info sed

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*/\
/

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 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

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