Join N number of lines recursively

awk is your friend:

$ awk 'ORS=NR%3?FS:RS' fileThis is line 1   This is line 2   This is line 3  This is line 4   This is line 5   This is line 6  This is line 7   This is line 8   This is line 9 

Which is the same as:

$ awk 'ORS=NR%3 ? " " : "\n"' fileThis is line 1   This is line 2   This is line 3  This is line 4   This is line 5   This is line 6  This is line 7   This is line 8   This is line 9  

Explanation

If number of record is not multiple of 3, then set the output record separator as space; otherwise, as new line.

• ORS defines the output record separator.
• NR defines the number of records (lines in this case).
• FS defines the fields separators. Default is " " (space).
• RS defines the records separators. Default is "\n" (new line).

More info and related examples in Idiomatic awk.

Try this,

paste -d " " - - - < file

This paste command with space as delimiter combines 3 consecutive lines into a single line.

sed version

sed -e '$ b printN$ b printN:prints/\n/ /g' YourFile

if there is only multiple of 3 lines, the $ b print and :print are not necessary

in one line version

• for non GNU sed (like my AIX)

  sed -e '$bprint' -e 'N;$bprint' -e 'N;:print' -e 's/\n/ /g' YourFile

• for GNU sed (no tested, missing linux here)

  sed -e '$bprint;N;$bprint;N;:print;s/\n/ /g' YourFile