UPDATE: Just a few days after hitting publish on this blog, a few folks from the Perforce community shared a better way to output JSON data from p4 commands using the -ztag and -Mj flags: p4 -ztag -Mj changes -m3 | jq -s '.'

I recently ran across this blog showing how to use jq to format git log output to JSON. Curious about jq and wanting to try a similar thing with Perforce, I was able to modify the example pretty easily.

This is what p4 changes output looks like:

$ p4 changes

Change 3 on 2023/07/20 by grepliz@test-local-01 'Add day1.pdf'
Change 2 on 2023/07/20 by grepliz@test-local-01 'Add crazy.psd.'
Change 1 on 2023/07/20 by grepliz@test-local-01 'Populated server with files '

Here is how you can format to JSON using jq:

$ p4 changes | jq -R -s '[split("\n")[:-1] | map(split("\u0020")) | .[] | { "change": .[1], "author": .[5], "date": .[3], "message": .[6:length] | join(" ") }]'

[
  {
    "change": "3",
    "author": "grepliz@test-local-01"
    "date": "2023/07/20",
    "message": "'Add day1.pdf'"
  },
  {
    "change": "2",
    "author": "grepliz@test-local-01",
    "date": "2023/07/20",
    "message": "'Add crazy.psd.'"
  },
  {
    "change": "1",
    "author": "grepliz@test-local-01",
    "date": "2023/07/20",
    "message": "'Populated server with files '"
  }
]

I think the original blog I referred to gives a pretty good explanation of what is going on, but I find it helpful to write out what is going on for future reference (and for my future self!).

The input: p4 changes

I know the output of p4 changes is being piped into the jq program and it is helpful that the output is delimited by a space character and is one line per changelist. This output produces the columns needed for programs like jq and awk.

The flags: jq -R -s

The jq flags being used are nicely documented here.

--slurp/-s:
Instead of running the filter for each JSON object in the input, read the entire input stream into a large array and run the filter just once.

--raw-input/-R:
Don't parse the input as JSON. Instead, each line of text is passed to the filter as a string. If combined with --slurp, then the entire input is passed to the filter as a single long string.

The jq filter

'[split("\n")[:-1] | map(split("\u0020")) | .[] | { "change": .[1], "author": .[5], "date": .[3], "message": .[6:length] | join(" ") }]'

This is where all the magic happens. There are basically 4 sections to this filter.

• split("\n")[:-1]

This says to split on newlines. If we were to stop here, we would get an array of the changelists.
Example:

p4 changes | jq -R -s 'split("\n")[:-1]'
[
  "Change 3 on 2023/07/20 by grepliz@test-local-01 'Add day1.pdf'",
  "Change 2 on 2023/07/20 by grepliz@test-local-01 'Add crazy.psd.'",
  "Change 1 on 2023/07/20 by grepliz@test-local-01 'Populated server with files '"
]

• map(split("\u0020"))

This tells the filter to map and split on the space character.
When combined with #1 above it would look like this:

p4 changes | jq -R -s 'split("\n")[:-1] | map(split("\u0020"))'

[
  [
    "Change",
    "3",
    "on",
    "2023/07/20",
    "by",
    "grepliz@test-local-01",
    "'Add",
    "day1.pdf'"
  ],
  [
    "Change",
    "2",
    "on",
    "2023/07/20",
    "by",
    "grepliz@test-local-01",
    "'Add",
    "crazy.psd.'"
  ],
  [
    "Change",
    "1",
    "on",
    "2023/07/20",
    "by",
    "grepliz@test-local-01",
    "'Populated",
    "server",
    "with",
    "files",
    "'"
  ]
]

• .[]

This creates arrays of the values such that we can iterate over it. The following is from the documentation:

Array/Object Value Iterator: .[]
If you use the .[index] syntax, but omit the index entirely, it will return all of the elements of an array. Running .[] with the input [1,2,3] will produce the numbers as three separate results, rather than as a single array.

You can also use this on an object, and it will return all the values of the object.

p4 changes | jq -R -s 'split("\n")[:-1] | map(split("\u0020")) | .[]'

[
  "Change",
  "3",
  "on",
  "2023/07/20",
  "by",
  "grepliz@test-local-01",
  "'Add",
  "day1.pdf'"
]
[
  "Change",
  "2",
  "on",
  "2023/07/20",
  "by",
  "grepliz@test-local-01",
  "'Add",
  "crazy.psd.'"
]
[
  "Change",
  "1",
  "on",
  "2023/07/20",
  "by",
  "grepliz@test-local-01",
  "'Populated",
  "server",
  "with",
  "files",
  "'"
]

• { "change": .[1], "author": .[5], "date": .[3], "message": .[6:length] | join(" ") }

Finally, this section gives us the desired JSON shape of each changelist and by wrapping the whole thing (the whole jq filter) with [], we get the final array.