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    <ol> <li><p>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.</p></li> <li><p><code>system</code> is convenient in two different cases:</p> <p>a. You have a long running program and you want the output to print as it runs (e.g. <code>system("tar zxvf some_big_tarball.tar.gz")</code>)</p> <p>b. <code>system</code> can bypass the shell expansion like <code>exec</code> (compare the output of <code>system "echo *"</code> and <code>system "echo", "*"</code>)</p> <p>system blocks until the subprocess has exited.</p></li> <li><p><code>fork</code> has a couple different use cases as well:</p> <p>a. You want to run some ruby code in a separate process (e.g. <code>fork { .... }</code></p> <p>b. You want to run a child process (or different program) without blocking progress of your script <code>fork { exec "bash" }</code>.</p> <p><code>fork</code> is your friend if you want to daemonize your program.</p></li> <li><p><code>IO.popen</code> 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 <code>2&gt;&amp;1</code> if you care about that.</p></li> <li><p><code>popen3</code> gives you a separate file descriptor for standard error (for when you need to capture that separately from standard out)</p></li> <li><p><code>PTY.spawn</code> is necessary when you want the spawned program to behave like you are running from the terminal. See the difference of <code>grep --color=auto pat file</code> when spawned with <code>system</code> vs <code>PTY.spawn</code></p></li> </ol>
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