SED command to delete empty lines till the first occurrence of sentence

Question

My input file will be

[emptyline]
[emptyline]
aaa
bbb
[emptyline]
cc
dd

Here [emptyline] indicates blanklines.

And I need an SED command to change this into

aaa
bbb
[emptyline]
cc
dd

That is, I need to delete all the blank lines at the top alone.

I need only SED command since i need to use that in bash script.

Additional info its MAC OSx

Answer 1

You can do it with branching in sed:

sed '/^ *$/d; :a; n; ba' file

A more efficient solution would be to use a range expression, see user2719058's answer for how to do this.

It is even more efficient if you can reduce the need for sed, see gniourf_gniourf's answer for alternatives.

This can be expressed in awk elegantly like this:

awk 'NF {f=1} f' file

Output in both cases:

aaa
bbb

cc
dd

Explanation

Both alternatives work by looking for the first non-empty line.

With sed the pattern /^ *$/d will delete all empty lines in the beginning of the file. What follows is a loop that prints the rest of the file.

awk will update NF for every line, when the line is empty NF is zero. This is exploited for setting the print-flag (f).