Filling empty spaces in a CSV file

I have a CSV file where some columns are empty such as

oski14,safe,0,13,53,4
oski15,Unknow,,,,0
oski16,Unknow,,,,0
oski17,Unknow,,,,0
oski18,unsafe,0.55,,1,2
oski19,unsafe,0.12,4,,56

How do I replace all the empty columns with the word “empty”. I have tried using awk(which is a command I am learning to use).

I want to have

oski14,safe,0,13,53,4
oski15,Unknow,empty,empty,empty,0
oski16,Unknow,empty,empty,empty,0
oski17,Unknow,empty,empty,empty,0
oski18,unsafe,0.55,empty,1,2
oski19,unsafe,0.12,4,empty,56

I tried to replace just the 3rd column to see if I was on the right track

    awk -F '[[:space:]]' '$2 && !$3{$3="empty"}1' file

this left me with

oski14,safe,0,13,53,4
oski15,Unknow,,,,0
oski16,Unknow,,,,0
oski17,Unknow,,,,0
oski18,unsafe,0.55,,1,2
oski19,unsafe,0.12,4,,56

I have also tried

    nawk -F, '{$3=" "?"empty":$3;print}' OFS="," file

this resulted in

oski14,safe,empty,13,53,4
oski15,Unknow,empty,,,0
oski16,Unknow,empty,,,0
oski17,Unknow,empty,,,0
oski18,unsafe,empty,,1,2
oski19,unsafe,empty,4,,56

Lastly I tried

    awk '{if (!$3) {print $1,$2,"empty"} else {print $1,$2,$3}}' file

this left me with

oski14,safe,empty,13,53,4 empty
oski15,Unknow,empty,,,0 empty
oski16,Unknow,empty,,,0 empty
oski17,Unknow,empty,,,0 empty
oski18,unsafe,empty,,1,2 empty
oski19,unsafe,empty,4,,56 empty

Answer

With a sed that supports EREs with a -E argument (e.g. GNU sed or OSX/BSD sed):

$ sed -E 's/(^|,)(,|$)/1empty2/g; s/(^|,)(,|$)/1empty2/g' file
oski14,safe,0,13,53,4
oski15,Unknow,empty,empty,empty,0
oski16,Unknow,empty,empty,empty,0
oski17,Unknow,empty,empty,empty,0
oski18,unsafe,0.55,empty,1,2
oski19,unsafe,0.12,4,empty,56

You need to do the substitution twice because given contiguous commas like ,,, one regexp match would use up the first 2 ,s and so you’d be left with ,empty,,.

The above would change a completely empty line into empty, let us know if that’s an issue.