sed – How to Delete Blank Lines Starting from Line 5

sed

I have this:

sed -i '/^$/d' temp_spec.rb

which is stripping blank lines and works well. How can I make it only do this for lines 5-999 (or ideally 5 to end-of-file).

I tried:

sed -n5,999 -i '/^$/d' temp_spec.rb
sed '5,999!d/^$/d' temp_spec.rb

but neither worked (no errors).

Best Answer

If you want to delete all blank lines starting with line 5 and keep lines 1 to 4, you can use

sed -i '5,${;/^$/d;}' temp_spec.rb

The { is the grouping operator, so the first command 5,${ means "from line 5 until the end of input ($) execute the following commands until the matching }". The commands between { and } can again be prefixed by addresses, so the inner command /^$/d means "if there is nothing between beginning (^) and end ($) of the line, delete it". Sed commands can be separated by ;. (This is a badly documented feature of sed. It's supported by most sed implementations, but it's not entirely portable.) As noted by Hauke, the ; after { is optional; the one before } is required, though.

If you want to delete all blank lines starting with line 5 and also delete lines 1 to 4, it's easier:

sed -i '1,4d;/^$/d' temp_spec.rb

Related Solutions

Sed command to replace a blank line with two lines of content

Sed matches entire lines but doesn't include the newline, so a blank line will just be an empty string. You can use ^ to match the beginning of a line and $ to match the end, so ^$ matches a blank line. Just replace that with % ghi\n%:

sed 's/^$/% ghi\n%/'

The newline that already existed will remain, so you'll end up with % ghi on one line and % on the next

Edit: If it needs to only match once then the expression is a bit more complicated. The easiest way I know to do it in sed is:

sed '0,/^$/ s/^$/% ghi\n%/'

The replacement is wrapped in an address range 0,/^$/, which means "only apply the following to the lines between 0 and the first line that matches ^$". Since the replacement expression checks for ^$ as well, the only line that's actually going to change is the first that matches ^$ -- the first blank line

Sed – How to Remove Multiple Blank Lines from a File

Case 1:

awk '!NF {if (++n <= 2) print; next}; {n=0;print}'

Case 2:

awk '!NF {s = s $0 "\n"; n++; next}
     {if (n>1) printf "%s", s; n=0; s=""; print}
     END {if (n>1) printf "%s", s}'

Best Answer

Related Solutions

Sed command to replace a blank line with two lines of content

Sed – How to Remove Multiple Blank Lines from a File

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