jq -r

Let’s break this shell line down piece by piece, from right to left, and then put it all together.

value=$(jq -r '.value // empty' "$CONFIG_FILE")

1️⃣ $( ... ) — command substitution

value=$( ... )

• $(...) means run the command inside and capture its output
• The output is assigned to the shell variable value

So whatever text the command prints to stdout becomes the value of value.

2️⃣ jq — JSON processor

jq is a command-line tool used to parse and query JSON files.

Here it is used to extract a field from a JSON file.

3️⃣ -r — raw output

jq -r ...

• -r means raw output
• Without -r, jq outputs JSON strings with quotes:

"value"

• With -r, jq outputs plain text:

value

This is important for shell scripting, because you usually don’t want quotes.

4️⃣ '.value // empty' — jq filter

This is the most important part.

.value

• Accesses the key named value at the top level of the JSON

Example JSON:

{
  "value": "hello"
}

.value → "hello"

// empty — fallback operator

• // is jq’s default / fallback operator

• It means:

  “If the left side is null or missing, use the right side instead”

empty is a jq builtin that outputs nothing at all.

So:

This avoids errors or "null" showing up.

5️⃣ "$CONFIG_FILE" — input file

• $CONFIG_FILE is a shell variable containing a path to a JSON file
• The quotes protect against spaces in the filename

Example:

CONFIG_FILE=config.json

🔁 Put it all together

This line means:

Read a JSON file, extract the .value field, print it as plain text if it exists, otherwise print nothing, and store the result in the shell variable value.

✅ Example walkthrough

JSON file

{
  "value": "production"
}

Result

value="production"

JSON file (missing key)

{
  "env": "production"
}

Result

value=""

(no error, no null, just empty)

🧠 Why this pattern is popular

• Safe for missing keys
• No noisy "null"
• No extra quotes
• Works well with set -e
• Ideal for config files in CI/CD or scripts