support curl, wget, perl timeout replacement, busybox timeout params

[DukeyToo]

Will fix #15, #16, #5 .

Adds command-line switches for busybox, and using curl and wget for more correct tests of http endpoints. If timeout command is not found, then will attempt to use a perl replacement.

These are all changes I made for my own needs in orchestrating the startup and configuration of a bunch of docker containers.

[dakl]

👍

[dequis]

# sh wait-for-it.sh
wait-for-it.sh: line 67: syntax error: unexpected "(" (expecting "fi")

BusyBox v1.24.2 (from docker:latest image). This sucks since that part of the if isn't even executed, but I guess shells aren't infinitely forgiving when parsing

[eigood]

I'd first attempt to use /dev/tcp(or /dev/udp) in a redirect first, before attempting external programs. There's the issue with not being able to set the timeout when doing it this way, however, that can be worked around by using a background process that sends a signal to the parent.

[jacobsvante]

/dev/tcp sounds like an interesting approach. @eigood Have you seen an implementation of this? Would be especially interesting to see HTTP status code parsing.