Info Catalog sed: uniq -u sed: Examples

sed: cat -s

 
 7.20 Squeezing Blank Lines
 ==========================
 
 As a final example, here are three scripts, of increasing complexity and
 speed, that implement the same function as 'cat -s', that is squeezing
 blank lines.
 
    The first leaves a blank line at the beginning and end if there are
 some already.
 
      #!/usr/bin/sed -f
 
      # on empty lines, join with next
      # Note there is a star in the regexp
      :x
      /^\n*$/ {
      N
      bx
      }
 
      # now, squeeze all '\n', this can be also done by:
      # s/^\(\n\)*/\1/
      s/\n*/\
      /
 
    This one is a bit more complex and removes all empty lines at the
 beginning.  It does leave a single blank line at end if one was there.
 
      #!/usr/bin/sed -f
 
      # delete all leading empty lines
      1,/^./{
      /./!d
      }
 
      # on an empty line we remove it and all the following
      # empty lines, but one
      :x
      /./!{
      N
      s/^\n$//
      tx
      }
 
    This removes leading and trailing blank lines.  It is also the
 fastest.  Note that loops are completely done with 'n' and 'b', without
 relying on 'sed' to restart the script automatically at the end of a
 line.
 
      #!/usr/bin/sed -nf
 
      # delete all (leading) blanks
      /./!d
 
      # get here: so there is a non empty
      :x
      # print it
      p
      # get next
      n
      # got chars? print it again, etc...
      /./bx
 
      # no, don't have chars: got an empty line
      :z
      # get next, if last line we finish here so no trailing
      # empty lines are written
      n
      # also empty? then ignore it, and get next... this will
      # remove ALL empty lines
      /./!bz
 
      # all empty lines were deleted/ignored, but we have a non empty.  As
      # what we want to do is to squeeze, insert a blank line artificially
      i\
 
      bx
 

Info Catalog sed: uniq -u sed: Examples