Note: This applies to Centos 7. If you are looking for a Debian answer, see . Those answers will not be duplicated here.  After an install of centos 7, I can't access man pages :  # man ls -bash: man: command not found   I tried to install it via yum   # yum install man-pages ... ok   But again:   # man ls -bash: man: command not found   Why?
On Debian systems (and derivatives):  $ dpkg --search /bin/ls coreutils: /bin/ls   That is, the file /bin/ls belongs to the Debian package named coreutils.  But this only works if the package is installed. What if it's not?
How can I recover photo files from a memory card with a corrupt filesystem? I know this is not specifically a photography issue but since many of us with prosumer/professional-level DLSRs use Compact Flash cards, it is bound to happen to others as well.  I have a Sandisk 16GB Ultra card that occasionally had the camera reporting inability to access the card. I had to reinsert the card to continue using it. I was able to read the card and transfer the photo files to my computer drive. Today, the same thing happened again, just that nearing the capacity of the card, it failed one last time and the camera was subsequently unable to read it anymore. Despite reinsertion.  My other CF cards never once exhibited this problem, so it's pretty much isolated to this card.  The card is similarly unrecognisable on my computer CF card reader. I think the filesystem is toast, but I suspect the photo files are still intact. I would like to find ways where I can inspect the raw binary data in the card and hopefully recover the photos? Can linear momentum be conserved before and after collision in the presence of an external force? I'm a physics tutor for 11th and 12th grade students. I taught my students that momentum is conserved only when the net external force on the system is zero. But in the books we are following, momentum was conserved in the following cases in spite of presence of an external force:   A body (mass m1) is dropped from a tower (height h) and another body (mass m2) is projected vertically upward with initial velocity u. Find the velocities just after collision. (Gravitational force is the external force) Two bodies projected towards each other on a rough table surface, with all required parameters given asked to find the velocities after collision. (frictional force is the external force) Body explodes in mid air. Momentum is conserved before and after explosion. (Again gravitational force is the external force)   I explained to my students in all these cases though an external force is present it has negligible effect. But I'm not satisfied with my answer.  EDIT: Thank you for the comments and answer. It is clear that contribution of external force to change in momentum is negligible. Can someone do the calculation of taking change in momentum into consideration and showing that it is negligible? What is the difference between public, protected, package-private and private in Java? Install Ubuntu, you can not continue I'm looking into a restful design and would like to use the HTTP methods (POST, GET, ...) and HTTP headers as much as possible. I already found out that the HTTP methods PUT and DELETE are not supported from the browser.  Now I'm looking to get different representations of the same resource and would like to do this by changing the Accept header of the request. Depending on this Accept header, the server can serve a different view on the same resource.  Problem is that I didn't find a way to tell my browser to change this header.   The &lt;a..&gt; tag has a , that can have a mime type, looked like a good candidate but the header was still the browser default (in Firefox it can be changed in about:config with the network.http.accept.default key). I have HTML code like this :  &lt;div&gt;        &lt;a&gt;Link A1&lt;/a&gt;        &lt;a&gt;Link A2&lt;/a&gt;        &lt;a&gt;Link A3&lt;/a&gt; &lt;/div&gt;  &lt;div&gt;        &lt;a&gt;Link B1&lt;/a&gt;        &lt;a&gt;Link B2&lt;/a&gt;        &lt;a&gt;Link B3&lt;/a&gt; &lt;/div&gt;   When user clicks a link from above HTML, I want to get the jQuery object of the corresponding &lt;a&gt; element, and then manipulate its sibling. I can't think of any way other than creating an ID for each &lt;a&gt; element, and passing that ID to an onclick event handler. I really don't want to use IDs.   Any suggestions? Insert distinct data into Combobox List I have 2 storage devices; classical slow HDD (750GB, /dev/sda) and faster SSD (128GB, /dev/sdb). Currently I have installed Ubuntu &amp; Mint on same btrfs partition on SSD (/dev/sdb5). My btrfs pool consists of /dev/sdb5.  What I want to achieve is data replication on HDD while keeping SSD performance: Something like this:  For read case it would be ideal for procedure to go like this:   read from SSD check checksum if it verifies.. awesome, give me the data if not, fetch data from HDD and perform error correction through duplicate data   I'd manually run (some tool I'm not sure which one... btrfs check --repair perhaps?) every month or so to check HDD checksums and correct them via SSD data against bitrot.  For write case:   write to SSD sync to HDD when write requirements are low, thus not slowing down the system.   Is this possible and how would I do it?  Bottom line is, I like btrfs and would like it to duplicate data across HDD &amp; SSD drive automatically while keeping SSD excellent performance.   Using SSD as write cache is not an option since it's missing data duplication and security. This is about that old chestnut, $\newcommand{\d}{\mathrm{d}} \int \sin^2 x\,\d x$.   OK, I know that ordinarily you're supposed to use the identity $\sin^2 x = (1 - \cos 2x)/2$ and integrating that is easy. But just for the heck of it, I tried using the $u$-$v$ substitution method (otherwise known as integration by parts).  $$ \int \sin^2 x\,\d x = \int \sin x \sin x\,\d x $$  We can say $u=\sin x$ and $\d u=\cos x\,\d x$ while $\d v = \sin x\,\d x$ and $v = -\cos x$. When we put it all together:   $$\int \sin^2 x\,\d x = u v - \int v\,\d u = -\sin x \cos x - \int -\cos^2 x\,\d x$$  and doing the same routine with $\cos^2 x$ we get $u = \cos x$, $\d u = -\sin x\,\d x$, $\d v = \cos x\,\d x $ and $v = -\sin x$, leading to:  $$\begin{align}   \int \sin^2 x\,\d x = uv - \int v\,\d u     &amp;= -\sin x \cos x - (-\cos x \sin x - \int -\sin^2 x\,\d x) \\     &amp;= -\sin x \cos x + \sin x \cos x - \int \sin^2 x\,\d x \end{align}$$  which eventually works out to  $$2\int \sin^2 x\,\d x = 0$$  So I wanted to get an idea why this didn't work. Maybe it's higher math and the why will be beyond me (I would think that might be the case), or maybe it's one of those proofs that looks absurdly simple when shown that I am just unaware of. Why use "the" for oceans/seas/rivers etc. but not lakes? Possibly two questions in here: Are these sentence constructions logical, and if they are, why are they different?     I swam across the Ocmulgee River.   I swam across the Pacific Ocean.   I swam across the Red Sea.   I swam across Lake Winnipeg.   I can't think of a way of phrasing the lake example while using an article with a proper noun. I thought at first it was because Lake Winnipeg is different; it's generic portion (the "Lake") comes first and the name of that body comes second, but I swam across Falls Lake is the same.   Yet the sentence I swam across the lake sounds right to me. ...Why? Libraries module. Where to extract? In my book about Logic, which is called 'Language, Proof and Logic', by the way, there is explained that the conditional $ P \rightarrow Q $ is equivalent with $\neg P \lor Q$.   that gives as answer: 'just compare the truth tables and you can see that they are equivalent'. However, I would like to know how to prove this using formal propositional logic. This should be possible, right? My textbook states that:  ...The capacitance $C$ depends only on the geometrical configuration (shape, size, separation)of the system of two conductors. [As we shall see, later, it also depends on the nature of the insulator (dielectric) separating the two conductors.]...  Now I wanted to know if these were the only two factors on which the capacitance of a conductor depends. So I tried thinking about it from the basic definition, which is:  Capacitance is the ratio of the change in electric charge of a system to the corresponding change in its electric potential. (Source: )  i.e., $$C= \frac {dQ}{dV}$$ where, $V$ is the potential of the conductor with respect to zero potential at infinity. Now, I can not think of why $C = f(V)$ isn't a possible scenario, assuming that shape size, etc remains constant. So:  Why cannot $C$ be a function of $V$? how to sort by the day of the week? I have two files.  file1:  Dave 734.838.9800   Bob 313.123.4567   Carol 248.344.5576   Mary 313.449.1390   Ted 248.496.2204   Alice 616.556.4458      file2:  Bob Tuesday   Carol Monday   Ted Sunday    Alice Wednesday   Dave Thursday     Mary Saturday     I merged the two files.  file3 should look like this:  Name      On-Call     Phone   Carol     MONDAY      248.344.5576   Bob       TUESDAY     313.123.4567   Alice     WEDNESDAY   616.556.4458   Dave      THURSDAY    734.838.9800   Nobody    FRIDAY      634.296.3356   Mary      SATURDAY    313.449.1390   Ted       SUNDAY      248.496.2204     But I cannot get the weekdays to be in order. How do I go about doing that? How do I disable a bottom panel with the list of open windows on GNOME 3? I want a green for "correct answer" and a red for "wrong answer". My current choice is rgb(144, 238, 144) for the green and rgb(250, 128, 114) for the red. (See wikilearner.net/wiki for the colours in action.)  I have been told that these colors are very close in contrast, making it difficult for a person with red-green color-blindness. What would be a better choice? App keeps crashing & rendering error since update to 1.0.68 I'm stuck with the proof of the following:  Suppose that $(s_n)$ converges to $s$, $(t_n)$ converges to $t$, and $s_n \leq t_n \: \forall \: n$. Prove that $s \leq t$.   I've tried starting with $s_n \leq t_n \: \forall : n$ and the definitions of each limit (i.e. $|s_n - s| \leq \epsilon \: \forall \: n &gt; N_1$), but I'm not really getting very far. Any help is appreciated! Any way to control light halos in the compositor based on emission level? I am wanting to animate a row of LED lights. I have separated the LED lights from the rest of my object with the pass index/ID mask functions; currently all of my LED lights are on pass index #1.  What I want is to have a minor halo effect around the lights, but only when they are active. Currently, when I add a halo effect to my pass index #1, all of the LED lights have a halo effect regardless if the LEDs are emitting light. All LEDs are diffuse and emission shaders, so I can animate by controlling the mix amount between diffuse and emission.   Is there any simple way to have the halo effect created through recognizing light emission values, or will I have to just animate each LED as its own layer of compositing? How exactly using VB6 can I can call any Windows shell command as you would from the command-line?  For example, something as trivial as:  echo foo