remove only the first blank line sed

Question

there are similar questions here but none matches my problem exactly.

How do I remove only the first blank line from a file using sed?

Let's say I have

a

b

c

And I want

a
b

c

As output.

Answer 1

See the sed FAQ here:

$ sed '0,/^$/{//d}' lines
a
b

c

d

Note this only removes truly empty lines, if you want to consider lines with whitespace you would use

$ sed '0,/^[[:space:]]*$/{//d}' lines

instead.

Answer 2

You can use sed to read up to the first blank line, and then use cat to read the rest which would be the most efficient for big files:

{ sed -n '/./!q;p'; cat; } < the-file

It only works with regular files though (not with pipes because sed reads data by blocks and can't seek back to the line after the one where q was called if the input is not seekable). As noted by @peterph, With GNU sed version 4.2.2 and above, you can add the -u flag which causes GNU sed to read its input one byte at a time (and output one line at a time) and removes the problem with pipes (though degrading performance).