How to make a custom bash tab completion?
An easy bash completion tutorial?
How do electromagnetic waves travel in a vacuum? This is perhaps a total newbie question, and I will try to formulate it the best I can, so here it goes. How does an electromagnetic wave travel through for example, the vacuum of space?   I usually see that waves are explained using analogies with water, pieces of rope, the strings of a guitar, etc, but it seems to me that all those waves need a medium to propagate. In fact, from my point of view, in those examples the wave as a "thing" does not exist, it's just the medium that moves (involuntary reference to The Matrix, sorry).   But in space there is no medium, so how does a wave travel? Are there free particles of some sort in this "vacuum" or something? I believe , so supposedly there isn't a medium for the wave to travel through.   Moreover, I've seen other answers that describe light as a perturbation of the electromagnetic field, but isn't the existence of the field, potential until disturbed? How can it travel through something it does not exist until it's disturbed by the traveling light in the first place? (this last sentence is probably a big misconception by me). Plugin or brush for transforming a photo to a fractal in gimp? Folks,  I've been looking all over for a plugin or brush that will help me take a photo and reduce it to that 'electric look' (or fractal?) that i'm seeing in various places. I'd vastly prefer one for Gimp.  Does anyone know how these are being done? I'm seening references to fractalius for photoshop, but what about gimp?  Examples: Delete files older than X days + I have found the command to delete files older than 5 days in a folder  find /path/to/files* -mtime +5 -exec rm {} \;   But how do I also do this for subdirectories in that folder? Define a relation and find its equivalence classes. Why do I have to enter my password to log in? Using the Stack Exchange login, why do I have to enter my password again and again to log in? Wasn’t the whole purpose of OpenID to do away with this?  This doesn’t even only happen when visiting a new Stack Exchange site – it also happens for sites where I already have an account but which I don’t visit regularly. Here’s what I mean:    Of course I don’t remember my password (it’s a long random string). And so I have to go to my password manager and manually copy the password from openid.stackexchange.com’s account into the login form. – And incidentally, this is what my password manager looks like now:    (And, by the way, those are the only entries in my keychain so far, but by far not the only SE sites where I have an account.)  … I feel that this defeats the purpose of site-wide login and OpenID. In the olden times when MyOpenID was still working properly, I’d simply enter my OpenID on the login form, and the software would take care of the rest – namely, redirect to the OpenID service, (automatically) verify that I was authorised to use it, and redirect back to the original website, having logged me in. It was literally one button click for me. It’s substantially more work now, and I’d like to understand why this regression is necessary.  (Note: of course once a site’s password has landed in my password manager I (probably?) won’t have to enter it again. But still: wasn’t site-wide login and OpenID supposed to solve this already?) $\\$   A normal subgroup $N$ is a subgroup where the left cosets are the same as the right cosets. $N$ is normal $\iff $ $xnx^{-1} \in N,  \forall x\in G$.   5.) Why is it that if $[G:H]=2 \implies $ $H$ is normal subgroup?  6.) Can we say that a factor group is just a group that has left cosets of $N$ (being a normal subgroup) as its elements? So if $N$ is a normal subgroup, then the left cosets of $N$ forms a group under coset multiplication given by $aNbN = abN$.   7.) The group of left cosets of $N$ in $G$ is called the factor group, why do we denote this by $G/N$? These are the same things as the integers modulo $n$ groups? How can I relate those exactly? Almost all passports I have seen have a place of birth field on them. For instance, on the US Passport,    Is there a specific reason to this? Is it possible to print to the Report window in the Info view? Why didn't Kenobi start training Luke earlier? When Anakin was only 9 years old, everyone in the Jedi Council was arguing that he was already too old for Jedi training.  When it comes to Luke, it was a pre-planned agenda to make him a powerful Jedi. So, why did Kenobi wait for Luke to be much older? He could have started Luke's training by himself. Let $f:\mathbb{R} \rightarrow \mathbb{R}$ be a differentiable function such that $f(0) = 0$ and $f'(x) = [f(x)]^{2}$, $\forall x \in \mathbb{R}$. Show that $f(x) = 0$,  $\forall x \in \mathbb{R} $.  I first (unsuccessfully) tried using the Mean Value Theorem, but this is in the Integrals chapter so the solution probably involves them. Can't really see where integrals come in here though.  What I've got so far:   (i) Since $f$ is differentiable, thus it is continuous and, hence,  integrable. Therefore $f^2$ is also integrable and as $f'=f^2$, $f'$ is too.   (ii) $f' \geq 0$, $\forall x \in \mathbb{R}$ Windows 7 USB install error I am trying to install Windows 7 Professional x64 from a bootable USB stick. I have used the Windows 7 USB DVD Download Tool to create the media from an ISO.  The machine boots from USB fine, and enters the Windows installation wizard but then I get an error stating:     A required CD/DVD drive device driver is missing. If you have a driver floppy disk, CD, DVD, or USB flash drive, please insert it now.   I have tried setting the SATA controller in the BIOS to both AHCI and ATA but neither appear make any difference.  I've also tried both the USB 3 and USB 2 ports with no difference.  I've also tried to use the files from an official Dell recovery CD as this is a Dell laptop but again, no dice.  All this because there's no optical drive in the machine!  Dell Vostro V131 / i5-2450M / BIOS A04  Can anyone offer any advice please? Inequality $\frac{1}{n+1}+\frac{1}{n+2}+...+\frac{1}{3n+1} \ge 1$ Steps to Reproduce   Tap faster on the code. or Tap once on a huge code.   but happens only when  setupForGeneric() is called, might be a bug related to   private void setupForCode()   { String str = getIntent().getStringExtra("com.stackexchange.marvin:code_block"); PrettyWebView localPrettyWebView = (PrettyWebView)findViewById(R.id.code); localPrettyWebView.setPostBody("&lt;pre&gt;&lt;code&gt;" + TextUtils.htmlEncode(str) + "&lt;/pre&gt;&lt;/code&gt;", false); localPrettyWebView.setBackgroundColor("#eeeeee"); }  private void setupForGeneric() { String str = getIntent().getStringExtra("com.stackexchange.marvin:web_url"); PrettyWebView localPrettyWebView = (PrettyWebView)findViewById(R.id.code); localPrettyWebView.getSettings().setBuiltInZoomControls(true); localPrettyWebView.getSettings().setLoadWithOverviewMode(true); localPrettyWebView.getSettings().setUseWideViewPort(true); localPrettyWebView.loadUrl(str); localPrettyWebView.setBackgroundColor("#eeeeee"); }   Log  09-05 03:54:30.312: E/AndroidRuntime(26264): FATAL EXCEPTION: WebViewCoreThread  09-05 03:09:39.522: E/Crittercism(18318): java.lang.NullPointerException: [v0.1.22]  09-05 03:09:39.522: E/Crittercism(18318):   at android.content.ComponentName.&lt;init&gt;(ComponentName.java:75) 09-05 03:09:39.522: E/Crittercism(18318):   at android.content.Intent.&lt;init&gt;(Intent.java:3156) 09-05 03:09:39.522: E/Crittercism(18318):   at com.stackexchange.marvin.ui.ViewPopupActivity.showCode(ViewPopupActivity.java:25) 09-05 03:09:39.522: E/Crittercism(18318):   at com.stackexchange.marvin.ui.question.QuestionFragment$1.onCodeClicked(QuestionFragment.java:67) 09-05 03:09:39.522: E/Crittercism(18318):   at com.stackexchange.marvin.ui.common.PrettyWebView$WebAppInterface.codeClicked(PrettyWebView.java:197) 09-05 03:09:39.522: E/Crittercism(18318):   at android.webkit.WebViewCore.nativeTouchUp(Native Method) 09-05 03:09:39.522: E/Crittercism(18318):   at android.webkit.WebViewCore.nativeTouchUp(Native Method) 09-05 03:09:39.522: E/Crittercism(18318):   at android.webkit.WebViewCore.access$4000(WebViewCore.java:59) 09-05 03:09:39.522: E/Crittercism(18318):   at android.webkit.WebViewCore$EventHub$1.handleMessage(WebViewCore.java:1449) 09-05 03:09:39.522: E/Crittercism(18318):   at android.os.Handler.dispatchMessage(Handler.java:99) 09-05 03:09:39.522: E/Crittercism(18318):   at android.os.Looper.loop(Looper.java:137) 09-05 03:09:39.522: E/Crittercism(18318):   at android.webkit.WebViewCore$WebCoreThread.run(WebViewCore.java:790) 09-05 03:09:39.522: E/Crittercism(18318):   at java.lang.Thread.run(Thread.java:856) I am trying to install  under Wine. The program setup continually requests proxy settings. My broker tells me this shouldn't happen, and is a problem on my end. Here is what I have tried to attempt to solve this error:   I have gone into Network settings and proxy is set to none. I have looked in Wine Regedit and proxy_enable is set to 0. I have checked the firewall settings on my modem. I have tried reinstalling wine. I have installed lib32nss-mdns I can log on to my modem in iexplore using the gateway ip but thats it I've set my DNS to 10.0.0.138 to reflect the gateway I've set up a 32 bit Wine prefix   How should I proceed in solving this? Using INSERT INTO...EXEC AT linked server into temp table fails with Msg 7391 While there be any issue applying for a new passport before stamping? Conditionals (Past Tense) In terms of conditional sentences, is it correct to say:   if you knew why I WAS here, then I wouldn't need to be here.   or   If you knew why I WERE here, then I wouldn't need to be here.   Which type of conditional is this? Type 2? I was thought at school that there are three types of conditionals. Are there more of them? A simple algebra that is not semisimple How to number and cross-reference \subsubsection-level headers How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves?  I have seen  given on Wikipedia, but unfortunately it did not help.