sed - remove comma from last line

Question

How do I remove the comma from the last line of the file? Here is the file:

# cat ox_data_archive_r_20120727.json
{"name": "secondary_ua","type":"STRING"},
{"name": "request_ip","type":"STRING"},
{"name": "cb","type":"STRING"},

The following will remove the comma from all 3 lines.

# sed 's/,$/ /' ox_data_archive_r_20120727.json
{"name": "secondary_ua","type":"STRING"}
{"name": "request_ip","type":"STRING"}
{"name": "cb","type":"STRING"}

I need to remove the last comma only. So the output should look something like this...

# cat newfile.json
{"name": "secondary_ua","type":"STRING"},
{"name": "request_ip","type":"STRING"},
{"name": "cb","type":"STRING"}

Answer 1

$ cat input.txt
{"name": "secondary_ua","type":"STRING"},
{"name": "request_ip","type":"STRING"},
{"name": "cb","type":"STRING"},
$ sed '$s/,$//' < input.txt
{"name": "secondary_ua","type":"STRING"},
{"name": "request_ip","type":"STRING"},
{"name": "cb","type":"STRING"}

From the documentation of GNU sed:

$: This address matches the last line of the last file of input, or the last line of each file when the -i or -s options are specified.