Managing large data transfers or processing pipelines on the command line often means dealing with temporary files. Process substitution is a lesser-known feature in Bash that lets you bypass these intermediate files and perform zero-disk streaming between commands. This technique not only aids in simple file comparisons but also serves as a foundation for advanced CLI-based archiving—such as streaming tar archives directly into cURL for remote uploads.

Understanding process substitution

Process substitution lets you redirect the output of a command to a file-like interface without actually creating an intermediate file. Using the syntax <(command), you generate a temporary FIFO (or a pseudo-file) that appears as a regular file to other commands. For example, you can compare the sorted contents of two files without the overhead of temporary files:

diff <(sort file1.txt) <(sort file2.txt)

Although this example demonstrates file comparison, the same principle applies when streaming large datasets or archives, allowing you to eliminate unnecessary disk I/O.

Benefits of zero-disk streaming

Eliminating temporary files offers several advantages:

  • Reduced disk I/O leads to faster operations.
  • Lower risk of disk space exhaustion, crucial when handling large datasets.
  • Minimized wear on storage media, especially beneficial for SSDs.
  • Cleaner scripts without manual cleanup of temporary files.

Streaming data between commands

A common scenario is archiving a directory and uploading it directly to a remote server without writing the archive to disk first. This method leverages direct piping, which is conceptually similar to process substitution:

# Basic streaming upload with progress indicator
tar czf - my-folder | curl -# -H "Content-Type: application/x-tar" -T - http://example.com/upload

You can further enhance reliability with error handling:

# With error handling
if ! tar czf - my-folder | curl -f -H "Content-Type: application/x-tar" -T - http://example.com/upload; then
    echo "Upload failed" >&2
    exit 1
fi

And if your upload requires authentication:

# With authentication
tar czf - my-folder | curl -u username:password -H "Content-Type: application/x-tar" -T - http://example.com/upload

Troubleshooting and best practices

When working with streaming transfers, consider these important points:

  • Size Limits: Check server upload limits before starting large transfers.
  • Memory Usage: Monitor system memory when processing large files.
  • Network Stability: Use curl's retry mechanism for unreliable connections:
    tar czf - my-folder | curl -f --retry 3 -H "Content-Type: application/x-tar" -T - http://example.com/upload
  • Debugging: Add the -v flag to curl for detailed transfer information:
    tar czf - my-folder | curl -v -H "Content-Type: application/x-tar" -T - http://example.com/upload
  • Server Limitations: Verify that the target server is configured to accept streaming uploads and can handle large payloads.
  • Resuming Transfers: Streaming uploads do not support resuming incomplete transfers. Ensure network stability or plan to restart the transfer if it is interrupted.
  • Shell Compatibility: Since process substitution is specific to Bash and some compatible shells, confirm that your environment supports it.

Advanced compression options

Optimizing your transfers may involve selecting different compression algorithms based on your needs. Here are examples using xz and zstd, which offer a balance between compression ratio and speed:

# Using xz for better compression ratio (May use more cpu)
tar cf - my-folder | xz -9 | curl -H "Content-Type: application/x-tar+xz" -T - http://example.com/upload

# Using zstd for faster compression
tar cf - my-folder | zstd | curl -H "Content-Type: application/x-tar+zst" -T - http://example.com/upload

Real-world applicability

Zero-disk streaming is particularly valuable in environments where disk space is limited or performance is critical. Common use cases include:

  • Backup operations to remote servers.
  • CI/CD pipeline artifact transfers.
  • Log file aggregation and processing.
  • Large dataset migrations.

In scenarios such as continuous integration or remote backups, eliminating intermediate storage can significantly simplify workflows and enhance performance. At Transloadit, similar principles power our file-compressing-service robot—minimizing disk operations while handling file compression and archiving.

Conclusion

Zero-disk streaming through process substitution and direct piping provides an efficient way to handle file transfers and processing workflows. By eliminating temporary files and incorporating proper error handling, you can build robust and efficient data pipelines. Experiment with these options to find the optimal balance between compression, speed, and reliability for your use case. For additional insights and tools, explore our File Compressing Service.