Get disk usage as a shell variable
Shell script to send an alert mail if disk usage exceeds 90%
Does umount calls sync to complete any pending writes We have BBB based custom board with kernel 3.12 running on it.  I have doubt regarding umount and &amp; sync.  Lets say a script is umounting a partition, Does it require to run sync command before umount to complete pending writes. ? Covariance of binary and continuous variable Suppose $y$ is a continuous random variable and $d$ is a binary random variable that takes the value $1$ with probability $p$ and $0$ with probability $1-p$.   How do I show that $\text{Cov}(y,d)=(E[y|d=1]-E[y|d=0])p(1-p)$? Simple question but I am confused.  What is the boundary of $S^2\times\mathbb{R}$? Is it just $S^2$?  What would be the general way to evaluate the boundary of a product manifold?  Thanks for the replies! Show that when $BA = I$, the solution of $Ax=b$ is unique I'm just getting back into having to do linear algebra and I am having some trouble with some elementary questions, any help is much appreciated.  Suppose that $A = [a_{ij}]$ is an $m\times n$ matrix and $B = [b_{ij}]$ and is an $n\times m$ matrix and $BA = I$ the identity matrix that is $n \times n$.  Show that if for some $b \in \Bbb{R}^m$ the equation $Ax = b$ has a solution, then the solution is unique. Can I disable full-disk encryption? Unable to correct problems, you have held broken packages How can I disable the Windows 8 charms bar? Can someone please tell me how to disable the annoying effect that pops up when you drag cursor to top- or bottom-right corners? Browsing Questions shows the same questions on different pages I have just been looking through the questions to answer those that I can and as I go to the next page (using the Next button at the bottom) I am presented with the same questions.  I imagine this is not a flaw and in fact questions are coming in fast filling up a page pushing my current page to the next one.  An idea to solve would be to set a variable for starting id when I start paging and only clicking an update button will reset the starting id. Cannot add a NOT NULL column with default value NULL in Sqlite3 Many of the Ringbearers (particularly those who carried it longer) used the word "precious" to describe it. Isildur described it as:     "But for my part I will risk no hurt to this thing: of all the works of Sauron the only fair. It is precious to me, though I buy it with great pain."   Bilbo himself used it at least once, when arguing with Gandalf about leaving it behind when he was leaving Bag End (this exchange and the lines were different in the movie, but the word is still there):     "Well, if you want my ring yourself, say so!" cried Bilbo. "But you won’t get it. I won’t give my precious away, I tell you."   And Gollum used it incredibly frequently, almost as if it was the ring's name. It was even his last words:     "Precious, precious, precious!" Gollum cried. "My Precious! O my Precious!"   So my question: Why "Precious"? It wasn't part of the inscription. It wasn't part of the poem that included the inscription. Was it somehow part of Sauron's crafting spell? Did Tolkien ever indicate why the Ringbearers would use that word in particular, other than that it sounded good? How can I prove that the additive group of rationals is not isomorphic to a direct product of two nontrivial groups? I am working through Paolo Aluffi's new GSM text on my own (self-study). On page 63, he asks the reader to     Prove that $\Bbb{Q}$ is not the direct product of two nontrivial groups.   For some context, this is an exercise following a section entitled "The category Grp".  I am assuming that he means "is not isomorphic to the direct product of two nontrivial groups", and I can see two possible ways to proceed with this proof, but have been unsuccessful with either approach.    Approach 1:  Show that the additive group of rationals has a property that is preserved by isomorphism that the direct product of two nontrivial groups does not have or vice-versa.  This seems challenging unless I can significantly narrow down the properties that a direct product of two nontrivial groups that was isomorphic to $\Bbb{Q}$ would necessarily have.  Approach 2:  Considering the section in which this question occurs, show that if $G$ and $H$ are nontrivial groups and $\Bbb{Q} \cong G \times H$, then there are homomorphisms $\varphi_{G}:\Bbb{Q} \rightarrow G$ and $\varphi_{H}:\Bbb{Q} \rightarrow H$ which do not factor or do not factor uniquely through the product $G \times H$.  This would be a contradiction, as $G \times H$ is a final object in the category Grp.  I would greatly appreciate suggestions on how to proceed further with either of these approaches or with alternate approaches. Picking a $\delta$ for a convenient $\varepsilon$? I'm studying some proofs in a book from the library  and  unlike my own book, it doesn't prove by showing that a certain expression is striclty less than $\varepsilon$ (given an  appropiate $\delta$ and $\vert x-a\vert &lt; \delta$, etc). Instead, it shows that it's less than some expression involving $\varepsilon$.   E.g. Given $\varepsilon &gt; 0$, we can pick a $\delta &gt; 0$ so $|f(x) - g(a)| &lt;  \text{insert stuff} &lt; k \cdot \varepsilon$ when $|x-a| &lt; \delta$, and thus $f(x) - g(a) \to 0 $ when $x \rightarrow a$.  How does this make sense formally? (Intuitively, I am convinced.)  I don't have much experience with $\varepsilon$-$\delta$ -proofs, so I might be missing something obvious. I have very little background in physics, so I apologize if this question is painfully naive.  Consider the following thought experiment: an observer is in a closed room whose walls, floor, and ceiling are made entirely of mirrors, with a single light source in the middle of the room. When the light is on, the observer can see many copies of his reflection all over the place.  Suddenly, the light source turns itself off. Intuitively, I would expect the observer to "instantly" see darkness. However, I can't figure out why that is the case under the "particle" interpretation of light. There are obviously lots of photons already in the room from before. Furthermore, we know that they don't get "consumed" when they hit a wall, because otherwise the observer wouldn't see so many reflections of himself. Basically, when the light goes off, what happens to the photons already in the room?  I suspect the answer goes something like this: the photons in the room lose a little bit of energy every time they bounce off a mirror, but it's so minuscule that we can still see more reflections than our eye can resolve anyway. When the light goes off, however, it takes them a very small fraction of a second to bounce around the room enough times to diffuse completely, which our eye cannot detect.  Is that about right? If we had a theoretical "perfect reflector", would the light remain trapped in the room forever? If we had instruments that could measure such things very finely, would it take (slightly) longer for the light to go out in a room made of mirrors as opposed to a room made of, say, black cloth? Aligning equations within multline I have this really long equation and I have used multline to break it up. I also want to align the broken bits of the equation to the left. When I use  align, for some reasons it aligns to the right when I want it to be on the left. How can I align to the left?  \begin{multline} \begin{align} \mathcal{H}_{q1} \mathcal{H}_{q2} \mathcal{H}_{q3}...\mathcal{H}_{qn}0=\frac{1}{2}H_{q1}\{\mathcal{H}_{q2}\mathcal{H}_{q3}...\mathcal{H}_{qn}0\}+\frac{1}{2} h_{q1} \\ =\frac{1}{2}H_{q1}\{\frac{1}{2} H_{q2}[\mathcal{H}_{q3}...\mathcal{H}_{qn} 0]+\frac{1}{2}h_{q2}\} +\frac{1}{2}h_{q1}=\\ \frac{1}{2}H_{q1} \frac{1}{2} H_{q2} [\frac{1}{2}H_{q3}(\mathcal{H}_{q4}..\mathcal{H}_{qn}0)+\frac{1}{2}h_{q3}]+ \frac{1}{2}h_{q2}+\frac{1}{2}h_{q1}=...\\          =(\frac{1}{2^n})H_{q1}H_{q2}H_{q3}..H_{qn}0+ (\frac{1}{2^n})H_{q1}H_{q2}H_{q3}..H_{qn-1}h_{q1}\\+(\frac{1}{2^n})H_{q1}H_{q2}H_{q3}..H_{qn-2}h_{qn-1}+.. \\+ (\frac{1}{2})h_{q1}=\sum^n_{j=1} (\frac{1}{2^j}) H_{qo}H_{q1} H_{q2}H_{q3}..H_{qj-1} h_{qj} \end{align} \end{multline} Make locking posts more granular My laptop which had Windows 7 on it crashed today. Being unable to access any file kept within, I found about the LiveCD. So I created a LiveUSB with Ubuntu in it, and managed to access my Windows desktop. I'm now backing up files important to me, but I have a question:  Now I'm cutting and pasting the files I want to save, using Ubuntu. Are these files being moved in Windows as well, or do the alterations I make through Ubuntu not affect the Windows state?   So let's say,if I cut and paste all of the D drive, and then format C and reboot with Windows, would the files in D have been gone (since they were cut and not copied) or still be there?  Thanks. Is there any built-in software or terminal method allowing me to view the hardware profiles on my system? Windows equivalent of such a feature would be Device Manager. Equation of the locus of the foot of perpendicular from any focus upon any tangent to the ellipse ${x^2\over a^2}+{y^2\over b^2}=1$ Find the equation of the locus of the foot of perpendicular from any focus upon any tangent to the ellipse ${x^2\over a^2}+{y^2\over b^2}=1$. will it also be an ellipse? I'm trying to determine if 1317 and 116 can be written as the sum of three squares?  I have the condition that if it is not of the form $4^{\alpha}(8k+7)$ then it can be written as a sum of three squares, but how do I use this condition to work it out?  Thank you How do deactivate plymouth boot screen? Lately, booting Ubuntu on my desktop has become seriously slow. We're talking two minutes. It used to take 10-20 seconds. Because of plymouth, I can't see what's going on. I would like to deactivate it, but not really uninstall it. What's the quickest way to do that? I'm using Precise, but I suspect a solution for 11.10 would work just as well.