Jo

jo

Build JSON in shell snippets with jo, not by hand-typing a quoted JSON literal. jo handles JSON quoting internally, so shell variables interpolate cleanly and you never juggle nested single/double quotes.

Authoritative documentation: https://github.com/jpmens/jo (README + jo.md man page). Verify any non-trivial form against the manual before relying on it.

Why

• Hand-written JSON in a shell string is a quote-escaping minefield. Inserting $VAR into '{"user":"x"}' either breaks the single quotes or forces an awkward '"$VAR"' pattern that is hard to read and easy to mistype.
• Typos in inline JSON (missing comma, stray quote, trailing comma) fail at the server, not at parse time — hard to spot in a long single-line payload.
• jo figures out types from the value: numbers, booleans, null, nested objects, and arrays are detected automatically; override with -s/-n/-b per word when the guess would be wrong (e.g. a numeric ID that must stay a string).
• jo composes: shell variables, command substitution, bracket-path shorthands, and nested jo calls all work without re-escaping.

How jo types a value

Per the man page, the value pipeline is:

1. Try to parse the value as JSON. 5 → number, true/false → boolean, null → null, {...} / [...] → embedded object/array, strings that look numeric but aren't valid floats (e.g. 3.14159.26) → string.
2. If JSON parse fails, check special value prefixes: @file, %file, :file (see Reading from files below).
3. Otherwise it's a literal string.

A missing/empty value (k=) becomes null. Pass -B to disable detection of the literal strings true/false/null. Globally, -n drops keys with empty values (not to be confused with the per-word -n coercion after --).

Direct substitutions

Bracket-path forms (key[]=, key[sub]=) are one level deep only — 'a[b][c]=1' silently drops [c] and produces {"a":{"b":1}}. For deeper nesting, use nested jo via command substitution.

Syntax cheatsheet

Value forms

• k=v — JSON-parsed then prefix-checked then literal string (see How jo types a value).
• k@v — boolean from a truthy value (T/t or non-zero numeric → true, else false); handy for flag@$ENABLED.
• k=@file — load file contents as a string value.
• k=%file — load file as base64-encoded string.
• k=:file — parse file as JSON and embed.
• k:=file or k:=- — same as k=:file but as an operator; - reads JSON from stdin.

To use a literal @, %, or : at the start of a string value, escape it with \: twitter='\@jpmens', vimcmd='\:split', uri='\\%20'.

Bracket paths (one level)

• 'name[]=v1' 'name[]=v2' — appends to the name array.
• 'obj[k]=v' — creates {"obj":{"k":v}}.
• Combine -d. (path separator) for dot notation: jo -d. geo.lat=10 geo.lon=20 is equivalent to jo 'geo[lat]=10' 'geo[lon]=20'.
• Always quote bracket forms — unquoted [ / ] get caught by zsh globbing (and bash too, if any expansion matches).

Arrays

• jo -a v1 v2 v3 — array of auto-typed values.
• jo -a -- -s 123 hello -n "five" — per-word coercion inside an array (here: ["123","hello",4] — -n on a non-numeric string yields its length).

Type coercion

• jo -- -s k=v — force string. Use for numeric IDs, version strings, phone numbers, ZIPs.
• jo -- -n k=v — force number (non-numeric strings become their length).
• jo -- -b k=v — force boolean (empty → false, anything else → true).
• -- is mandatory before the word list whenever the first word starts with -; pass it always for consistency with the manual.

Global options

The two most-used:

• -p — pretty-print (drop for production payloads).
• -a — emit an array instead of an object.

For the full option list (-B, -D, -n global, -d sep, -f file, -e, -o, -v/-V, plus per-word coercion semantics after --) see references/options.md.

Idioms

curl POST with a body:

curl -X POST https://api.example.com/v1/users \
    -H "Authorization: Bearer $TOKEN" \
    -H "Content-Type: application/json" \
    -d "$(jo name="$NAME" email="$EMAIL" active=true)"

Bracket shorthand for one-level nesting (no nested jo substitution needed):

curl -X POST https://api.example.com/v1/orders \
    -H "Authorization: Bearer $TOKEN" \
    -d "$(jo \
        'customer[id]='"$CUSTOMER_ID" \
        'customer[name]='"$CUSTOMER_NAME" \
        'items[]='"$SKU_A" \
        'items[]='"$SKU_B")"

Nested deeper than one level — build via command substitution (or stash in a variable):

payload=$(jo \
    name="$NAME" \
    address="$(jo street="$STREET" city="$CITY")" \
    tags="$(jo -a admin user)")

curl -X POST -d "$payload" https://api.example.com/v1/users

kubectl patch:

kubectl patch deployment web \
    --type=merge \
    --patch "$(jo spec="$(jo replicas=3)")"

Numeric ID that must stay a string (otherwise jo emits "id":12345 as a number):

curl -X POST -d "$(jo -- -s id=12345 name="$NAME")" \
    https://api.example.com/v1/users

Modify an existing JSON document:

curl -sSL https://api.example.com/v1/config \
    | jo -f - updated_at="$(date -Iseconds)" version="$NEW_VERSION"

When NOT to use jo

Large, fixed JSON document

A heredoc with literal JSON is clearer than a long chain of jo calls, and the JSON can be copy-pasted from API docs without translation:

curl -X POST -d @- https://api.example.com/v1/config <<'JSON'
{
  "feature_flags": {
    "alpha": true,
    "beta": false
  },
  "limits": {
    "rps": 100,
    "burst": 200
  }
}
JSON

jo not available on the host

Some minimal CI images or locked-down servers lack it. Fall back to a heredoc; do not fall back to fragile inline quoted JSON.

Installing jo

• macOS: brew install jo or sudo port install jo
• Debian/Ubuntu: apt install jo
• Fedora: dnf install jo
• Alpine: apk add jo
• Gentoo: emerge jo
• Arch: pacman -S jo (in extra)
• Snap: snap install jo
• Windows: scoop install jo
• Docker: docker run --rm jpmens/jo ...
• From source: https://github.com/jpmens/jo

jo is not in the default aqua/mise short-name registry; on a mise-managed host install via the package manager or via ubi:jpmens/jo.

Scope

Applies anywhere a shell snippet emits JSON:

• Bash tool invocations.
• Snippets in SKILL.md / README.md / docs.
• Scripts under scripts/.
• Makefile recipes.
• CI configs.
• API smoke-test instructions.

If you encounter a hand-written inline JSON string while editing a file, replace it with the equivalent jo form as part of the same edit.