Convert Separator Character While Escaping Fields in CSV

Another POSIX one:

awk -F , 'NF == 11' <file

If the line has 10 commas, then there will be 11 fields in this line. So we simply make awk use , as the field delimiter. If the number of fields is 11, the condition NF == 11 is true, awk then performs the default action print $0.

Regular expressions and sed aren't a great match for this kind of task, but it's fairly simple using jq, a JSON processing tool:

$ jq -r '.list[]|[.dt, .main[]] | @csv' < data.json
1519333200,271.62,271.62,272.921,1028.3,1037.2,1028.3,88,-1.3
1519344000,271.22,271.22,272.193,1028.11,1037.04,1028.11,100,-0.98

jq takes in a JSON data stream and filters it to extract the data you want, analogously to sed but for structured data rather than text. This command takes each element of the "list" array (.list[]) and plucks out the "dt" field (.dt) of the object inside along with everything in the "main" object too (.main[]), putting the whole lot into an array and converting that to a CSV row (@csv).

If you're interested in particular fields, or you want to reorder them, you can list them out too:

$ jq -r '.list[]|[.dt, .main.temp_min, .main.humidity] | @csv' < data.json
1519333200,271.62,88
1519344000,271.22,100

Just list out each one you care about and ignore the rest.

If you want to get into other parts of the data, you can put that in there too: .clouds will look into the clouds field, or you can read the wind speed with .wind.speed - always a dot before the name, like you were writing JavaScript. The jq manual is quite comprehensive for whatever custom behaviour you need.