gron Session 01: Basics

gron transforms JSON into discrete assignments. Each line is a complete path to a value. This makes JSON greppable — the whole point of the tool.

Pre-Session State

Can read JSON structure
Know basic grep usage
Have gron installed (sudo pacman -S gron)

Setup

cat > /tmp/gron-01.json << 'EOF'
{
  "hostname": "ise-01",
  "ip": "10.50.1.20",
  "domain": "inside.domusdigitalis.dev",
  "roles": ["pan", "mnt", "psn"],
  "config": {
    "version": "3.3",
    "patches": ["patch1", "patch2"],
    "tls": {
      "enabled": true,
      "port": 443,
      "cert_path": "/etc/ssl/certs/ise.pem"
    }
  },
  "interfaces": [
    {"name": "eth0", "ip": "10.50.1.20", "vlan": 50},
    {"name": "eth1", "ip": "10.50.10.20", "vlan": 10}
  ]
}
EOF

Lesson 1: Flatten JSON

Concept: gron converts JSON into JavaScript-style assignment statements. Each line is a complete path.

Exercise 1.1: Flatten the file

gron /tmp/gron-01.json

Output: Every value becomes its own line:

json = {};
json.hostname = "ise-01";
json.ip = "10.50.1.20";
json.config = {};
json.config.tls = {};
json.config.tls.enabled = true;
json.config.tls.port = 443;
...

Each line is self-contained. That’s the power — you can grep any of them.

Exercise 1.2: From stdin

cat /tmp/gron-01.json | gron

Same result. gron reads files or stdin.

Lesson 2: Grep the Paths

Concept: Once flattened, standard grep finds what you need.

Exercise 2.1: Find TLS-related fields

gron /tmp/gron-01.json | grep tls

Output: Only lines containing "tls" — complete paths to every TLS-related value.

Exercise 2.2: Find all IPs

gron /tmp/gron-01.json | grep -i ip

Exercise 2.3: Find array elements

gron /tmp/gron-01.json | grep 'interfaces\['

Output: All interface entries with their full paths.

Exercise 2.4: Find specific values

gron /tmp/gron-01.json | grep '= 443'

Output: json.config.tls.port = 443; — now you know the exact path.

Lesson 3: ungron — Reconstruct JSON

Concept: gron --ungron (or ungron) reverses the flattening. Filter lines with grep, then reconstruct valid JSON from the subset.

Exercise 3.1: Round-trip (identity)

gron /tmp/gron-01.json | gron --ungron

Output: Original JSON reconstructed. gron | ungron is the identity operation.

Exercise 3.2: Filter and reconstruct

gron /tmp/gron-01.json | grep config | gron --ungron

Output: A valid JSON document containing only the config subtree.

Exercise 3.3: Extract interfaces only

gron /tmp/gron-01.json | grep interfaces | gron --ungron

Output: Valid JSON with just the interfaces array.

Lesson 4: The Discovery Workflow

Concept: gron → grep → discover path → use jq. This is the workflow.

Exercise 4.1: Discover, then extract

# Step 1: What paths mention "cert"?
gron /tmp/gron-01.json | grep cert
# Output: json.config.tls.cert_path = "/etc/ssl/certs/ise.pem";

# Step 2: Now use jq with the known path
jq '.config.tls.cert_path' /tmp/gron-01.json

The gron path json.config.tls.cert_path maps directly to jq’s .config.tls.cert_path — just drop the json prefix.

Exercise 4.2: Discover nested structure

# What's inside config?
gron /tmp/gron-01.json | grep '^json\.config\.' | head -10

Shows every leaf under config with full paths.

Lesson 5: Flags

Exercise 5.1: Colorized output

gron --color /tmp/gron-01.json | head -10

Exercise 5.2: Stream mode

echo '{"a":1}{"b":2}' | gron -s

-s handles streaming JSON (newline-delimited objects).

Summary: What You Learned

Concept Syntax Example

Concept	Syntax	Example
Flatten	`gron file.json`	Lines of `json.path = value;`
Search	`gron \| grep pattern`	`gron f.json \| grep tls`
Reconstruct	`gron \| grep \| gron --ungron`	Filter to valid JSON subset
Round-trip	`gron \| gron --ungron`	Identity operation
Discovery	gron → grep → jq	Find path, then extract

Flatten

gron file.json

Lines of json.path = value;

gron | grep pattern

gron f.json | grep tls

Reconstruct

gron | grep | gron --ungron

Filter to valid JSON subset

Round-trip

gron | gron --ungron

Identity operation

Discovery

gron → grep → jq

Find path, then extract

Exercises to Complete

[ ] Find all fields containing "10.50" in the test data
[ ] Extract only the TLS config as valid JSON
[ ] Find the cert_path using gron, then extract it with jq
[ ] Flatten, remove all array entries, reconstruct (what remains?)

Next Session

Session 02: API Discovery - Using gron to explore GitHub and other APIs.

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