loader

A minimal CLI load-testing tool that runs a command repeatedly in parallel and reports timing statistics.

Install

go install github.com/boyvinall/loader@latest

Or build locally:

make build

Usage

loader [options] COMMAND [ARGS...]

Options

Flag	Short	Default	Description
--rate	-r	1s	Interval between launching new commands
--max-parallel	-p	20	Maximum number of simultaneous processes
--max-count	-n	0	Total processes to launch before stopping (0 = unlimited)
--duration	-d	0	Stop launching after this duration (0 = unlimited)
--verbose		off	Show stdout/stderr from each process

At least one of --max-count or --duration must be set, otherwise the tool runs until interrupted.

Examples

Launch curl 100 times, at most 10 in parallel, one per second:

loader -n 100 -p 10 curl -s https://example.com

Run for 30 seconds at 5 launches per second, capped at 50 parallel:

loader -d 30s -r 200ms -p 50 ./my-script.sh

Stress test a local server with no rate limit between launches:

loader -d 60s -r 0s -p 100 curl -s http://localhost:8080/health

Behaviour

• A new process is launched on every --rate tick. If all --max-parallel slots are occupied the launch loop blocks until one frees up.
• When --duration expires, no new processes are launched but any already-running processes are allowed to finish.
• Ctrl-C once — stops launching new processes, waits for running ones to finish.
• Ctrl-C twice — sends SIGKILL to all running processes and exits immediately.
• Subprocess stdout/stderr is discarded by default; use --verbose to see it.

Output

A live status line is printed to stderr during the run:

launched=42      running=8       completed=34      failed=0

A summary is printed on exit:

=== Summary ===
Launched:  100
Completed: 100
Successes: 98
Failures:  2
Duration:
  min: 142ms
  avg: 187ms
  p50: 183ms
  p95: 241ms
  p99: 267ms
  max: 312ms