Question

I have a CSV file that shows the statistics for links on a half an hour basis. The link name only appears on the 00:00 line.

link1,0:00,0,0,0,0
,00:30,0,0,0,0
,01:00,0,0,0,0
,01:30,0,0,0,0
,02:00,0,0,0,0
,02:30,0,0,0,0
,03:00,0,0,0,0
,03:30,0,0,0,0
,23:30,0,0,0,0
....
....
link2,00:00,0,0,0,0

How do I copy the link name to every other line until the link name is different, using sed or awk?

Answer 1

With awk, just keep track of the last seen non-empty link name, and always use that.

awk -F, -v OFS=, '$1 != "" { link=$1 } { $1 = link; print $0 }'

Omitting the ellipses, this gives:

link1,0:00,0,0,0,0
link1,00:30,0,0,0,0
link1,01:00,0,0,0,0
link1,01:30,0,0,0,0
link1,02:00,0,0,0,0
link1,02:30,0,0,0,0
link1,03:00,0,0,0,0
link1,03:30,0,0,0,0
link1,23:30,0,0,0,0
link2,00:00,0,0,0,0