Accidentally in /var/ directory I ran chown root . -R;chgrp root . -R. Now services like mysql don't start. Is there a way to restore default owner and groups?  If there is no way, what is the default owner and group of /var/lib/mysql directory and it's files?
I accidentally changed /var owner/group to my username and then changed it back to root, but not all the /var folders' owners are root, so is there anyway to change back owner/group of files/folders to default state?  Or at least those files/folders that are created by packages?
How do I derive the Lorentz contraction from the invariant interval? Reviewing some basic special relativity, and I stumbled upon this problem:  From the definition of the proper time: $$c^2d\tau^2=c^2dt^2-dx^2$$ I was able to derive the time dilation formula by using $x=vt$: $$c^2d\tau^2=c^2dt^2-v^2dt^2=c^2dt^2\left(1-\frac{v^2}{c^2}\right)\rightarrow d\tau = dt\sqrt{1-\frac{v^2}{c^2}}=t/\gamma$$  Now, I would very much like to be able to derive the length contraction formula in a similar fashion, and feel strongly that this should be possible. The definition of the invariant interval is: $$ds^2=dx^2-c^2dt^2$$ using $t=\frac{x}{v}$ I tried: $$ds^2=dx^2-\frac{c^2}{v^2}dx^2=dx^2\left(1-\frac{c^2}{v^2}\right)\rightarrow ds=dx\sqrt{1-\frac{c^2}{v^2}}$$  This is where I'm stuck: I don't see how this can be converted to a Lorentz factor...  Any help that will allow me to arrive at the desired result $ds=\gamma dx$ would be very much appreciated. How to input open intervals I want to write open and half-open intervals using the following notation:  ]a, b[ ]a, b] ]-∞, b]   When writing them just like that in the LaTeX source, the spacing doesn't come out right. For example, for this equation:   \(X = ]-\frac{π}{2}, +\frac{π}{2}[\)   the minus sign is typeset as a binary operator. I can solve this by surrounding the whole value in braces, but then the spacing around the equal sign is still not correct. For other combinations of intervals and operators, different spacing inconsistencies arise.  What is the correct and easy  way to input intervals in order to avoid having to take care of this at all? Repairing corrupt MP4 I took a video from my Android phone (Sony Xperia P, if you need to know), and before I could stop recording it, the phone memory got full and the phone got hanged too. I rebooted the phone, the video was there, but I couldn't play it from any media player on Android. It's in mp4 format.    So I tried to open it on PC and Mac with different video player, like VLC, quick Time, Windows Media Player etc., but none of them can play it. I know that the video is corrupted, but I believe it's repairable, since it got more than a GB in size. Is there anything that I can try  to repair the video file? The video is really important. Why are accepted answers not included in the tag badge calculation? For the 400 votes tag badge, does it take into consideration answers marked as the accepted answer?  The description indicates it does not.  I would think an accepted answer would be worth more, or at least the same amount, towards getting a tag badge than just a simple upvote?  Lots of users mark answers as accepted and do not bother giving an upvote since the 15 points is like and upvote and a tip.  So it's possible for a user to have 400 accepted answers with no upvotes and not get a badge where a user with 400 answers (which could be right or wrong) with a single upvote and no accepted answers ever would get the badge.  Just seems a little odd or does accounting for accepted answers make the badge calculation too complicated / slow?  Edit: Questions you ask and submit an answer to shouldn't be counted towards the total for accepted answers.  Edit: It just seems to me that the most important thing in the system is an accepted answer yet there are no badges for achieving more than one accepted answer.  If up votes were more important, and therefore far more worthy of a badge, then why do the top voted answers not appear above the accepted answer?  When it come to badges, accepted answers are second class citizens as compared to a possibly random upvote.   I have seen many upvotes on completely wrong answers but rarely do I see an accepted answer that was completely wrong. How to capture all std outputs What is covered by CDIC savings deposit insurance? In the unlikely event that a financial institution fails in Canada, what is covered by the Canadian Deposit Insurance Corporation?  What is the limit of coverage? Cannot locate Ubuntu Software Center via Firefox to open APT links I'm trying to locate where the Ubuntu Software Center is to choose as default for handling APT links in Firefox.  I can click on the links, and I get the pop-up, but Ubuntu Software Center is not there.  I tried to choose an application through Firefox's settings and through the pop-up, but I don't know the path.  As you can see by my screen shot for the pop-up I chose the wrong one as that doesn't work.  What is the correct path I need to select the correct file?  EDIT - I added a fourth screen shot.  I don't have it in that location.  This is what I have.  My view is in ABC order, and it's showing hidden files. How to know if Google Sheets IMPORTDATA, IMPORTFEED, IMPORTHTML or IMPORTXML functions are able to get data from a resource hosted on a website? How do you calculate this limit $\lim_{n\to\infty}\sum_{k=1}^{n} \frac{k}{n^2+k^2}$? How to find the value of $\lim_{n\to\infty}S(n)$, where $S(n)$ is given by $$S(n)=\displaystyle\sum_{k=1}^{n} \dfrac{k}{n^2+k^2}$$   is unable to calculate it.  This is a question from a questions booklet, and the options for the answer are--  $\begin{align} &amp;A) \dfrac{\pi}{2} \\ &amp;B) \log 2 \\ &amp;C) \dfrac{\pi}{4} \\ &amp;D) \dfrac{1}{2} \log 2 \end{align}$ Prove that a subset of a separable set is itself separable Can the Force be used to fly? File permissions and saving Let's say you open a file on which you have write permission. Meanwhile you change permissions and remove write permission while you still have the file open in some editor.  What will happen if you edit and save it? What is the remainder of $\,3^{3^{3^{\:\!\phantom{}^{.^{.^.}}}}}\!\!\!$ divided by $46$?  The level of powers is $2020$.  First there is no parenthesis so it means 3 power of 3 which is also power 3 and so on 2020 times  Second I think that we can use Fermat's Little Theorem but I don't know how, and maybe there is a better way. How to add click event to an element? I am trying to understand how to see the total size of my hard drive &amp; how much is left. It was recommended on various searches to use the command below   df -h Filesystem      Size  Used Avail Use% Mounted on udev            7.8G     0  7.8G   0% /dev tmpfs           1.6G  9.6M  1.6G   1% /run /dev/sda1       219G   29G  180G  14% / tmpfs           7.8G   18M  7.8G   1% /dev/shm tmpfs           5.0M  4.0K  5.0M   1% /run/lock tmpfs           7.8G     0  7.8G   0% /sys/fs/cgroup cgmfs           100K     0  100K   0% /run/cgmanager/fs tmpfs           1.6G   64K  1.6G   1% /run/user/1000   However I don't understand any of these directories. I mean what are they, are these all directories or partitions or what? And why do they appear under "mounted on"? How does that bit work. I assume the /dev/sda1 is my solid state drive? So now if I install applications, which one of these directories does my software get installed  into? I assume it just gets installs in the / directory (I haven't even got a clue what the / directory is). Can someone explain what the other directories are and what they are used for? Is there anyway to find and remove ligatures from tex file? Sometimes when I copy/paste a text to Winedt, it shows perfect, but when I compile it to Pdf, some issues appear, e.g. finding becomes nding.  I am wondering if there is any way to search and find these characters?  Here is an example file (needs utf8 encoding).  \documentclass{article} \begin{document} ﬁ \end{document} I have a problem with the endfloat package and unicode characters.  When I compile the following, everything works as expected:  \documentclass{article} \usepackage[utf8]{inputenc} \usepackage[T1]{fontenc} \usepackage{graphicx}  \begin{document}  \begin{figure} \caption{Ö} \end{figure}  \end{document}   However, when I also load the endfloat package (\usepackage{endfloat}), I get the following error message:  ! Package inputenc Error: Unicode char \u8:ÖÖ not set up for use with LaTeX.   This could be a bug, but my suspicion is that I'm making a mistake somewhere.  I'm hardly the first person to use special characters with the endfloat package.  However, the above example is so simple, I don't see where I could have made a mistake.  Any advice is appreciated.  Edit: The file is of course encoded using utf-8. Let $p(x)$ be a polynomial of degree $n$ satisfying $p(x)\geq 0$ for all $x$. That is, for all $x$, $p(x) = a_n x^n + a_{n-1} x^{n-1} + \cdots + a_1 x + a_0 \geq 0$, $a_n\neq 0$.  Show that $p(x)+p&#39;(x)+p&#39;&#39;(x)+\cdots+p^{(n)}(x)\geq 0$ for all $x$ where $p^{(i)}(x)$ is the $i^\text{th}$  derivative.  My interest: I know that, we can rewrite the LDE as follows  $$ p(x)+p&#39;(x)+p&#39;&#39;(x)+\cdots+p^{(n)}(x) = Lp(x) $$  where $L := I + D + D^2  + \cdots + D^{(n)}$. Can we say anything about a linear operator of this kind so that it does not change the sign of the input it takes? I can try to solve the question by writing for all the derivatives and factoring them using $p(x)$, but I think there should be a clever way of showing this by the properties of $L$. Can I figure out a solution by just looking at $L$ and the sign of $p(x)$ as in the question? Where should I look for that? How to remove blank lines from a file (including tab and spaces)? I want to remove all empty lines from a file. Even if the line contains spaces or tabs it should also be removed. Showing that two definitions of $\limsup$ are equivalent