Concurrent Shell Command Queues

11 points by speckx 2 hours ago · 5 comments

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js2 an hour ago

> 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>

> My original use case had to do with making a large number of web requests

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 $@ $(BASE)/$@

Or, to emulate one of the examples in the blog post:

  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

Run with:

  $ make -O -j 3

> You do not need to know all the commands that you want executed up-front, you can submit commands to it over time.

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

tacostakohashi an hour ago

I guess that works.

I've always been fond of using make for things to get free parallelism / ability to interrupt / restart / incremental updates.

kernc an hour ago

> language with poor concurrency controls

Few other languages have as many builtins/keywords for concurrency control: &, wait, jobs ...

Joker_vD an hour ago
Yeah, and all those concurrency controls of the shell suck. Here's a snippet to run N commands in parallel, wait for all of them, while supporting interrupting them with simple Ctrl+C:
```
    (
        trap 'kill 0' SIGINT;
        for i in {1..$N}
        do
            whatever_command $i &
        done
        wait
    )
```
The parentheses are absolutely crucial there for some reason, the { } won't give you pass-through of SIGINT. No wonder people keep re-inventing PID files, manually tracking $! and all kinds of other crazy bullshit that work 90% of the time, but sometimes fail to properly terminate the runaway children.
nagaiaida 4 minutes ago

perhaps that's more an indictment of most programming languages

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