yq YAML Processing

Expert knowledge for processing, querying, and transforming YAML data using yq v4+, the lightweight command-line YAML processor with jq-like syntax.

Core Expertise

YAML Operations

• Query and filter YAML with path expressions
• Transform YAML structure and shape
• Multi-document YAML support
• Convert between YAML, JSON, XML, and other formats

Configuration Management

• Modify Kubernetes manifests
• Update GitHub Actions workflows
• Process Helm values files
• Manage application configurations

Essential Commands

Basic Querying

# Pretty-print YAML
yq '.' file.yaml

# Evaluate and format
yq eval '.' file.yaml

# Extract specific field
yq '.fieldName' file.yaml

# Extract nested field
yq '.spec.containers[0].name' pod.yaml

# Access array element
yq '.[0]' array.yaml
yq '.items[2]' data.yaml

Reading YAML

# Read specific field
yq '.metadata.name' deployment.yaml

# Read nested structure
yq '.spec.template.spec.containers[].image' deployment.yaml

# Read with default value
yq '.optional // "default"' file.yaml

# Check if field exists
yq 'has("fieldName")' file.yaml

Array Operations

# Get array length
yq '.items | length' file.yaml

# Map over array
yq '.items[].name' file.yaml

# Filter array
yq '.items[] | select(.active == true)' file.yaml
yq '.users[] | select(.age > 18)' file.yaml

# Sort array
yq '.items | sort_by(.name)' file.yaml
yq '.items | sort_by(.date) | reverse' file.yaml

# Get first/last elements
yq '.items | first' file.yaml
yq '.items | last' file.yaml

# Unique values
yq '.tags | unique' file.yaml

# Filter and collect
yq '[.items[] | select(.status == "active")]' file.yaml

Modifying YAML (In-Place)

# Update field value
yq -i '.metadata.name = "new-name"' file.yaml

# Update nested field
yq -i '.spec.replicas = 3' deployment.yaml

# Add new field
yq -i '.metadata.labels.app = "myapp"' file.yaml

# Delete field
yq -i 'del(.spec.nodeSelector)' file.yaml

# Append to array
yq -i '.items += {"name": "new-item"}' file.yaml

# Update array element
yq -i '.items[0].status = "updated"' file.yaml

# Update all matching elements
yq -i '(.items[] | select(.name == "foo")).status = "active"' file.yaml

Multi-Document YAML

# Split multi-document YAML
yq -s '.metadata.name' multi-doc.yaml
# Creates files: doc-0.yaml, doc-1.yaml, etc.

# Select specific document (0-indexed)
yq 'select(document_index == 0)' multi-doc.yaml

# Process all documents
yq eval-all '.' multi-doc.yaml

# Filter documents
yq 'select(.kind == "Deployment")' k8s-resources.yaml

# Combine YAML files
yq eval-all '. as $item ireduce ({}; . * $item)' file1.yaml file2.yaml

Object Operations

# Get all keys
yq 'keys' file.yaml

# Select specific fields
yq '{name, version}' file.yaml

# Rename field
yq '{newName: .oldName, other: .other}' file.yaml

# Merge objects
yq '. * {"updated": true}' file.yaml
yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' base.yaml override.yaml

# Deep merge
yq eval-all '. as $item ireduce ({}; . *+ $item)' base.yaml override.yaml

Filtering and Conditions

# Select with conditions
yq 'select(.status == "active")' file.yaml
yq '.[] | select(.price < 100)' file.yaml

# Multiple conditions (AND)
yq '.[] | select(.active and .verified)' file.yaml
yq '.[] | select(.age > 18 and .country == "US")' file.yaml

# Multiple conditions (OR)
yq '.[] | select(.type == "admin" or .type == "moderator")' file.yaml

# Has field
yq '.[] | select(has("email"))' file.yaml

# Field not null
yq 'select(.optional != null)' file.yaml

# Not condition
yq '.[] | select(.status != "deleted")' file.yaml

# Contains (arrays)
yq '.tags | contains(["important"])' file.yaml

String Operations

# String interpolation
yq '"Hello, \(.name)"' file.yaml

# Convert to string
yq '.port | tostring' file.yaml

# String contains
yq '.[] | select(.email | contains("@gmail.com"))' file.yaml

# String starts/ends with
yq '.[] | select(.name | test("^A"))' file.yaml
yq '.[] | select(.file | test("\\.yaml$"))' file.yaml

# Split string
yq '.path | split("/")' file.yaml

# Join array to string
yq '.tags | join(", ")' file.yaml

# Lowercase/uppercase
yq '.name | downcase' file.yaml
yq '.name | upcase' file.yaml

# String substitution
yq '.image | sub("v1.0"; "v2.0")' file.yaml

Format Conversion

# YAML to JSON
yq -o=json '.' file.yaml

# JSON to YAML
yq -p=json '.' file.json

# YAML to XML
yq -o=xml '.' file.yaml

# YAML to CSV/TSV
yq -o=csv '.' file.yaml
yq -o=tsv '.' file.yaml

# Properties format
yq -o=props '.' file.yaml

# Compact JSON
yq -o=json -I=0 '.' file.yaml

Output Formatting

# Compact output
yq -o=yaml -I=0 '.' file.yaml

# Custom indentation
yq -I=4 '.' file.yaml

# No colors
yq --no-colors '.' file.yaml

# Raw output (no quotes)
yq -r '.message' file.yaml

# Null input (create YAML from scratch)
yq -n '.name = "example" | .version = "1.0"'

Real-World Examples

# Kubernetes: Get all container images
yq '.spec.template.spec.containers[].image' deployment.yaml

# Kubernetes: Update image version
yq -i '.spec.template.spec.containers[0].image = "myapp:v2.0"' deployment.yaml

# Kubernetes: Add label to all deployments
yq -i '(.metadata.labels.environment = "production")' deployment.yaml

# GitHub Actions: Extract job names
yq '.jobs | keys' .github/workflows/ci.yml

# GitHub Actions: Update checkout action version
yq -i '(.jobs.*.steps[] | select(.uses == "actions/checkout*")).uses = "actions/checkout@v5"' workflow.yml

# Docker Compose: Get all service names
yq '.services | keys' docker-compose.yml

# Docker Compose: Update service image
yq -i '.services.web.image = "nginx:latest"' docker-compose.yml

# Helm: Override values
yq -i '.replicaCount = 5 | .image.tag = "v2.0"' values.yaml

# Config: Merge environment-specific configs
yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' base-config.yaml prod-config.yaml

# Extract all environment variables
yq '.spec.template.spec.containers[].env[].name' deployment.yaml

# Create ConfigMap from key-value pairs
yq -n '.apiVersion = "v1" | .kind = "ConfigMap" | .metadata.name = "myconfig" |
  .data.key1 = "value1" | .data.key2 = "value2"'

# Batch update: Change all port references
yq -i '(.. | select(has("port"))).port = 8080' config.yaml

Advanced Patterns

# Recursive descent (find all images)
yq '.. | select(has("image")) | .image' file.yaml

# Variable assignment
yq '.items[] | . as $item | $item.name + " - " + ($item.price | tostring)' file.yaml

# Conditional updates
yq -i '(.spec.replicas) |= if . < 3 then 3 else . end' deployment.yaml

# Reduce (sum, accumulate)
yq '[.items[].price] | add' file.yaml

# Group by
yq 'group_by(.category)' items.yaml

# Flatten nested arrays
yq '.items | flatten' file.yaml

# Alternative operator (default values)
yq '.optional // "default-value"' file.yaml

# Error handling with alternative
yq '.mayNotExist // empty' file.yaml

Best Practices

Query Construction

• Use yq eval for complex expressions
• Test on small files first before modifying production configs
• Use -i flag carefully (always backup first)
• Prefer specific paths over recursive descent for performance

Configuration Management

• Version control before in-place edits
• Use yq eval-all for merging configurations
• Validate YAML after modifications
• Document complex transformations in scripts

Multi-Document YAML

• Use select(document_index == N) for specific documents
• Use eval-all when processing multiple documents together
• Split large multi-doc files with -s for easier management

Performance

• Use targeted queries for large files (instead of .. recursive descent)
• Use specific paths when possible
• Process files in parallel when operating on multiple files
• Stream large files if yq supports it

Common Patterns

Kubernetes Manifests

# Update deployment image
yq -i '.spec.template.spec.containers[0].image = "app:v2.0"' deployment.yaml

# Scale deployment
yq -i '.spec.replicas = 5' deployment.yaml

# Add environment variable
yq -i '.spec.template.spec.containers[0].env += [{"name": "DEBUG", "value": "true"}]' deployment.yaml

# Get all resource names
yq '.metadata.name' *.yaml

# Filter by kind
yq 'select(.kind == "Service")' *.yaml

# Extract all namespaces
yq '.metadata.namespace' *.yaml | sort -u

GitHub Actions Workflows

# List all jobs
yq '.jobs | keys' .github/workflows/ci.yml

# Get job steps
yq '.jobs.build.steps[].name' .github/workflows/ci.yml

# Update action version
yq -i '(.jobs.*.steps[] | select(.uses | test("actions/checkout"))).uses = "actions/checkout@v5"' workflow.yml

# Add environment variable to job
yq -i '.jobs.build.env.NODE_ENV = "production"' .github/workflows/ci.yml

# Extract trigger events
yq '.on | keys' .github/workflows/*.yml

Configuration Files

# Environment-specific config merge
yq eval-all 'select(fileIndex == 0) *+ select(fileIndex == 1)' base.yaml production.yaml

# Update nested configuration
yq -i '.database.host = "prod-db.example.com"' config.yaml

# Add to array configuration
yq -i '.allowed_hosts += ["example.com"]' config.yaml

# Remove sensitive fields
yq -i 'del(.secrets)' config.yaml

Docker Compose

# List services
yq '.services | keys' docker-compose.yml

# Update service image
yq -i '.services.app.image = "myapp:latest"' docker-compose.yml

# Add port mapping
yq -i '.services.web.ports += ["8080:80"]' docker-compose.yml

# Set environment variables
yq -i '.services.app.environment.DEBUG = "true"' docker-compose.yml

Troubleshooting

Invalid YAML

# Validate YAML syntax
yq '.' file.yaml  # Will show parse errors

# Check YAML structure
yq eval '.' file.yaml

Empty Results

# Debug: Show entire structure
yq '.' file.yaml

# Debug: Check keys
yq 'keys' file.yaml

# Debug: Check type
yq 'type' file.yaml

# Debug: Show all scalar values
yq '.. | select(tag == "!!str")' file.yaml

Unexpected In-Place Edits

# Preview changes (without -i)
yq '.field = "new-value"' file.yaml

# Backup before edit
cp file.yaml file.yaml.bak
yq -i '.field = "new-value"' file.yaml

# Use version control
git diff file.yaml

Multi-Document Issues

# Count documents
yq 'document_index' multi-doc.yaml | sort -u | wc -l

# Process specific document
yq 'select(document_index == 0)' multi-doc.yaml

# Process all documents
yq eval-all '.' multi-doc.yaml

Integration with Other Tools

# With kubectl (Kubernetes)
kubectl get deployment myapp -o yaml | yq '.spec.replicas'

# With helm
helm template myapp ./chart | yq 'select(.kind == "Service")'

# With git (version control)
git diff deployment.yaml | grep '^+' | yq -p yaml '.'

# With find (batch processing)
find . -name "*.yaml" -exec yq '.version' {} \;

# With jq (JSON processing)
yq -o=json '.' file.yaml | jq '.specific.field'

# With envsubst (environment variable substitution)
yq '.database.host = strenv(DB_HOST)' config.yaml

yq vs jq Comparison

Operation	jq	yq
Read field	jq '.field'	yq '.field'
Update in-place	Not supported	yq -i '.field = "value"'
Format output	jq .	yq .
Multi-file merge	jq -s	yq eval-all
String interpolation	"\(.var)"	"\(.var)"
Convert to JSON	N/A	yq -o=json

Quick Reference

Operators

• .field - Access field
• .[] - Iterate array/object
• | - Pipe (chain operations)
• , - Multiple outputs
• // - Alternative operator (default value)
• * - Merge objects (shallow)
• *+ - Deep merge objects

Functions

• keys, values - Object keys/values
• length - Array/object/string length
• select() - Filter
• sort_by() - Sort
• group_by() - Group
• unique - Remove duplicates
• has() - Check field existence
• type - Get value type
• add - Sum or concatenate
• del() - Delete field

Flags

• -i - In-place edit
• -o - Output format (json, yaml, xml, csv, tsv, props)
• -p - Input format (json, yaml, xml)
• -I - Indent (default: 2)
• -r - Raw output (no quotes)
• -n - Null input (create from scratch)
• -s - Split multi-document YAML

String Functions

• split(), join() - String/array conversion
• contains() - String/array contains
• test() - Regex match
• sub(), gsub() - String substitution
• upcase, downcase - Case conversion

Installation

# macOS (Homebrew)
brew install yq

# Note: Install mikefarah/yq (v4+), not the Python yq
brew install yq

# Ubuntu/Debian (snap)
sudo snap install yq

# Verify installation (should be v4+)
yq --version

Important: This skill assumes yq v4+ (mikefarah/yq). The older Python-based yq has different syntax.

Resources

• Official Documentation: https://mikefarah.gitbook.io/yq/
• GitHub Repository: https://github.com/mikefarah/yq
• Try with Docker: docker run --rm -i mikefarah/yq '.key' <<< 'key: value'
• Operators Reference: https://mikefarah.gitbook.io/yq/operators/