I have output like this:

USER_ID
12/31/69 19:00:00
12/31/69 19:00:00
USER_ID
12/31/69 19:00:00
12/31/69 19:00:00
USER_ID
12/31/69 19:00:00
12/31/69 19:00:00
USER_ID
12/31/69 19:00:00
12/31/69 19:00:00
...

USER_ID 12/31/69 19:00:00 12/31/69 19:00:00

Originally Posted by http://sed.sourceforge.net/sed1line.txt # if a line ends with a backslash, append the next line to it sed -e :a -e '/\\$/N; s/\\\n//; ta'

sed -e :a -e '/:[0-9][0-9]$/N; s/[0-9][0-9]$\n//; ta'

USER_ID
12/31/69 19:00:12/31/69 19:00:USER_ID
12/31/69 19:00:12/31/69 19:00:USER_ID
12/31/69 19:00:12/31/69 19:00:USER_ID
12/31/69 19:00:12/31/69 19:00:USER_ID

$ paste - - - < infile
USER_ID 12/31/69 19:00:00       12/31/69 19:00:00
USER_ID 12/31/69 19:00:00       12/31/69 19:00:00
USER_ID 12/31/69 19:00:00       12/31/69 19:00:00
USER_ID 12/31/69 19:00:00       12/31/69 19:00:00
$

This is easy to explain:

/:[0-9][0-9]$/N

/:[0-9][0-9]$/! {...}

/:[0-9][0-9]$/! {N;N;s/\n//g;}

sed '/:[0-9][0-9]$/! {N;N;s/\n/ /g;}' /path/to/input > path/to/output

Thanks, jay. I was unaware of the paste command. I was man -k "append" but if I had used the keyword "merge" I would have found it. I'm still dissecting bakunin's explanation.

Just another sed statement if your whole file is formatted the same way than in your example :

sed 'N;N;s/\n/ /g' yourfile

This one's even better because it doesn't add the extra-large space (perhaps a tab?) before the last set. Either way I have to read up on this "N" business in sed.

You can also specify a delimiter (here, a space) with the paste alternative (to avoid the tabs that are used by default):

paste -d" " - - - <yourfile