Why do we double-quote a dollar sign evaluation in Bash?
When is double-quoting necessary?
Why is "union" an exception to the "a/an" rule? The following is the first part of Proposition 2.9 in "Introduction to Commutative Algebra" by Atiyah &amp; Macdonald.     Let $A$ be a commutative ring with $1$. Let $$M' \overset{u}{\longrightarrow}M\overset{v}{\longrightarrow}M''\longrightarrow  0\tag{1} $$ be sequence of $A$-modules and homomorphisms. Then the sequence (1) is exact if and only if for all $A$-modules $N$, the   sequence  $$0\longrightarrow \operatorname{Hom}(M'', N)  \overset{\overline{v}}{\longrightarrow}\operatorname{Hom}(M,  N)\overset{\overline{u}}{\longrightarrow}\operatorname{Hom}(M', N)  \tag{2} $$ is exact.   Here, $\overline{v}$ is the map defined by $\overline{v}(f)=f\circ v$ for every $f\in\operatorname{Hom}(M'', N)$ and $\overline{u}$ is defined likewise.   The proof one of direction, namely $(2)\Rightarrow (1)$ is given in the book, which I am having some trouble understanding. So assuming (2) is exact sequence, the authors remark that "since $\overline{v}$ is injective for all $N$, it follows that $v$ is surjective". Could someone explain why this follows?  Given that $\overline{v}$ is injective, we know that whenever $f(v(x))=g(v(x))$ for all $x\in M$, we have $f=g$. I am not sure how we conclude from this surjectivity of $v$.  Thanks! Terminal only shows up as a black window Problem about the definition of Euclidean domain I encountered the integral $$\int_0^1 \frac{\log \left(1-x^2\right) \sin ^{-1}(x)^2}{x^2} \, dx = -0.9393323982...$$ while researching the evaluation of harmonic sums. Mathematica 11 is not able to evaluate this integral, and is not able to evaluate the underlying indefinite integral.  I considered using the Maclaurin series for the expression $\sin ^{-1}(x)^2$ to evaluate this integral. Using this Maclaurin series, it is easily seen that the problem of evaluating the above integral is equivalent to the problem of evaluating the series $$\sum _{n=1}^{\infty } \frac{2^{2 n-1} H_{n-\frac{1}{2}}}{(1-2 n) n^2 \binom{2 n}{n}}=-0.9393323982...$$ which Mathematica 11 is not able to evaluate directly.  I also considered using the Maclaurin series for the expression $\log \left(1-x^2\right)$ to evaluate the above integral. Using this Macluarin series, it is easily seen that this integral is equal to $$\frac{1}{4} \left(16 \pi  C-\pi ^3-\pi ^2 \log (4)\right)+\sum _{n=1}^{\infty } \frac{\, _3F_2\left(\frac{1}{2},\frac{1}{2},1;\frac{3}{2},n+1;1\right)}{n^2-2 n^3}=-0.939332...,$$ but Mathematica is unable to evaluate the above infinite series.  What is a closed-form evaluation of $\int_0^1 \frac{\log \left(1-x^2\right) \sin ^{-1}(x)^2}{x^2} \, dx$? What techniques can be applied to evaluate this definite integral? How can I resize an ext root partition at runtime? How can I increase the size of the root partition of a system at runtime?   I have a partition that is not allocated after the root partition (which is also ext4), how can I add that unallocated space to the space allocated to the root partition without having to shutdown the server? running a bash file states:date: command not found I've installed a version of Ubuntu Server and although I'm fairly still fresh with Ubuntu, I'm trying to create a bash file to run a program in the background. More importantly, I'm trying to get a file nameto display correct and create it before I start running the program (I like my debug stuff).  However, here is my bash script:    #!/bin/bash  PATH="/var/log/servers/Game/"  FILENAME="startup"  EXT=".log"  DATE=$(date +%d-%m-%Y)  NEWFILE=${PATH}${FILENAME}_${DATE}${EXT}   I have verified that the file does exist in the PATH directory. However, the issue is with the date command. When I run the script bash -x game.sh this is what is displays.   + PATH=/var/log/servers/Game/  + FILENAME=startup  + EXT=.log  ++ date +%d-%m-%Y  game.sh: line 5: date: command not found  + DATE=  + NEWFILE=/var/log/servers/Game/startup_.log  + echo /var/log/servers/Game/startup_.log   I don't understand why it would say the command doesn't work, when I can go to the main console and run date and get a valid response.  Any help is greatly appreciated! is it safe to keep database connections open for long time I have a .net client application which is connected to a remote database. Is it safe to keep a single connection open for the lifetime of the client (hours)? Does the answer hold if I have multiple (10 or 100) clients running? The biggest peeve is sometimes I vote up an answer mid-way through reading it, and the "don't forget to accept" message blocks the remaining text, so I scramble and triple-click it until it closes.  Anyways I frequently scan my question list and search for SO's reminders, ("Consider accepting an answer") so the "don't forget to accept" is wholly unnecessary to some, and there should be a way to disable it. Do you think so too? What is The Rule of Three? How can I draw a horizontal line spanning only some of the table cells? In a table, how can I draw a horizontal line that goes only through some of the cells? I mean a line like the one in the Foo and in the Bar area of this table:  +----+-----+ |Foo |1 |2 | |    |1 |2 | |    +--+--+ |    |1 |2 | |    |1 |2 | +----+-----+ |Bar |1 |2 | |    |1 |2 | |    +--+--+ |    |1 |2 | |    |1 |2 | +----+--+--+ Free way to clone HDD to SSD? Why is gksu no longer installed by default? I have always recommended pressing ALT+F2 and typing for example gksu nautilus or gksudo nautilus both are equivalent; to run graphical programs with root privileges.  However, I have just installed raring on a test system and this no longer works.  gksu and gksudo are not installed as part of a default raring install; though they are in the repositories. In 12.04 at least gksudo was just a symbolic link to gksu  The reasons I have always done this are because while it is often safe to use sudo under some circumstances it can cause problems with graphical applications because gksu has you running the program as root while with sudo you are running as yourself but with elevated privileges.  This can mean you inadvertently change the owner of critical system files.  This can cause problems.  This reasoning is explained better here:         Why was command removed from a fresh install and what should we be using instead? Ppening an HTML file on localhost in XAMMP/apache 2.2 using subdirectories under htdocs I have a site directory for html (dreamweaver CSS3) with lots of subdirectories that contain HTML files for a number of drop down menus. They work fine where they are, but they all have menus that are JavaScript and could be kept in a separate file. So, I want to use Include. I don't want to have to FTP my files to HostGator every time I want to test the includes. I want to have this on my local machine. So, I am attempting for the first time to setup XAMMP and then run my html files (site) on localhost, so I can see what they look like before ftping everything to HostGator and putting it live.  I tried copying the main site folder to htdocs, and then copying the path to the file I want to run in front of the localhost in Firefox, but it gave me file not found:   Do I have too many subdirectories? Why isn't the file being found? It is there.  Tried this too:   No go. How does a non-mathematician go about publishing a proof in a way that ensures it to be up to the mathematical community's standards? I'm a computer science student who is a maths hobbyist. I'm convinced that I've proven a major conjecture. The problem lies in that I've never published anything before and am not a mathematician by profession. Knowing full well that my proof may be fallacious, erroneous, or simply lacking mathematical formality, what advice would you give me? How do I split a string, breaking at a particular character? I have this string  'john smith~123 Street~Apt 4~New York~NY~12345'   Using JavaScript, what is the fastest way to parse this into  var name = "john smith"; var street= "123 Street"; //etc... The website I'm working is allegedly going to have a massive hit rate soon after launch. The client is talking about the possibility  of around 2500 hits per second over a day or so.   Ignoring the fact that this hit rate is probably wild client optimism and apart from getting the largest servers possible, what is the best way that Drupal should be configured to support a large hit rate.  I've read Scaling the , ,  and many other pages, but what I'm looking for is real experience of doing this, what works, what doesn't, and what to expect. SF Short Story about resetting the world every day Trying to identify a short story. I recall it was quite short, shorter than a novella. I read it within the last few years, but my impression is that it's from the second half of the twentieth century, maybe the sixties. I can't recall if I read it online, in an anthology, or collection. Set in America, a middle-aged man stumbles on a team (maybe like dwarves or gnomes) whose job it is to set up the world for the next day by removing all the previous day's &quot;stuff&quot;, like street furniture? (I know that makes no sense). He disrupts this process and somehow prevents progressing to the next day. I also remember the story was quite lighthearted with the team behaving in a very grumpy way like interrupted workmen. I thought the title was something like &quot;What happened to Tuesday&quot; or similar. For some reason, I thought it was by Keith Laumer, but a search doesn't reveal anything. The trope was similar to &quot;The Tunnel Under the World&quot; by Pohl, and &quot;The Adjustment Team&quot; by Dick, in that reality was constantly changed and controlled by an organisation of some kind. What could cause exposure problems after my camera got wet? I had an unfortunate accident where my DSLR camera (and me) briefly ended up in a river. The camera wasn't fully submerged but it did get very wet. It still works, sort of, although the electrics are doing some random things. I'm hoping that's just a bit of dampness doing that, that will fix itself when it dries out fully.  More concerning is that the documents exposure metering seems to be all wrong when using the viewfinder. Pictures are extremely overexposed and nearly white. But if I use live view, then they are fine.  Why could cause this? I assume the light is routed a different way when using the viewfinder and maybe it is going through something with water/condensation on it? Is it correct to say "I write children books" (not possessive case)? Although Children's books is what everybody says, I would like to understand why the genitive case is applied in such case.  If I write books for children, children is an adjective here; not the owners of my book! The word "children" just defines or characterizes the type of books I write. Therefore, it's an adjective.   So, I understand that genitive/possessive case ("I write children's book")  is incorrect grammar.  My question is: is the genitive case here really accepted as right? If I use "I write children books" (following the grammar principle) as as I say "I write pets books" (books about pets, and not possessions of pets) - would I be incorrect? Why?