectively it gets discarded. If we apply this to your input file then the first match is :8:, which gets substituted with :8|. At the end of the expression is the letter g, which is the "global" flag, which means that the operation should be repeated for every occurrence on the line.

So this will be repeated, and crucial here, is that the next match will start after the previous match, so next up will be :4: which becomes :4| and then :14: becomes :14| and so on...
Last edited by Scrutinizer; 06-25-2013 at 01:49 AM..
 

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LEARN ABOUT DEBIAN

keyhead

keyhead(1)						      General Commands Manual							keyhead(1)

Name
       keyhead - Change FITS or IRAF header keyword names

Synopsis
       keyhead [-hknv] <FITS or IRAF file> kw1=kw1a kw2=kwd2a ... kwn=kwdna

Description
       Change the names of keywords in FITS or IRAF image headers.  Each current image keyword whose entry is to be modified should be followed by
       an equal sign and a second keyword, with no intervening spaces.	If the -r option is used, the value of the second keyword is transfered to
       that  of the first. Otherwise, the name of the first keyword is changed to the second keyword.  To change keywords in a list of files, sub-
       stitute @<listfile> for the file names on the command line.  To change a lot of keywords, put them, one pair separated by an = sign with no
       spaces  per  line, in a file and substitute @<keylistfile> on the command line.	If two @ commands are present, the program will figure out
       which contains file names and which contains keywords.  Lines in a keyword list file which do not contain an = are ignored.

Options
       -h     Write HISTORY line into output header with sethead version, current date and time, and names of keywords set.

       -k     Write KEYHEAD keyword into output header with the keyhead version, current date and time, and names  of  keywords  whose	names  are
	      changed.	If a KEYHEAD keyword already exists, move the old value to a HISTORY line before writing the new value

       -n     Write a new file with an added "e" before the extension.

       -r letter
	      Replace  value of 1st keyword with value of 2nd keyword instead of changing the name of the 1st keyword to the 2nd keyword.  The 2nd
	      keyword remains in the header.  If the 1st keyword does not exist in the header, it is created.

       -v     Print confirmations of each keyword change.

Web Page
       http://tdc-www.harvard.edu/software/wcstools/keyhead.html

Author
       Doug Mink, SAO (dmink@cfa.harvard.edu)

6 July 2001							     WCSTools								keyhead(1)
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