Merge lines between two patterns using sed

Question

I have an output file that looks like this:

HEADER 1:
server1 server2 server3
server4 server5 server6
server7 server8 server9
HEADER 2:

HEADER 1:
server10 s         


        

Answer 1

Using sed:

sed '/HEADER 1/{n;:l N;/HEADER 2/b; s/\n//; bl}' input

• n skips/prints the current line (HEADER 1), clears buffer
• l is a label for looping (goto label)
• N adds (appends) lines to buffer (preserving newlines)
• /HEADER 2/b, where b is branch (without the label, it is break), break out when HEADER2 is seen
• s/\n// removes the newlines in the buffer
• bl jumps back to label l

Answer 2

This shows how it could be done using awk
You can download Gawk for Windows

awk '/HEADER/ {printf "\n%s\n",$0;next} {printf "%s ",$0}' file

HEADER 1:
server1 server2 server3 server4 server5 server6 server7 server8 server9
HEADER 2:

HEADER 1:
server10 server11 server12 server13 server14 server15 server16 server17 server18
HEADER 2: