This variable is used to set the output field separator which is a space by default.
awk -F'=' 'BEGIN { OFS=":" } { print $1 }' file
Example:
$ cat file.csv
col1,col2,col3,col4
col1,col2,col3
col1,col2
col1
col1,col2,col3,col4,col5
$ awk -F',' 'BEGIN { OFS="|" } { $...