Filter inputs away

[stroan]

I might well be reading the docs wrong, but I don't think there's a way to perform the following operation. Given the input

{"foo": "bar", "baz": 10}
{"foo": "blap", "baz": 11}
{"foo": "bar", "baz": 12}

Get only the objects where .foo == "bar", resulting in the output:

{"foo": "bar", "baz": 10}
{"foo": "bar", "baz": 12}

Currently the syntax for conditionals, requires an else case. If this were changed to allow only an if case and to not output a value if the case that doesn't match then the program which supplies the above could be:

./jq 'if .foo == "bar" then . end'

My current work around involves using the else case to output a sentinel value which I grep away before piping back into jq again.

[stedolan]

./jq 'if .foo == "bar" then empty end'

would do what you want. empty is a builtin that produces zero results. You can in theory implement it yourself with something like [][] (each element of an empty array).

You should probably use the (sadly undocumented) builtin function select instead:

./jq 'select(.foo == "bar")'

I intentionally didn't implement if-without-else, since I'm not sure whether the best default for "else" behaviour is produce-no-output or dont-change-the-input (that is, whether the implicit else block should be . or empty). This sounds like a vote for empty.

[stroan]

Ah. Brilliant. I don't think there's any need for if-without-else in the presence of select and empty. Closing this ticket because there's no issue with jq, just a gap in the docs. Cheers.