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Overview

Script variables let you write reusable markdown scripts with customizable parameters. Users can override variables from the command line without editing the script itself.

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YAML Front-Matter Syntax

Declare variables in a YAML front-matter block at the top of your script: 
#!/usr/bin/env -S ai --haiku
---
vars:
  topic: "machine learning"
  style: casual
  audience:
---
Write a summary of {{topic}} in {{style}} style.
Target audience: {{audience}}.

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Syntax Rules

  • Opt-in: Variables only activate when front-matter contains vars:. No vars: = no behavior change.
  • Front-matter stripped: The --- block is removed from the prompt when vars: is present.
  • Default values: Variables can have default values (like topic: "machine learning")
  • No default: Variables declared with just the key (like audience:) have no default value

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Placeholder Substitution

Use {{varname}} syntax to insert variable values into your prompt: 
Write a {{length}} summary of {{topic}} in a {{style}} tone.
The {{placeholder}} syntax:
  • Doesn’t collide with shell variables ($VAR)
  • Doesn’t collide with markdown syntax
  • Doesn’t collide with Claude syntax
  • Is visually distinct and easy to read

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Unset Variables

A variable declared with no default (just key:) and no CLI override leaves {{key}} as-is in the prompt: 
---
vars:
  topic:
---
Summarize {{topic}}.
Without --topic flag: “Summarize .” (literal text) With --topic "AI": “Summarize AI.”

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CLI Overrides

Override variables from the command line using flag syntax: 
./script.md --varname value
./script.md --varname="value with spaces"

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Examples

Given this script:
summarize-topic.md
#!/usr/bin/env -S ai --haiku
---
vars:
  topic: "machine learning"
  style: casual
  length: short
---
Write a {{length}} summary of {{topic}} in a {{style}} tone.
Run with defaults: 
./summarize-topic.md
# Uses: topic="machine learning", style=casual, length=short
Override one variable: 
./summarize-topic.md --topic "AI safety"
# Uses: topic="AI safety", style=casual, length=short
Override multiple variables: 
./summarize-topic.md --topic "AI safety" --style formal
# Uses: topic="AI safety", style=formal, length=short
Equals form works too: 
./summarize-topic.md --topic="robotics" --length="100 words"

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Mixing with AI Runner Flags

Variable overrides mix freely with AI Runner flags and provider overrides: 
# --live is an AI Runner flag, --topic is a variable override
./summarize-topic.md --live --topic "quantum computing"

# Override provider + model alongside variables
ai --aws --opus summarize-topic.md --topic "the fall of rome"

# All together
./summarize-topic.md --live --length "100 words" --topic "the fall of rome" --style "peter griffin"

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How Flag Consumption Works

Override flags matching declared var names are consumed by the variable system — they don’t pass through to Claude Code:
  • --topic, --style, --length are consumed (match declared vars)
  • --live, --aws, --opus pass through to AI Runner
  • Unrecognized flags (like --verbose) pass through to Claude Code

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Complete Example

Here’s a real-world script with variables:
summarize-topic.md
#!/usr/bin/env -S ai --haiku
---
vars:
  topic: "machine learning"
  style: casual
  length: short
---
Write a {{length}} summary of {{topic}} in a {{style}} tone.

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Usage Examples

Default behavior: 
./summarize-topic.md
Output: Short, casual summary of machine learning Override topic: 
./summarize-topic.md --topic "quantum computing"
Output: Short, casual summary of quantum computing Override all variables: 
./summarize-topic.md \
  --topic "the fall of rome" \
  --style "peter griffin" \
  --length "100 words"
Output: 100-word summary of the fall of Rome in Peter Griffin’s style With live streaming: 
./summarize-topic.md --live --topic "AI safety"
Output: Streams the summary in real-time With provider override: 
ai --aws --opus summarize-topic.md --topic "climate change" --style formal
Uses AWS Bedrock with Claude Opus, formal style summary of climate change

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Advanced Patterns

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Optional vs Required Variables

Optional (with default): 
---
vars:
  format: markdown
  verbose: false
---
Generate output in {{format}} format. Verbose mode: {{verbose}}.
Required (no default): 
---
vars:
  target_file:
  operation:
---
Perform {{operation}} on {{target_file}}.
Users must provide --target_file and --operation or the placeholders remain literal.

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Multiple Scripts with Shared Variables

Create a family of scripts that accept the same variables:
analyze.md
---
vars:
  language: python
  depth: basic
---
Analyze {{language}} code with {{depth}} depth.
review.md
---
vars:
  language: python
  depth: basic
---
Review {{language}} code for {{depth}} issues.
Both scripts accept the same flags: 
./analyze.md --language go --depth thorough
./review.md --language go --depth thorough

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Pipeline with Variables

Pass variables through a pipeline: 
./extract.md --source data.json | \
./transform.md --format csv | \
./validate.md --schema schema.json > output.csv
Each script in the pipeline can have its own variables.

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Common Use Cases

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1. Documentation Generator

generate-docs.md
#!/usr/bin/env -S ai --skip
---
vars:
  source_dir: src
  output_file: ARCHITECTURE.md
  style: technical
---
Read files in {{source_dir}}/
Generate {{output_file}} documenting the codebase.
Use {{style}} writing style.
Usage: 
./generate-docs.md
./generate-docs.md --source_dir lib --output_file README.md --style casual

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2. Test Runner

run-tests.md
#!/usr/bin/env -S ai --skip
---
vars:
  test_pattern: "*.test.js"
  framework: jest
  coverage: false
---
Run tests matching {{test_pattern}} using {{framework}}.
Generate coverage report: {{coverage}}.
Usage: 
./run-tests.md
./run-tests.md --test_pattern "integration/*.test.ts" --coverage true

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3. Code Reviewer

review.md
#!/usr/bin/env -S ai --opus
---
vars:
  focus: security
  severity: high
  output_format: markdown
---
Review code for {{focus}} issues.
Only report {{severity}} severity and above.
Output in {{output_format}} format.
Usage: 
./review.md
./review.md --focus performance --severity medium --output_format json

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4. Data Analyzer

analyze-data.md
#!/usr/bin/env ai
---
vars:
  metric: revenue
  period: monthly
  visualization: false
---
Analyze {{metric}} trends over {{period}} periods.
Include visualization: {{visualization}}.
Usage: 
cat data.csv | ./analyze-data.md --metric users --period weekly

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Best Practices

Do:
  • Provide sensible defaults for optional parameters
  • Use descriptive variable names (source_dir not src)
  • Document expected values in comments
  • Keep variable names lowercase with underscores
  • Use boolean variables for feature flags (verbose: false)
Don’t:
  • Use variable names that conflict with AI Runner flags (--live, --aws)
  • Put sensitive data in default values (use environment variables instead)
  • Create too many variables (makes scripts hard to use)
  • Use complex variable names (target_file_for_processing → target_file)

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Troubleshooting

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Variables Not Substituting

Problem: {{topic}} appears literally in output Solutions:
  1. Check YAML syntax (must have vars: key)
  2. Ensure front-matter is at the top of the file
  3. Verify closing --- after front-matter
  4. Check variable is declared in vars: block

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CLI Override Not Working

Problem: --topic "AI" doesn’t override the default Solutions:
  1. Check variable is declared in front-matter
  2. Verify flag syntax: --topic value or --topic="value"
  3. Check for typos in variable name
  4. Ensure flag comes after script name: ./script.md --topic "AI"

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Conflicts with Claude Code Flags

Problem: --format json is interpreted as a variable instead of a Claude Code flag Solution: Don’t declare variables with names that match Claude Code flags. Common conflicts to avoid:
  • --live
  • --quiet
  • --output-format
  • --max-turns

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Next Steps

Writing Scripts

Learn script basics and common patterns

Live Output

Stream progress in real-time

CI/CD Automation

Use variables in CI/CD pipelines

Permissions

Control what scripts can do