Both give the output of the command, so what is the semantic difference between the two? Some reading led me to suspect is that $(command) is Bash syntax, and the back quotes are integrated into Unix somehow; is there any truth to this?
There are two syntaxes for command substitution: with dollar-parentheses and with backticks. Running top -p $(pidof init) and top -p `pidof init` gives the same output. Are these two ways of doing the same thing, or are there differences?
$\DeclareMathOperator{\tr}{tr}$Let's consider a $n \times n$ matrix and the sequence of traces $\tr(A)=\tr(A^{2})= \ldots = \tr(A^{n})=0$. How to prove that $A$ is a nilpotent matrix (a matrix so that $A^{k} \times u = 0$ for all $u \in V$ and for some $k$)?  Maybe it would be reasonable to consider $A$'s Jordan form (we assume that the action occurs over an algebraically closed field, such as $\mathbb{C}$). For example, suppose that we have found it and $A'$ consists of $m$ blocks of sizes $r_{1}, \ldots r_{m}$ ($r_{1}+r_{2}+\ldots+r_{m}=m, r_{i} \in \mathbb{Z}_{+}$) with $\alpha_{1}, \ldots \alpha_{m}$ as corresponding eigenvalues to each of the blocks. Then, according to the problem will get the system  $$\begin{align} r_{1}\alpha_{1}+&amp;\ldots+r_{m}\alpha_{m}=0 \\ r_{1}{\alpha_{1}}^{2}+&amp;\ldots+r_{m} {\alpha_{m}} ^{2}=0\\ &amp;\ ~\,\vdots \\ r_{1}{\alpha_{1}}^{n}+&amp;\ldots +r_{m}{\alpha_{m}}^{n}=0  \end{align}$$  But the system's analysis over $\mathbb{C}$ seems to be too much complicated.  Actually, the problem must have more or less simple solution.  Any piece of advice would be much appreciated. Ping faster than light I just discovered very strange thing while testing my internet connection. My ping is smaller than it should be. For example ping time to Arizona State University is about 14ms.  eryk@eryk-pc:~$ ping www.asu.edu PING www.asu.edu.cdn.cloudflare.net (104.16.51.14) 56(84) bytes of data. 64 bytes from 104.16.51.14: icmp_seq=1 ttl=60 time=13.8 ms   I'm living in Poznań in Poland so my distance to Arizona State University in a straight line (very optimistic assumption) is about 10000 km. Considering the fact that ping time is a time for two directions (to the target and back to home). So my ping packet have to cross 20000 km distance. Speed of light is 300 000 km per second that is 300km per millisecond. So the smallest possible time to ping Arizona State University with my packet travelling at a speed of light is  20000/300=67ms   I have noticed similar results for servers located in Australia and some other States of America. Is my internet connection five times faster than light? Compute $\int_0^\infty\frac{\cos(xt)}{1+t^2}dt$ Let $x \in \mathbb R$  Find a closed form of $\int_0^\infty\dfrac{\cos(xt)}{1+t^2}dt$ .   Let me give some context: this is an exercise from an improper integrals course for undergraduates. My teacher was not able to give a valid solution without  resorting to Fourier series.   He actually wanted to prove first that the function is a solution of the ODE $$ y-y'' =0$$ without succeeding in proving it.   Anyway the result sould be $\dfrac{\pi}{2}e^{-x}$  Thanks for any help on this subject. Delete all files to which no corresponding file with another extension exists My goal is to find a Linux command sequence, which deletes some files if a special condition is fulfilled:  All files with the extensions .cut or .cut.bak in the current directory shall be removed, IF there is no file having the same name, but extension .rec or .mpg in the current directory.  Background: I am developing a tool for a Linux-based PVR, that allows cutting of recorded programs. For each recording (extension .rec or .mpg) the segment markers are stored in a .cut-file with the same name. When some recording gets moved/renamed/deleted, then the corresponding cut-file remains orphaned and shall be deleted. I already have implemented the removing of those useless cut-files in C. But I am wondering, whether there may be a (simple) system based solution. In this case it could be run via 'system' and &amp; in the background, which would make my application more responsive...  I already asked a similar question for the recursive case (browsing also into subdirectories), . There has been a (nearly) perfect answer by kos - but unfortunately it removes too many files, so it is not usable, and I was not able to figure it out... But the same idea can be used here, IF it is possible to parse and pipe the result of 'ls' linewise... STM32 send from UART3 and receive on UART2 I thought to create a simple test to send and receive a buffer via UART. I succeeded in sending bytes from my PC to the nucleo bord and receiving them in blocking and non-blocking mode. But for some reason I can't succeed to send data from UART3 and receive the bytes on UART2.  I have a nucleo-f103rb and have the following pins setup:   UART2: tx=PA2;rx=PA3 UART3: tx=PB10;rx=PB11   And then the following code:    while (1)   {     /* USER CODE END WHILE */     /* USER CODE BEGIN 3 */     if (HAL_UART_Transmit(&amp;huart3, send, 2, 1000) == HAL_OK)     {         if (HAL_UART_Receive(&amp;huart2, receive, 2, 1000) == HAL_OK)         {             HAL_GPIO_TogglePin(GPIOA, LD2_Pin);         }     }   }   All the boiler plate is generated using cube mx. So the UART 2 and 3 are all default, same for the button pins.    I connected the transmit pin of uart3 to the receiving pin of uart2 I connected the receive pin of uart3 to the transmitting pin of uart2 (while not used in this trivial example though)   What am I missing here?  Update  As comments and answers suggested, I shouldn't be using the blocking mode approach. So I rewrote it slightly to rely more on interrupts.  I attached a callback to the nucleo blue button  void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin) {     if (GPIO_Pin == B1_Pin)     {         HAL_UART_Transmit_IT(&amp;huart3, (uint8_t *)send, 2);     } }   And I added a callback for receiving stuff over uart  void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {   /* Prevent unused argument(s) compilation warning */   UNUSED(huart);    /* NOTE : This function should not be modified, when the callback is needed,             the HAL_UART_RxCpltCallback can be implemented in the user file    */       // Toggle the pin to see some proof that I received something      HAL_GPIO_TogglePin(LD2_GPIO_Port, LD2_Pin);       // Resubscribe uart2      HAL_UART_Receive_IT(&amp;huart2, (uint8_t *)receive, 2); }   In the main function I call the interrupt so that the callback is active.  while (HAL_UART_Receive_IT(&amp;huart2, receive, 2) != HAL_OK);  __NOP(); while (1) { }   Again, when debugging through I do end up in the button released callback, but the callback on the uart receive isn't triggered. When I send stuff from the PC, it does. So it seems that sending stuff via uart3 isn't working. So I guess that must be the wiring.     But I really can't see what I am missing here, the pins are clearly defined in the pinout here: .  I hope the photo is clear enough, but this is how I connected them. Inequality involving the integral of vector valued function Suppose $f :[a,b]\to \mathbb{R}^n$ is given by $f(t)=\langle f_1(t), \ldots, f_n(t)\rangle$, where each of the $n$ component functions is integrable over $[a,b]$. I think the following inequality   $$\left|\int _a^b f(t) \, dt\right|\leq\int_a^b\left|f(t)\right| \, dt$$  still hold but I don't know how to prove it. Any idea?  Thanks What are the recommended textbooks for introductory calculus? I've already taken my calculus sequence and I'm interested in brushing up and staying sharp on the basics. So far, my calculus background is limited to single-variable calculus, which I applied in my physics sequence and engineering statistics. I learned a little bit of vector math in physics as well, but I'm not strong in it. My mathematical strengths lie in the discrete mathematics.  I'm particularly interested in studying for my IEEE Certified Software Development Associate exam - 10% of the exam is mathematics (calculus, differential equations, and statistics). Calculus is one of the mathematics knowledge areas that a software engineer should be competent in, so I'm looking for not only solidifying the courses that I've already taken, but moving forward with slightly more advanced topics, at an undergraduate mathematics level.  I'm currently using the textbook from my calculus sequence, but I'm interested to know what books other people used to learn calculus and if the book is potentially worth checking out.  What books are available to help me? Does the OS X log notifications from the Notification Center? Image of the union is the union of the images $$ f\left(\bigcup\limits_{\lambda \in \wedge} A_{\lambda}\right) = \bigcup\limits_{\lambda \in \wedge} f(A_\lambda)$$  Let $b \in f(\bigcup\limits_{\lambda \in \wedge} A_{\lambda})$ $\rightarrow b=f(a)$ for some $a \in (\bigcup\limits_{\lambda \in \wedge} A_{\lambda})$.  Since $a \in (\bigcup\limits_{\lambda \in \wedge} A_{\lambda}) \rightarrow a \in A_{\lambda}$ for some $\lambda \in \wedge$.  Since $b=f(a)$, then $b \in f(A_{\lambda})$ for some $\lambda \in \wedge$.  $\rightarrow b \in \bigcup\limits_{\lambda \in \wedge} f(A_\lambda)$ $\rightarrow f(\bigcup\limits_{\lambda \in \wedge} A_{\lambda}) \subset \bigcup\limits_{\lambda \in \wedge} f(A_\lambda)$.  I used a similar argument to prove $\bigcup\limits_{\lambda \in \wedge} f(A_\lambda) \subset f(\bigcup\limits_{\lambda \in \wedge} A_{\lambda})$.  Which shows $ f(\bigcup\limits_{\lambda \in \wedge} A_{\lambda}) = \bigcup\limits_{\lambda \in \wedge} f(A_\lambda)$.  Please let me know if this is valid. In other words, why don't we always use Schottky diodes if they are so much better? What diode properties do Schottky diodes have that makes them unfit for certain applications? I just want to know if it's possible to get Cain and abel installed in ubuntu, it would really help my work instead of dual booting back to my windows 7 just to use it. THanks! Don't let the association bonus be given away as bounties When a user with at least 200 rep on any Stack Exchange site registers a new account on another SE site, they immediately receive 100 rep on that site as an .  This is a deliberate feature to give users who are already familiar with how the SE platform and community works a bit of an extra boost (e.g. letting them upvote and comment immediately), and it's pretty much working as intended. What's probably not intended is that, currently, this bonus rep can be given away as . In fact, because awarding bounties currently requires only 75 rep, any user with 200+ rep on at least one site can join another site, receive the 100 rep bonus, and immediately award it all as a bounty to another user, without having to earn any actual rep on the site at all.  Not only does this seem contrary to the design intent of bounties (and of the association bonus), but it also seems ripe for potential abuse. (In fact, the only thing stopping a sockpuppet ring from gaining unlimited rep this way is the fact that the bounties would likely be sooner or later noticed by mods.) It's also possible for experienced users to (more or less) accidentally bounty their reputation so low that that they lose access to basic site features that they'd normally have due to the bonus.  Arguably, of course, this is entirely the user's own fault, but it can still be .  Also, this behavior , which currently require 110 rep to answer for users with the association bonus. The fix I'd like to suggest is simple:  If a user has been awarded the association bonus, don't let them award a bounty that would take their rep below 100.  Of course, it would still be possible for users with the bonus to drop below 100 rep via other means, e.g. by having their posts downvoted.  But they wouldn't be able to just give away this baseline rep to other users any more. How do you clone an Array of Objects in Javascript? Why does my query run fast in Environment A, but slow in Environment B? I have a piece of SQL that seems to run really fast in Environment A, but the exact same query runs really slow in Environment B!  Environments are supposed to be the same, so what should I do and/or where should I look in order to see why the query doesn't perform the same? $\lim_{x \to 0} f(x)=\frac{\tan 3x}{\tan 2x}$ without L'Hospital I have a hazy notion of some stuff in differential geometry and a better, but still not quite rigorous understanding of basics of differential topology.  I have decided to fix this lacuna once for all. Unfortunately I cannot attend a course right now. I must teach myself all the stuff by reading books.  Towards this purpose I want to know what are the most important basic theorems in differential geometry and differential topology. For a start, for differential topology, I think I must read Stokes' theorem and de Rham theorem with complete proofs.   Differential geometry is a bit more difficult. What is a connection? Which notion should I use? I want to know about parallel transport and holonomy. What are the most important and basic theorems here? Are there concise books which can teach me the stuff faster than the voluminous Spivak books?  Also finally I want to read into some algebraic geometry and Hodge/Kähler stuff.  Suggestions about important theorems and concepts to learn, and book references, will be most helpful. Amy: What day is it today?    Tom: It's Monday. Hmm... I like Monday.   Amy: What do you have today?   Tom: We have math, science and art.What about you?   Amy: We have English and P.E. What do you have tomorrow?    Tom: We have art and P.E.tomorrow.    Amy: Oh, great.      I can understand that "What about you" means "What do you have today?"  But I wonder whether "What about you" is an elliptical sentence. If it is, what is the full sentence? If it isn't, how to parse this structure? How can I add a PPA from a previous release? Why do I get 'UnknownHostException: session.minecraft.net' when attempting to log in to Minecraft servers? Direct Link to a Comment