Parallel Download Tools For Linux: Aria2, Axel, And Curl

When you need to download large files faster, parallelizing across multiple connections can significantly improve throughput. Standard tools like wget and curl use single connections by default, but several alternatives support multi-connection downloads that can achieve 3-5x speedups on capable networks.

aria2c

aria2 is the most powerful option. It handles HTTP, HTTPS, FTP, BitTorrent, and Metalink protocols, supports multiple simultaneous sources, and offers granular control over connection behavior.

Basic multi-connection download:

aria2c -x 10 -s 10 -k 1M http://example.com/largefile.iso

Flag breakdown:

-x 10 — maximum 10 connections per host
-s 10 — split file into 10 segments
-k 1M — minimum 1MB per segment (prevents excessive fragmentation)

With custom output directory and filename:

aria2c -x 10 -s 10 -k 1M -d /path/to/dir/ -o custom_name.iso http://example.com/largefile.iso

For resumable downloads with crash recovery:

aria2c -x 10 -s 10 -k 1M --continue=true --allow-overwrite=false \
  -d /downloads/ http://example.com/file.iso

When downloading multiple files concurrently:

aria2c -x 5 -s 5 -j 3 http://example.com/file1.iso http://example.com/file2.iso http://example.com/file3.iso

The -j 3 flag processes 3 files in parallel. When running multiple concurrent downloads, reduce -x to avoid overwhelming the server — 5 connections per file across 3 parallel jobs is typically safe.

aria2 also maintains a session file, which is useful for resuming interrupted batch downloads:

aria2c --save-session=/tmp/session.txt -x 5 -s 5 http://example.com/file1.iso http://example.com/file2.iso

# Resume later with:
aria2c --load-session=/tmp/session.txt

axel

axel is lightweight and straightforward for simple HTTP/HTTPS downloads. It has minimal dependencies and lower memory overhead than aria2, making it suitable for resource-constrained systems or quick one-off downloads.

axel -n 10 http://example.com/largefile.zip

The -n 10 flag opens 10 parallel connections. axel automatically handles reassembly, so there’s no manual cleanup needed. It also supports resume with -a flag:

axel -n 10 -a http://example.com/largefile.zip

curl with GNU Parallel

For integration into shell scripts or pipelines, combine curl with GNU Parallel. This approach offers flexibility but requires manual file size calculation and range header setup.

First, determine file size:

curl -sI http://example.com/file | grep -i content-length

Then split and download:

FILESIZE=1073741824
PARTS=10
PARTSIZE=$((FILESIZE / PARTS))

seq 0 $((PARTS - 1)) | parallel -j 10 \
  "curl -r \$(({}*PARTSIZE))-\$((({} + 1)*PARTSIZE - 1)) \
  http://example.com/file -o file.part.{}"

cat file.part.* > file && rm file.part.*

This approach works well when you need to integrate downloads into larger automation workflows, but it’s more verbose than dedicated tools.

Choosing the Right Tool

aria2c: Use for complex scenarios, multiple file downloads, BitTorrent support, or when you need session persistence and metadata handling. Best for production automation and large-scale batch operations.

axel: Pick for straightforward HTTP/HTTPS downloads on systems with tight resource constraints. Ideal for shell scripts and embedded environments.

curl + parallel: Useful for custom pipelines where you need full control over the download logic or must integrate with existing curl-based workflows.

Server Compatibility

Not all servers support multi-connection downloads. Check before attempting:

curl -I http://example.com/file | grep -i accept-ranges

If the response shows Accept-Ranges: bytes, the server supports range requests. If it’s absent or shows Accept-Ranges: none, multi-connection downloads won’t help — you’ll receive the full file from each connection.

Connection Limits and Rate Limiting

Most servers limit concurrent connections per client to prevent abuse. Start conservative:

Use -x 5 or -x 10 initially
If you receive 429 Too Many Requests responses, reduce connection count
Respect Retry-After headers in server responses
Add delays between requests if the server rate-limits aggressively

For aria2, configure a global connection limit to avoid accidental DOS-like behavior:

aria2c -x 5 -s 5 --max-overall-download-limit=10M http://example.com/file

Performance Expectations

On modern broadband with cooperative servers, multi-connection downloads typically provide 3-5x speedup over single-connection transfers. Actual gains depend on:

Server bandwidth capacity
Network latency and stability
Number of available connections allowed
File size (larger files benefit more from parallelization)

For files under 100MB, single-connection downloads are often sufficient. Multi-connection approaches shine with files in the gigabyte range or when bandwidth is shared across many competing transfers.

Parallel Download Tools for Linux: aria2, axel, and curl