[GRASS-dev] Re: [GRASS GIS] #1164: newlines in sed scripts not portable for wingrass

[GRASS GIS]

#1164: newlines in sed scripts not portable for wingrass
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 Reporter:  hamish         |       Owner:  grass-dev@…              
     Type:  defect         |      Status:  new                      
 Priority:  minor          |   Milestone:  6.4.1                    
Component:  Default        |     Version:  svn-develbranch6         
 Keywords:  wingrass, sed  |    Platform:  MSWindows XP             
      Cpu:  x86-64         |  
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Comment(by glynn):

 Replying to [comment:2 hamish]:
 > > If you want to insert a literal newline, you're supposed to
 > > use backslash-newline, i.e.:

 > Only gotcha is that the sed expression needs 'single' quotes not
 "double" ones to get that to work:

 The sed expression requires backslash-newline. How to pass such an
 expression to sed depends upon which shell (if any) you are using. With
 bash, it's good practice to use single quotes unless you specifically need

 > committed to 6.5svn r.in.wms with r43549, a couple of left-hand-side
 s/// '\n's remain, will wait and see how those behave

 A \n in the pattern is valid.

 > (since sed is line by line I'm kinda surprised those would work at all
 even when right-hand-side replacement was functional.. shrug).

 sed commands work on the "pattern space". sed operates by reading each
 line into the pattern space in turn. However, certain commands (e.g. G, N)
 append data preceded by a literal newline to the pattern space; this is
 one way that newlines can get into the pattern space (substitutions can
 also introduce them).

 A common sed idiom is "{;N;<commands>;P;D;}". N appends the next line to
 the pattern space preceded by a newline, P prints everything up to the
 first newline, D deletes everything up to the newline (and suppresses the
 automatic reading of the next line). This allows <commands> to work on a
 2-line "window", so you can e.g. use "s" commands to match phrases which
 might be split over two lines.

 > I guess the right hand side was working before because it was using
 "double" quotes and the shell was doing the newline replacement, not sed.
 Sort of like "echo -e" without asking for it..

 No, bash itself doesn't interpret \n as a newline. Without quotes, \n will
 evaluate to just n. With single or double quotes, it will evaluate to \n.
 It appears that some versions of GNU sed interpret a \n in the replacement
 as a newline, contrary to the documentation and the standard (and they do
 this even with --posix).

 FWIW, sed 4.2 on Linux interprets \n, sed 3.02 on Windows doesn't. I
 suspect that it may be a case of Linux systems typically having newer
 versions of sed than MSys.

 Hang on; the current sed manual says this, under "Reporting Bugs":
 {{{
 Here are a few commonly reported bugs that are not bugs.
 ...
 Regex syntax clashes (problems with backslashes)
 ...
 In addition, this version of sed  supports several escape characters (some
 of which are multi-character) to insert non-printable characters in
 scripts (\a, \c, \d, \o, \r, \t, \v, \x). These can cause similar problems
 with scripts written for other seds.
 }}}

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Ticket URL: <https://trac.osgeo.org/grass/ticket/1164#comment:3>
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