data-transformation

Data Transformation with Dasel v3

<when_to_use>

Activate this skill when:

• Modifying values in config files (YAML, TOML, JSON, XML, HCL, INI)
• Converting between data formats (JSON to YAML, TOML to JSON, etc.)
• Batch-updating array elements (prices, versions, flags)
• Appending, removing, or reshaping data structures
• Merging objects or constructing new structures from existing data

</when_to_use>

Safety Rules

NEVER redirect output to the same input file. This truncates the file before dasel reads it, resulting in data loss:

# WRONG — destroys input
cat config.yaml | dasel -i yaml --root 'port = 9090' > config.yaml

# CORRECT — write to temp, then rename
cat config.yaml | dasel -i yaml --root 'port = 9090' > config_tmp.yaml && mv config_tmp.yaml config.yaml

Always verify before overwriting. Preview the transformation output first, then redirect:

# Preview
cat config.yaml | dasel -i yaml --root 'server.port = 9090'
# Apply
cat config.yaml | dasel -i yaml --root 'server.port = 9090' > config_tmp.yaml && mv config_tmp.yaml config.yaml

Core Modification Syntax

In dasel v3, put and delete subcommands do not exist. All modifications use assignment expressions with --root to output the full document.

Set a Value

# Output full document with one field changed
cat config.yaml | dasel -i yaml --root 'server.port = 9090'

# Numeric, boolean, and string assignments
echo '{"count": 1}' | dasel -i json --root 'count = 42'
echo '{"enabled": false}' | dasel -i json --root 'enabled = true'
echo '{"name": "old"}' | dasel -i json --root 'name = "new"'

Set Nested Values

cat config.yaml | dasel -i yaml --root 'database.connection.host = "db.example.com"'
cat config.yaml | dasel -i yaml --root 'database.connection.port = 5432'

In-Place Modification (Safe Pattern)

# Single field update
cat config.yaml | dasel -i yaml --root 'server.port = 9090' > config_tmp.yaml && mv config_tmp.yaml config.yaml

# Multiple fields — chain with semicolons
cat config.yaml | dasel -i yaml --root 'server.port = 9090; server.host = "0.0.0.0"' > config_tmp.yaml && mv config_tmp.yaml config.yaml