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Full Discussion: What is the meaning of this awk expression using sub?
Top Forums UNIX for Dummies Questions & Answers What is the meaning of this awk expression using sub? Post 302591494 by kristinu on Thursday 19th of January 2012 04:12:50 PM
Old 01-19-2012
What would be the difference between these two expressions? They seem to do the same.

Code:
{sub(/^/, s "&")}

Code:
{sub(/^/, "&" s)}

---------- Post updated at 04:11 PM ---------- Previous update was at 04:04 PM ----------

Quote:
Originally Posted by Corona688
It does a search/replace in the current line( $0 ), stopping after the first replace.

The regular expression /^/ is what's being searched for. ^ has a special meaning in expressions, "the beginning of the line".

It replaces it with the literal string "&" S. So what it actually puts in there depends on what value S holds.

So if S was "asdf", this command would add "&asdf" to the beginning of the line.

It seems that & prints the entire text when used in a replacement string.

---------- Post updated at 04:12 PM ---------- Previous update was at 04:11 PM ----------

I have done like this and the results are shown. Not exactly what you are saying.
Code:
echo "Hello" | awk '{s="test";sub ( /^/, "&" s );print}'
testHello

echo "Hello" | awk '{s="test";sub ( /^/, "&" s );print}'
testHello

Also, the one below gives also same result

Code:
echo "Hello" | awk '{s="test";sub ( /^/, s );print}'
testHello

 

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LEARN ABOUT REDHAT

ppmtosixel

ppmtosixel(1)						      General Commands Manual						     ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.	The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
	      pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order	of  magni-
	      tude larger than a compressed file and prints much slower.

       -margin
	      If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
	      fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater	than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

								   26 April 1991						     ppmtosixel(1)
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