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Currently, when I want to pretty-print a json file using jq, I use:

cat file.json | jq .

Yet I would expect for jq to accept a file name as argument without having to fallback on cat.

The man page says:

jq [options...] filter [files...]

... By default, jq reads a stream of JSON objects (whitespace separated) from stdin. One or more files may be specified, in which case jq will read input from those instead.

Yet running:

jq file.json

fails by throwing compile errors that the keys are not defined.

How am I supposed to add the file when running jq?

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1 Answer 1

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198 
jq . file.json

is what I was looking for. I didn't realize that the . is a filter and not a placeholder for the piped in content:

.

The absolute simplest (and least interesting) filter is .. This is a filter that takes its input and produces it unchanged as output.

And the man page makes it clear that the filter is a required argument.

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4
  • 7
    To modify a file in place, use jq . file.json | sponge file.json (this requires sponge from moreutils)
    – Flimm
    Jun 8, 2022 at 18:29
  • 2
    How is this different from jq . file.json > file.json?
    – Steve Anderson
    Oct 31, 2022 at 18:43
  • 4
    @SteveAnderson - if a process opens a file, processes it line-by-line, and outputs each line, then the output pipe will clobber that file, potentially while it's in use. This is sometimes fine, but to be 100% certain it's OK, use sponge, which buffers.
    – Mark McDonald
    Nov 16, 2022 at 4:21
  • This technically worked for me, but I don't know why it slowed my (relatively new w/ no other problems) computer down to nearly a standstill. File size was only 5kb. Maybe because it had Chinese characters in it?
    – Joe Flack
    Jan 17 at 20:58

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