I run the command: ll /dev/null and got this output:  crw-rw-rw- 1 root root 1, 3 Feb 19 10:20 /dev/null   I know d means directory. Can someone please explain what the c special flag means?
I've never really got how chmod worked up until today. I followed a tutorial that explained a big deal to me.   For example, I've read that you've got three different permission groups:   owner (u) group (g) everyone (o)   Based on these three groups, I now know that:   If the file is owned by the user, the user permissions determine the access. If the group of the file is the same as the user's group, the group permission determine the access. If the user is not the file owner, and is not in the group, then the other permission is used.   I've also learned that you've got the following permissions:   read (r) write (w) execute (x)   I created a directory to test my newly acquired knowledge:  mkdir test   Then I did some tests:  chmod u+rwx test/ # drwx------ chmod g+rx test/ # drwxr-x--- chmod u-x test/ # drw-r-x---   After fooling around for some time I think I finally got the hang of chmod and the way you set permission using this command.    But...  I still have a few questions:   What does the d at the start stand for? What's the name and use of the containing slot and what other values can it hold? How can I set and unset it? What is the value for this d? (As you only have 7=4+2+1 7=4+2+1 7=4+2+1) Why do people sometimes use 0777 instead of 777 to set their permissions?   But as I shouldn't be asking multiple questions, I'll try to ask it in one question.  In UNIX based system such as all Linux distributions, concerning the permissions, what does the first part (d) stand for and what's the use for this part of the permissions?
Why must an English learners should know Grammar first? Why must everyone understand Grammar first before learning to speak? I am trying to find the formula for the number $f(n,k)$ which is defined as the number of $k$-subsets of $[n]$ in which no two elements are consecutive numbers in $[n]$.  From the simple thinking I had made up following recursion:  $$f(n,k) = f(n-1,k) + f(n-2, k-1) $$  This is true since I can construct without last element of $[n]$ then make up $k$-subsets, then otherwise must contain the last element of $[n]$ then construct remaining $k-1$ elements among $[n-2]$ since with the last one, I can't use $n-1$.  However, I had never learned any techniques regarding changing those recursion into a well-summarized formula in terms of $n$ and $k$.  Any guidance would be appreciate. In what cases I should skip 's in possessive case? Solving the Kolmogorov forward equation for transition probabilities Let $\lambda \mu &gt; 0$ and let $X$ be a Markov chain on $\{1,2\}$ with generators  $$ Q = \begin{pmatrix} -\mu &amp; \mu \\ \lambda &amp; -\lambda \end{pmatrix}$$  Write down the forward equations and solve them fro the transition probabilities, $p_{i,j}(t), i,j = 1, 2$.  The answers say:  "We have that   $$p_{11}' = -\mu p_{11} + \lambda p_{12}, \, \, \, p_{22}' = -\lambda p_{22} + \mu p_{21}$$  where $p_{12} = 1 - p_{11}$ and $p_{21} = 1 - p_{22}$. Solve these subject to $p_{ij}(t) = \delta_{ij}$, the Kronecker delta, to obtain that the matrix $P_t = \{p_{ij}(t)\}$."  The bits I don't understand are these  1) How do they get those equations in the first place? For example, why have they added $\lambda p_{12}$ when the coefficient for $p_{12} = \mu$?  2) How do you solve these equations? What do you do with that Kronecker delta to solve them? Random operators Let $(\Omega, \mathcal F,P)$ be a probability spaces and $H$ be a Hilbert space. By a random operator $A$ from $H$ to $H$ we mean a linear continuous mapping from $H$ into the Frechet space $L_0^H (\Omega, \mathcal F,P)$ of all $H$-valued random variables. My question is: How Can we define $|A|$ and the adjoint $A^*$? Have iOS apps. Can I use them on my Mac Mini? I bought many iOS Apps on various iPads and iPod touchs. This Wednesday I will have my first Mac, a Mac Mini, so my question is: Will I be able to download these Apps without any charge, if they are available on Mac Store?  I ask this because I have been watching some videos and have seen GarageBand on the Mac but I don't know if it's the same App. I know that the App Store and Mac Store are different, but, like some Apps on AppStore have a plus symbol on their icons corners saying these Apps can be run on iPad, iPod touch and iPhone I wonder whether there will be some Apps for both iOS and the Mac.  Also, I don't know if GarageBand was released on Mac before iOS devices. How to change the starting reference number in Natbib in a given bibunit? I'm using bibunit and Natbib packages with a numerical numbering.   \begin{bibunit}[plainnat]     \nocite{ref1,ref2}     \putbib[mybiblio] \end{bibunit}  \begin{bibunit}[plainnat]     \nocite{ref3,ref4}     \putbib[mybiblio]  \end{bibunit}   How can I set the starting number for ref3 and ref4 to be different from 1 (say 15), without affecting the starting number for ref1 and ref 2? The desired result is the following:   [1] ref 1 [2] ref 2 [15] ref3 [16] ref4 I'm using an iPhone, but I would imagine it's the same for other mobile devices as well.  If you browse to any stack exchange site, you get a very well made mobile site.  The only gripe that I have is that I can't find any way to view the number of visitors to my questions?  Am I missing something or could this possibly be integrated into the mobile renderer? How does an arcane focus work with an Eldritch Knight? My PC is an Eldritch Knight. I was wondering what is an Arcane Focus? Do I need to have it in my hand? If I get the War Caster Feat does my arcane focus become obsolete? How do you make a torus 'telescope' open? How to make beveled 90 degree corners on a curve better? I know the title is a mouthful, but this is the problem: I made a curve with 90 degree corners, like so:  I then beveled it, and this was the result:  As you can imagine, this isn't very pretty. I'm looking for something like this:  How can I achieve this look? Calculate $E(Y)$ and $Var(Y)$ given the random variable which is exponentially distributed The random variable $X$ is exponentially distributed with parameter $\lambda = \dfrac{1}{2}$. Also, define $Y = -2X + 1$. Calculate $E(Y)$ and $Var(Y)$.  So we have   $$f_X(x) = \dfrac{1}{2} e^{-\dfrac{1}{2}x} $$  if $x\ge0$ and $0$ otherwise.  But how to calculate $E(Y)$ and $Var(Y)$ ? What does "Baking" mean? I want to make some cool items for Dota 2 and I'm reading a guide. There are some tips about baking  etc. but I can't use them, because I don't even know what baking means. Can somebody explain this, please?  Thanks for all answers. Maximum value of $\sin A+\sin B+\sin C$? What is the possessive of "you guys"? Changing the color of the path in Terminal.app I would like the CasebashMac:bfgenv chris$ part of the following line to appear in a different color in my terminal.  CasebashMac:bfgenv chris$ python Similar to my old question about  are there any limits to the animal forms an Animagus can be?     We know that the animal form can be much smaller than human (cat, rat, beetle) so I don't think size is a factor.   I'm not aware of any Animagus who transforms into a magical creature (e.g. unicorn or phoenix) so maybe those aren't possible.   Ultimately, what I'd like to know is whether there could be a T-Rex Animagus... (prompted by ) These two operators commute...but their eigenvectors aren't all the same. Why? How to reinstall kernels after accidental deletion with a separate boot partition and an encrypted LVM volume? How to type the Indian Rupee Symbol? I want to type the Indian Rupee Symbol. So I went to System Settings >> Keyboard Layout and clicked on the Options button. Under the Adding Currency signs to keys sections, I checked the Rupee on 4 option.    Later when I opened the Layout Chart of the Keyboard there was Rupee sign in the 4 key but I do not know how to type the symbol. What combination of the keys will type the Rupee symbol on the screen?