How to make asynchronous function calls in shell scripts

Question

I have a collection of curl commands to be executed by a shell script. Now what i want is all these commands have to be executed at a regular interval of time ( which is different for every curl url ) so what i want to do is make asynchronous calls to

wait [sec]

command and execute different functions for different wait periods like

start 5 timers one for 120s, 2 for 30s, 3 for 3000s etc. and then as soon as they get completed i want to trigger the execution of the handler function attached to every timeout. I can do this in javascript and nodejs easily as they are event driven programming language. But i have little knowledge about shell scripting. So, how else can i implement this or hotto make such asynchronous calls in the shell script? I dont know if i am clear enough, what other details should i mention if i am not?

Solution

Something to experiment with:

delayed_ajax() {
  local url=$1
  local callback=$2
  local seconds=$3

  sleep $seconds
  curl -s "$url" | "$callback"
}

my_handler() {
  # Read from stdin and do something.
  # E.g. just append to a file:
  cat >> /tmp/some_file.txt
}

for delay in 120 30 30 3000 3000; do
  delayed_ajax http://www.example.com/api/something my_handler $delay &
done

Comments

[Deleted User]

is 60 seconds resolution too course, which is why you don't run them as cron jobs?

[Andreas Kalin]

Ah, yes, cron is probably the right tool for this if minute resolution is sufficient.

[Andreas Kalin]

And, similarily, the at command is useful if you want to run the command at a certain time rather than in certain intervals. Also minute resolution. Neither cron, nor at requires nohup, which you may want if you use the scripted solution I hinted. A scripted solution will be killed by the HUP signal at logout unless started with nohup (or disowned with disown -h or similar).

[Harshit Laddha]

Thanks a lot, they did work for my purpose, only problem in future i might have is too many processes assigned for every timeout as i will be having a lot of them in near future

[Andreas Kalin]

You saw that I missed the ampersand first? Will not be async w/o it.

[Andreas Kalin]

Yes, bash tend to produce loads of processes. Since you're into node and js (and perhaps coffeescript) you could maybe consider doing it in node?

[Harshit Laddha]

I know, i thought of finding some module to do it in node, but as it was needed urgently i though of implementing it in shell script first and then with node later