cat -s (sed, a stream editor) (original) (raw)

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 andb, 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