Xvfb
Let's sit in on a conversation between a couple of techies named Remus and Romulus on the subject of Xvfb and learn what Remus eventually does: that Java boosts the importance of this old utility
REMUS: Hey, Romulus, I was just reading about this thing called Xvfb. Have you ever heard of it?
ROMULUS: Yeah. It's actually an acronym; it stands for X virtual frame buffer.
REMUS: And that is...?
REMUS: A dummy load. The X Window graphical user interface (GUI) defines an architecture of client applications that send graphical instructions to a display server.
REMUS: A punching bag that just hangs there and takes it? What's the good in that?
ROMULUS: Lots of good. There are plenty of useful tests you can run on a graphical application without ever seeing its display: timing tests, tests that make sure that it adjusts to different color depths and environment parameters, tests of command-line arguments and other external interfaces, load testing, code-coverage profiles, and so on. Smart quality-assurance engineers certainly know about Xvfb.
REMUS: So testers like it. Is that all?
ROMULUS: Only the beginning. Here's where it comes up most often: Someone says to his buddy at work, "I've got this cool image converter/graph generator/network monitor/whatever that I want to use to generate daily reports for my boss. I've got it all figured out, and it runs automatically, but when I put it in crontab it doesn't work. Now what?" That person probably needs Xvfb. There are a lot of situations like that: an application insists on a DISPLAY, but doesn't really require anyone to see what's there. Xvfb is great for such jobs.
REMUS: Sounds cool. What else?
ROMULUS: Well, it's not precisely true that Xvfb simply throws away all the graphical messages it receives. In fact, it can be configured to render its data into a kind of image that's kept in a specific location in memory. An engineer could start a big headless aerodynamic calculation (i.e., one without a display), walk away from it for a week, then connect the memory area to a real display screen to peek at a snapshot of the calculation in progress.
REMUS: Cool. That sounds a little like virtual network computing (VNC).
ROMULUS: You're right; users get some of the same benefits -- persistence and relocatability across processes -- from VNC. In fact, many administrators also recommend VNC for automation. When someone has an application that insists on a DISPLAY, one solution is to configure VNC as a dummy X server. Even in domains such as this, where the capabilities of VNC and Xvfb overlap, each also has unique features. They're both valuable.
REMUS: You make it sound as though there's something wrong with using VNC in a crontab.
ROMULUS: If it works for you, I'm all in favor of
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