How to add column when the number of columns in file is 2

Question

I am trying to write awk for a file I have. Example of the dataset is

S,CV0110,1235
S,1234
D,CQ120,3245
P,7894

Desired outcome is as follows (added empty field when the number of fields in a row is 2)

S,CV0110,1235
S,,1234
D,CQ120,3245
P,,7894

I tried this but it is adding value for all data rows instead of those that have 2 fields.

printf 'S,CV010,1235\r\n' | awk 'BEGIN{FS=OFS="fs"}{$n = $n OFS value}1'

Answer 1

Given

$ cat file
S,CV0110,1235
S,1234
D,CQ120,3245
P,7894

then

$ awk -F, 'BEGIN{OFS=FS} NF<3{$1 = $1 OFS} 1' file
S,CV0110,1235
S,,1234
D,CQ120,3245
P,,7894

Answer 2

sed '/,.*,/!s/,/,,/' file

• /,.*,/! If line does not contain two commas,
• s/,/,,/ Substitute the first comma by two commas.

This can be generalized to more fields. E.g., to add an empty field if some line is missing the 7th field,

sed -E '/,(.*,){5}/!s/,/,,/' file

Answer 3

Assuming the values en each column are always similar.

If a single letter at the start of the line is followed by 4 numbers and the line ends, add a comma:

sed '/^[A-Z],[0-9]\{4\}$/s/,/,,/' file

Output:

S,CV0110,1235
S,,1234
D,CQ120,3245
P,,7894