When to use each method of launching a subprocess in Ruby

1. `` The Backtick

defined in Kernel

1. a) `%x{}` Percent X < alternate syntax for The Backtick

defined in parse.y, see discussion

2. `system()`

Kernel#system

3. `fork()`

Kernel#fork, Process#fork

4. `open()`

open a pipe
Kernel#open

4.a. `IO.popen()` < behaves the same as `open()`

open a pipe
IO#popen

4.b. `open("|-")`

fork to a pipe

4.c. `IO.popen("-")` < behaves the same as `open("|-")`

fork to a pipe
see discussion

5. `Open3.popen3()`

require 'open3'
stdlib Open3

6. `PTY.spawn()`

require 'pty'
stdlib PTY

7. `Shell.transact()`

require 'shell'
stdlib Shell

When should one forsake the trusty back-tick for one of the more complex methods?

Edit 1. Big thanks to Avdi Grimm for his posts describing example usage of each method: #1 (& gist); #2 (& gist); #3.

They are fantastic resources to answer How, but are not explicitly composed to answer when each should be used or Why, and as such IMHO are not complete answers to this question.

use backticks when you want to easily capture the output of a program in a variable. you probably only want to use this for short-running programs, because this will block.
system is convenient in two different cases:

a. You have a long running program and you want the output to print as it runs (e.g. system("tar zxvf some_big_tarball.tar.gz"))

b. system can bypass the shell expansion like exec (compare the output of system "echo *" and system "echo", "*")

system blocks until the subprocess has exited.
fork has a couple different use cases as well:

a. You want to run some ruby code in a separate process (e.g. fork { .... }

b. You want to run a child process (or different program) without blocking progress of your script fork { exec "bash" }.

fork is your friend if you want to daemonize your program.
IO.popen is useful when you need to interact with the standard out and standard in of a program. Note that it doesn't capture standard err, so you need to redirect that with 2>&1 if you care about that.
popen3 gives you a separate file descriptor for standard error (for when you need to capture that separately from standard out)
PTY.spawn is necessary when you want the spawned program to behave like you are running from the terminal. See the difference of grep --color=auto pat file when spawned with system vs PTY.spawn