sed ':a;N;$!ba;s/\n/ /g' file
:a create a label 'a'
N append the next line to the pattern space
$! if not the last line, ba branch (go to) label 'a'
s substitute, /\n/ regex for new line, / / by a space, /g global match (as many times as it can)# sed is intended to be used on line-based input. Although it can do what you need.
#A better option here is to use the tr command as follows:
tr '\n' ' ' < input_filename
#or remove the newline characters entirely:
tr -d '\n' < input.txt > output.txt
#or if you have the GNU version (with its long options)
tr --delete '\n' < input.txt > output.txt
#All alternatives, unlike sed will not need to reach the last line to begin the process
#with bash, slow
while read line; do printf "%s" "$line "; done < file
#with perl, sed-like speed
perl -p -e 's/\n/ /' file
#with tr, faster than sed, can replace by one character only
tr '\n' ' ' < file
#with paste, tr-like speed, can replace by one character only
paste -s -d ' ' file
#with awk, tr-like speed
awk 1 ORS=' ' file
#Other alternative like "echo $(< file)" is slow, works only on small files and needs to process the whole file to begin the process.$ echo 'foo
bar
baz
foo2
bar2
baz2' \
| tr '\n' '\000' \
| sed 's:\x00\x00.*:\n:g' \
| tr '\000' '\n'