> How to run shell commands in parallel while limiting their concurrency.

Depending upon the task, generate a Makefile and run it with:

  make -j <concurrency>

This is in fact the use case that caused me to think about using a Makefile for concurrency probably 20 years ago. A rule for downloading each URL to a file. Then a simple `make -j <jobs>` with whatever concurrency I desired and bonus: automatically skipping already downloaded files. e.g.:

  BASE  := https://example.com/files
  FILES := a.txt b.txt c.txt d.txt e.txt

  .DELETE_ON_ERROR:

  all: $(FILES)

  $(FILES):
        curl -sS --fail --max-time 30 -o $@ $(BASE)/$@

  SHELL := bash
  .SHELLFLAGS := -o pipefail -c

  N    := 1 2 3 4 5
  REQS := $(N:%=req-%)

  .PHONY: all $(REQS)
  all: $(REQS)

  $(REQS): req-%:
        @curl -sS --fail-with-body --max-time 10 -X POST -H 'Content-Type: application/json' \
        -d '{"request": $*}' https://postman-echo.com/post | jq -c .json

  $ make -O -j 3

Ah, yeah, that does put Make in the same bucket as `xargs -P` / GNU `parallel` and requires something more clever.