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Bash Output

Retrieves output from a running or completed background bash shell.

Common Properties​

  • Name - The custom name of the node.
  • Color - The custom color of the node.
  • Delay Before (sec) - Waits in seconds before executing the node.
  • Delay After (sec) - Waits in seconds after executing node.
  • Continue On Error - Automation will continue regardless of any error. The default value is false.

Inputs​

  • Shell ID - string - The ID of the background shell to retrieve output from (required).
  • Filter - string - Optional regex pattern to filter output lines (optional).

Outputs​

  • New Stdout - string - New standard output since last check.
  • New Stderr - string - New standard error since last check.
  • Status - string - Current shell status: running, completed, or failed.
  • Exit Code - int - Exit code if shell has completed (0 = success).
  • Shell Info - object - Detailed shell information:
    • shellID - The shell ID
    • command - Original command
    • status - Current status
    • exitCode - Exit code (if completed)
    • totalOutputLines - Total lines of stdout captured
    • totalErrorLines - Total lines of stderr captured
    • newOutputLines - Lines of stdout returned this call
    • newErrorLines - Lines of stderr returned this call

How It Works​

The Bash Output node retrieves output from background shells started by the Shell node. When executed, the node:

  1. Validates the shell ID is provided
  2. Looks up the background shell in the registry
  3. Retrieves new output since the last read:
    • Tracks read position for stdout
    • Tracks read position for stderr
    • Returns only new lines since last call
  4. Applies optional regex filter to output lines
  5. Returns new output, status, and metadata
  6. Updates read positions for next call

Requirements​

  • Valid shell ID from a Shell node background execution
  • Background shell must exist in the registry

Error Handling​

The node will return specific errors in the following cases:

  • Missing shell ID - "Shell ID is required"
  • Empty shell ID - "Shell ID cannot be empty"
  • Shell not found - "No background shell found with ID: {{shellID}}"
  • Invalid filter - "Invalid filter regex: {{error}}"

Usage Examples​

Basic Output Retrieval​

Shell ID: abc123-xyz789

Filtered Output​

Shell ID: abc123-xyz789
Filter: ERROR|WARNING

Continuous Monitoring​

Use in a loop to monitor long-running processes:

1. Shell node (background): python long_process.py
2. Loop:
   - Delay 5 seconds
   - Bash Output: retrieve new output
   - Check status
   - If status = completed, break

Error Detection​

Shell ID: abc123-xyz789
Filter: ^ERROR:

Usage Notes​

  • Each call returns only NEW output since the last call
  • The node maintains separate read positions for stdout and stderr
  • Filter regex is applied to individual lines
  • Empty result means no new output since last check
  • Status values: "running", "completed", "failed"
  • Exit code is only meaningful when status is "completed" or "failed"
  • The shell must be started with "Run in Background: true" in the Shell node
  • Output is line-buffered - complete lines are returned
  • Very long lines may be split

Status Values​

  • running - Process is still executing
  • completed - Process finished successfully (exit code 0)
  • failed - Process finished with error (non-zero exit code)

Output Tracking​

The node tracks what has been read:

  • First call: Returns all output since process started
  • Second call: Returns only new output since first call
  • Third call: Returns only new output since second call
  • And so on...

This allows you to:

  • Monitor progress in real-time
  • Avoid processing the same output twice
  • Track incremental changes

Common Use Cases​

Long-Running Process Monitoring​

Monitor build processes, data processing, or batch jobs.

Log Streaming​

Stream logs from background services in real-time.

Error Detection​

Watch for errors or warnings in running processes.

Progress Tracking​

Monitor progress messages from long-running scripts.

Test Execution​

Watch test output as tests run in the background.

Deployment Monitoring​

Track deployment script output and status.

Best Practices​

  • Poll regularly for running processes (e.g., every 5-10 seconds)
  • Check status to know when process completes
  • Use filters to focus on important output (errors, warnings)
  • Handle both stdout and stderr - important messages may be in either
  • Check exit code when status is completed/failed
  • Clean up completed processes with Kill Bash
  • Store Shell ID in a variable for repeated checks
  • Implement timeout logic for processes that may hang
  • Log output for debugging and audit trails

Example Workflow​

Complete Background Process Flow​

1. Shell Node (Background)
   - Command: npm run build
   - Run in Background: true
   - Output: shell_id

2. Loop (while status != "completed" and != "failed")
   - Delay: 5 seconds
   - Bash Output:
     - Shell ID: `{{shell_id}}`
     - Output: new_stdout, new_stderr, status
   - Log Output:
     - Log: `{{new_stdout}}` `{{new_stderr}}`
   - Check Status:
     - If status = "completed": break
     - If status = "failed": handle error

3. Bash Output (Final)
   - Shell ID: `{{shell_id}}`
   - Get any remaining output

4. Kill Bash (Cleanup)
   - Shell ID: `{{shell_id}}`

Filter Examples​

Filter Errors Only​

Filter: ^ERROR

Filter Warnings and Errors​

Filter: (ERROR|WARNING|FATAL)

Filter Progress Updates​

Filter: Progress:|Complete:|\d+%

Filter Specific Keywords​

Filter: (success|failed|completed)

Comparison with Other Nodes​

  • Bash Output vs Shell: Bash Output reads from background shells; Shell executes commands
  • Bash Output vs Kill Bash: Bash Output reads output; Kill Bash terminates processes
  • Bash Output vs Read: Bash Output reads process output; Read reads files