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4.17 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*/\
/
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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
}
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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
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 |