Bash tool

The bash tool enables Claude to execute shell commands in a persistent bash session, allowing system operations, script execution, and command-line automation.

Overview

The bash tool provides Claude with:

• Persistent bash session that maintains state
• Ability to run any shell command
• Access to environment variables and working directory
• Command chaining and scripting capabilities

Model compatibility

Model	Tool Version
Claude 4 models and Sonnet 3.7 (deprecated)	bash_20250124

Older tool versions are not guaranteed to be backwards-compatible with newer models. Always use the tool version that corresponds to your model version.

Use cases

• Development workflows: Run build commands, tests, and development tools
• System automation: Execute scripts, manage files, automate tasks
• Data processing: Process files, run analysis scripts, manage datasets
• Environment setup: Install packages, configure environments

Quick start

Python

import anthropic

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    tools=[
        {
            "type": "bash_20250124",
            "name": "bash"
        }
    ],
    messages=[
        {"role": "user", "content": "List all Python files in the current directory."}
    ]
)

How it works

The bash tool maintains a persistent session:

1. Claude determines what command to run
2. You execute the command in a bash shell
3. Return the output (stdout and stderr) to Claude
4. Session state persists between commands (environment variables, working directory)

Parameters

Parameter	Required	Description
command	Yes*	The bash command to run
restart	No	Set to true to restart the bash session

*Required unless using restart

Example usage

// Run a command
{
  "command": "ls -la *.py"
}

// Restart the session
{
  "restart": true
}

Example: Multi-step automation

Claude can chain commands to complete complex tasks:

# User request
"Install the requests library and create a simple Python script that fetches a joke from an API, then run it."

# Claude's tool uses:
# 1. Install package
{"command": "pip install requests"}

# 2. Create script
{"command": "cat > fetch_joke.py << 'EOF'\nimport requests\nresponse = requests.get('https://official-joke-api.appspot.com/random_joke')\njoke = response.json()\nprint(f\"Setup: {joke['setup']}\")\nprint(f\"Punchline: {joke['punchline']}\")\nEOF"}

# 3. Run script
{"command": "python fetch_joke.py"}

The session maintains state between commands, so files created in step 2 are available in step 3.

---

Implement the bash tool

The bash tool is implemented as a schema-less tool. When using this tool, you don’t need to provide an input schema as with other tools; the schema is built into Claude’s model and can’t be modified.

1. Set up a bash environment

   Create a persistent bash session that Claude can interact with:

   import subprocess
   import threading
   import queue
   
   class BashSession:
       def __init__(self):
           self.process = subprocess.Popen(
               ['/bin/bash'],
               stdin=subprocess.PIPE,
               stdout=subprocess.PIPE,
               stderr=subprocess.PIPE,
               text=True,
               bufsize=0
           )
           self.output_queue = queue.Queue()
           self.error_queue = queue.Queue()
           self._start_readers()
   

2. Handle command execution

   Create a function to execute commands and capture output:

   def execute_command(self, command):
       # Send command to bash
       self.process.stdin.write(command + '\n')
       self.process.stdin.flush()
       
       # Capture output with timeout
       output = self._read_output(timeout=10)
       return output
   

3. Process Claude's tool calls

   Extract and execute commands from Claude’s responses:

   for content in response.content:
       if content.type == "tool_use" and content.name == "bash":
           if content.input.get("restart"):
               bash_session.restart()
               result = "Bash session restarted"
           else:
               command = content.input.get("command")
               result = bash_session.execute_command(command)
           
           # Return result to Claude
           tool_result = {
               "type": "tool_result",
               "tool_use_id": content.id,
               "content": result
           }
   

4. Implement safety measures

   Add validation and restrictions:

   def validate_command(command):
       # Block dangerous commands
       dangerous_patterns = ['rm -rf /', 'format', ':(){:|:&};:']
       for pattern in dangerous_patterns:
           if pattern in command:
               return False, f"Command contains dangerous pattern: {pattern}"
       
       # Add more validation as needed
       return True, None
   

Handle errors

When implementing the bash tool, handle various error scenarios:

Command execution timeout

If a command takes too long to execute:

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01A09q90qw90lq917835lq9",
      "content": "Error: Command timed out after 30 seconds",
      "is_error": true
    }
  ]
}

Command not found

If a command doesn’t exist:

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01A09q90qw90lq917835lq9",
      "content": "bash: nonexistentcommand: command not found",
      "is_error": true
    }
  ]
}

Permission denied

If there are permission issues:

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01A09q90qw90lq917835lq9",
      "content": "bash: /root/sensitive-file: Permission denied",
      "is_error": true
    }
  ]
}

Follow implementation best practices

Use command timeouts

Implement timeouts to prevent hanging commands:

def execute_with_timeout(command, timeout=30):
    try:
        result = subprocess.run(
            command, 
            shell=True, 
            capture_output=True, 
            text=True, 
            timeout=timeout
        )
        return result.stdout + result.stderr
    except subprocess.TimeoutExpired:
        return f"Command timed out after {timeout} seconds"

Maintain session state

Keep the bash session persistent to maintain environment variables and working directory:

# Commands run in the same session maintain state
commands = [
    "cd /tmp",
    "echo 'Hello' > test.txt",
    "cat test.txt"  # This works because we're still in /tmp
]

Handle large outputs

Truncate very large outputs to prevent token limit issues:

def truncate_output(output, max_lines=100):
    lines = output.split('\n')
    if len(lines) > max_lines:
        truncated = '\n'.join(lines[:max_lines])
        return f"{truncated}\n\n... Output truncated ({len(lines)} total lines) ..."
    return output

Log all commands

Keep an audit trail of executed commands:

import logging

def log_command(command, output, user_id):
    logging.info(f"User {user_id} executed: {command}")
    logging.info(f"Output: {output[:200]}...")  # Log first 200 chars

Sanitize outputs

Remove sensitive information from command outputs:

def sanitize_output(output):
    # Remove potential secrets or credentials
    import re
    # Example: Remove AWS credentials
    output = re.sub(r'aws_access_key_id\s*=\s*\S+', 'aws_access_key_id=***', output)
    output = re.sub(r'aws_secret_access_key\s*=\s*\S+', 'aws_secret_access_key=***', output)
    return output

Security

The bash tool provides direct system access. Implement these essential safety measures:

• Running in isolated environments (Docker/VM)
• Implementing command filtering and allowlists
• Setting resource limits (CPU, memory, disk)
• Logging all executed commands

Key recommendations

• Use ulimit to set resource constraints
• Filter dangerous commands (sudo, rm -rf, etc.)
• Run with minimal user permissions
• Monitor and log all command execution

Pricing

The bash tool adds 245 input tokens to your API calls.

Additional tokens are consumed by:

• Command outputs (stdout/stderr)
• Error messages
• Large file contents

See tool use pricing for complete pricing details.

Common patterns

Development workflows

• Running tests: pytest && coverage report
• Building projects: npm install && npm run build
• Git operations: git status && git add . && git commit -m "message"

File operations

• Processing data: wc -l *.csv && ls -lh *.csv
• Searching files: find . -name "*.py" | xargs grep "pattern"
• Creating backups: tar -czf backup.tar.gz ./data

System tasks

• Checking resources: df -h && free -m
• Process management: ps aux | grep python
• Environment setup: export PATH=$PATH:/new/path && echo $PATH

Limitations

• No interactive commands: Cannot handle vim, less, or password prompts
• No GUI applications: Command-line only
• Session scope: Persists within conversation, lost between API calls
• Output limits: Large outputs may be truncated
• No streaming: Results returned after completion

Combining with other tools

The bash tool is most powerful when combined with the text editor and other tools.

Next steps

Tool use overview

Learn about tool use with Claude

Text editor tool

View and edit text files with Claude