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
dasel -f config.yaml --root 'port = 9090' > config.yaml

# CORRECT — write to temp, then rename
dasel -f config.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
dasel -f config.yaml --root 'server.port = 9090'
# Apply
dasel -f config.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
dasel -f config.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

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

In-Place Modification (Safe Pattern)

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

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

Format Conversion

Pipe through dasel with different input/output format flags:

# JSON to YAML
cat data.json | dasel -i json -o yaml > data.yaml

# YAML to TOML
cat config.yaml | dasel -i yaml -o toml > config.toml

# JSON to XML
cat data.json | dasel -i json -o xml > data.xml

# TOML to JSON
cat config.toml | dasel -i toml -o json > config.json

# CSV to JSON
cat data.csv | dasel -i csv -o json > data.json

Array Operations

Append to Array

# Add element to end of array
echo '[1,2,3]' | dasel -i json --root '[$this..., 4]'
# Output: [1, 2, 3, 4]

# Append object to array
dasel -f data.json --root 'items = [$root.items..., {"name": "new", "value": 42}]'

Batch Update with each

# Multiply all prices by 1.1 (10% increase)
dasel -f data.json --root 'items.each(price = price * 1.1)'

# Set a flag on all elements
dasel -f data.json --root 'users.each(active = true)'

# Increment all values
echo '[1,2,3]' | dasel -i json --root 'each($this = $this + 1)'

Filter Then Transform

# Get only active users, then extract names
dasel -f data.json 'users.filter(active == true).map(name)'

Object Operations

Merge via Spread

# Add new key to existing object
dasel -f base.json --root '{ $root..., "newKey": "value" }'

# Merge two objects
dasel -f base.json --root '{ $root..., "extra": true, "version": 2 }'

Field Deletion via Reconstruction

Since v3 has no delete command, remove fields by constructing a new object with only the desired keys:

# Keep only "name" and "email", drop everything else
echo '{"name":"a","email":"b","password":"c"}' | dasel -i json --root '{ name, email }'

Field Rename via Reconstruction

echo '{"old_name": "value"}' | dasel -i json --root '{ "new_name": old_name }'

Multi-Step Transformations

Use variable assignment and semicolons for complex operations:

# Store intermediate result in variable, then transform
dasel -f data.json '$active = users.filter(active == true); $active.map(name)'

# Multiple variables
dasel -f data.json '$a = items.filter(price > 100); $b = $a.map(name); $b'

Conditional Transformation

# Set value based on condition
dasel -f data.json --root 'if(count > 5) { status = "many" } else { status = "few" }'

Transformation Patterns

For detailed per-use-case transformation patterns (config updates, batch processing, format migration, data reshaping, error handling), see Transformation Patterns.

References

• Dasel v3 Documentation (fetched 2026-02-19)
• Dasel Spread Operator (fetched 2026-02-19)
• Dasel each() Function (fetched 2026-02-19)
• Dasel filter() Function (fetched 2026-02-19)
• Dasel CHANGELOG (fetched 2026-02-19)