Ubuntu 16.04 - Sources.list Problem I have installed Ubuntu 16.04.02 a couple of minutes ago. When I want to update my system:  $ sudo apt-get update &amp;&amp; sudo apt-get upgrade   Ubuntu does not update itself. The error message is above:    The problem is with sources.list. But how can i solve this problem? Please help me!
Restore default apt repositories in sources.list from command line I am looking for the best method to restore /etc/apt/sources.list to default from the command line.  Is there no way to reference the source code of the package which generates this file or something like that? I want a trusted and version-independent way of restoring this file.  Solutions Ruled Out  Before you mark this as a duplicate, note that I have reviewed  already. It is only applicable if you have the Ubuntu GUI available. This question is specific to the command line.  I have also reviewed  where the accepted solution is someone pasted the contents of their sources.list file. This is not an appropriate way to restore the file as the intentions of the person providing the file contents cannot be verified and the file is subject to change with new releases.  I checked out the , but this is also not from Ubuntu so I do not plan to use it.
Sending & Receiving SMS via WiFi Is there a way to send and receive SMS when a cellular signal is not available, but WiFi is?  I'm often on-site at locations where the building has a strong WiFi (Internet) connection, but a simple cell signal is spotty at best. Finite abelian group contains an element with order equal to the lcm of the orders of its elements I will quote a question from my textbook, to prevent misinterpretation:     Let $G$ be a finite abelian group and let $m$ be the least common multiple of the orders of its elements. Prove that $G$ contains an element of order $m$.   I figured that, if $|G|=n$, then I should interpret the part with the least common multiple as $lcm(|x_1|,\dots,|x_n|)=m$, where $x_i\in G$ for $0\leq i\leq n$, thus, for all such $x_i$, $\exists a_i\in\mathbb{N}$ such that $m=|x_i|a_i$. I guess I should use the fact that $|x_i|$ divides $|G|$, so $\exists k\in \mathbb{N}$ such that $|G|=k|x_i|$ for all $x_i\in G$. I'm not really sure how to go from here, in particular how I should use the fact that $G$ is abelian. When did The Doctor start running so much? Thanks to Netflix Streaming, I've been watching some random classic Doctor Who stories.  One thing that strikes me as different between the modern Who and old Who was that in modern Who, they do an awful lot of running.  It's become a sort of running gag that they've referenced in-universe.  In the classic Who I've seen, he tends to drive around quite a bit instead.  In addition, The Doctor in modern Who is incredibly reliant on his sonic screwdriver, which I often don't see at all in classic Who stories.  Did the Doctor running and the sonic screwdriver get introduced over time in classic Who, or were they both added as part of the modern Who revival? installing rtl8814AU wifi adapter driver on ubuntu 16.04 Package Cache File corrupted From advice on  , I've opened a new question about the same issue, as it turned out my problem was not the one described in the question, information I did not know at the time. I'm running Ubuntu 16.04.  $ sudo apt-get update  Hit:1 http://archive.ubuntu.com/ubuntu xenial InRelease Get:2 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB] Get:3 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB] Get:4 http://archive.ubuntu.com/ubuntu xenial-security InRelease [102 kB] Get:5 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 DEP-11 Metadata [307 kB] Get:6 http://archive.ubuntu.com/ubuntu xenial-updates/main DEP-11 64x64 Icons [226 kB] Get:7 http://archive.ubuntu.com/ubuntu xenial-updates/universe amd64 DEP-11 Metadata [185 kB] Get:8 http://archive.ubuntu.com/ubuntu xenial-updates/universe DEP-11 64x64 Icons [268 kB] Get:9 http://archive.ubuntu.com/ubuntu xenial-updates/multiverse amd64 DEP-11 Metadata [5,888 B] Get:10 http://archive.ubuntu.com/ubuntu xenial-backports/main amd64 DEP-11 Metadata [3,324 B] Get:11 http://archive.ubuntu.com/ubuntu xenial-backports/universe amd64 DEP-11 Metadata [4,712 B] Get:12 http://archive.ubuntu.com/ubuntu xenial-security/main amd64 DEP-11 Metadata [62.7 kB] Get:13 http://archive.ubuntu.com/ubuntu xenial-security/main DEP-11 64x64 Icons [71.2 kB] Get:14 http://archive.ubuntu.com/ubuntu xenial-security/universe amd64 DEP-11 Metadata [51.3 kB] Get:15 http://archive.ubuntu.com/ubuntu xenial-security/universe DEP-11 64x64 Icons [85.1 kB] Fetched 1,578 kB in 1s (1,529 kB/s)                                     Reading package lists... Error! E: Read error - read (5: Input/output error) W: You may want to run apt-get update to correct these problems E: The package cache file is corrupted   If there's any more information you need, don't hesitate to ask in the comments.  Edit 1:  $ sudo df -h Filesystem      Size  Used Avail Use% Mounted on udev            5.9G     0  5.9G   0% /dev tmpfs           1.2G  9.4M  1.2G   1% /run /dev/sda1       135G  7.2G  121G   6% / tmpfs           5.9G   14M  5.9G   1% /dev/shm tmpfs           5.0M  4.0K  5.0M   1% /run/lock tmpfs           5.9G     0  5.9G   0% /sys/fs/cgroup tmpfs           1.2G   88K  1.2G   1% /run/user/1000 /dev/sdb1       932G  300G  633G  33% /media/adoria298/MDrive1   Edit 2 SMART Test results: What does "static" mean in C? What does the 'static' keyword do in a class? To be specific, I was trying this code:  package hello;  public class Hello {      Clock clock = new Clock();      public static void main(String args[]) {         clock.sayTime();     } }   But it gave the error      Cannot access non-static field in static method main   So I changed the declaration of clock to this:  static Clock clock = new Clock();   And it worked. What does it mean to put that keyword before the declaration? What exactly will it do and/or restrict in terms of what can be done to that object? MSO Review Queue Privileges are Misleading When I visit Stack Overflow, I see the review option in the bar at the top of the screen.    However when on MSO, I don't see it. I assumed this was because I didn't have enough reputation to access the queue. I was curious what the requirement was on meta, so I moseyed on over to the privileges page. To my surprise, I saw that the privilege is listed as only requiring 500 rep and informs me I already have it:    Confused, I manually typed in meta.stackoverflow.com/review to see what would happen. The page did in fact load, but as you can see meta has no "First Posts" or "Late Answers" queue.    Seeing on that page that a minimum of 2k reputation is required to access any queues on MSO, I report the following bug:  1) On the privileges list on MSO, the 500 rep privilege of "Access first posts and late answers review queues" should not appear.  I also have the following feature request:  2) On the privileges list on MSO, it would be useful if instead there was a 2k privilege that informed you that you have access to the "Suggested Edits" and "Low Quality Posts" queues. Page title with substitution on views page I have a view page that requires a dynamic page title. It is a simple substitution of the label field.  If I use {{ label }} it works fine for the title displayed on the page, but it doesn't substitute the label for the actual page title (in meta)  I was able to fix this by using hook_views_post_render using the following code  // Set the route title. $route = \Drupal::routeMatch()-&gt;getCurrentRouteMatch()-&gt;getRouteObject(); $route-&gt;setDefault('_title', $title);  $view-&gt;setTitle($title);   However, the title only works if the page is not cached. For the cached pages, the page title is missing. This feels like a bug in drupal, but I am hoping that there is a simple way to get this done. Write $\frac 11 + \frac12 + ...\frac1{(p-1)} =$ $\frac ab$.Such that $(a,b)=1$. Show that $p^2 \mid a$ $\text{if}$ $p\geq 5$.  I was trying to apply something with prime modulus, but I am unsure how exactly to go about it. What is the most accurate way to retrieve a user's correct IP address in PHP? Computing: $\int_0^1 \sqrt[3]{x\log{\frac{1}x}} \, \mathrm dx$ Tsunami dampening mechanisms Encouraged by the zeitgeist let me ask the following:  Is it feasible (now or in the future) to build systems a certain distance of a vulnerable coastline which can serve to dampen a tsunami before it reaches the coast itself?  The following picture comes to mind. We have some device which is small enough to be mounted on a buoy. An array of such buoys can be anchored some distance off the coastline, controlled by computers which are linked into the global tsunami warning network. This setup is quite feasible given what a cell phone can do today.  On the detection of an oceanic earthquake or other tsunami generating event (such as a piece of the Rock of Gibraltar falling into the sea) and given present environmental conditions one can calculate the expected path of the tsunami and its intensity. If a TDS (Tsunami Dampening System) of the type I describe above happens to be deployed in range of the tsunami, all relevant information is conveyed to the TDS controllers by the global tsunami warning system. All this happen in a matter of moments.  This leaves plenty of time (25min - 2hr) for the devices in the TDS array to be activated and to do their job. The question in this case would be - what form could such devices take? Perhaps mini-turbines which could dissipate the incoming energy into the ocean by first converting it to electrical energy.  Any such undertaking involves physical estimates for the size and shape of such an array, the precise capabilities of each device, etc. Many of these concerns involve simple physical questions.  Can you come up with the details of such a system or alternatively outline your own design? If you think that this idea would never work in the first place - for technological, physical or some other reasons - please explain what you think would be the prime obstacle and why? Making Ctrl+C copy text in gnome-terminal? Is it possible for me to make Ctrl+C perform a copy command if there is highlighted text in the terminal? Otherwise, it should retain its normal behavior.  (If there is a terminal that can do this other than gnome-terminal, that's probably fine too.) Is there a way in Java to have a map where the type parameter of a value is tied to the type parameter of a key? What I want to write is something like the following:  public class Foo {     // This declaration won't compile - what should it be?     private static Map&lt;Class&lt;T&gt;, T&gt; defaultValues;      // These two methods are just fine     public static &lt;T&gt; void setDefaultValue(Class&lt;T&gt; clazz, T value) {         defaultValues.put(clazz, value);     }      public static &lt;T&gt; T getDefaultValue(Class&lt;T&gt; clazz) {         return defaultValues.get(clazz);     } }   That is, I can store any default value against a Class object, provided the value's type matches that of the Class object. I don't see why this shouldn't be allowed since I can ensure when setting/getting values that the types are correct.  EDIT: Thanks to cletus for his answer. I don't actually need the type parameters on the map itself since I can ensure consistency in the methods which get/set values, even if it means using some slightly ugly casts. I am looking for a proof possibly using the sandwich theorem and/or Bernoulli's inequality for proving the following statement:  If $x&gt;0$ then $x^{1/n}$ tends to $1$. How to calculate the difference between two dates using PHP? shell: keep trailing newlines ('\n') in command substitution I want to be able to capture the exact output of a command substitution, including the trailing new line characters.  I realise that they are stripped by default, so some manipulation may be required to keep them, and I want to keep the original exit code.  For example, given a command with a variable number of trailing newlines and exit code:  f(){ for i in $(seq "$((RANDOM % 3))"); do echo; done; return $((RANDOM % 256));} export -f f   I want to run something like:  exact_output f   And have the output be:  Output: $'\n\n' Exit: 5   I'm interested in both bash and POSIX sh. Command Options on Command Line - Details and Functions? Does equality of characteristic polynomials guarantee equivalence of matrices? I have a qualifying exam coming up in a couple days and I am just trying to understand some pathological examples I have in my notes. I will list a similar problem which I know the solution to and then the question.  True or False:   let $A,B \in \mathbb{Q}^{n \times n }$  Suppose $xI-A$ and $xI-B$ are equivalent then $\det(xI-A) = \det(xI-B)$.     I wrote in my notes this is true because any two matrices are equivalent in $\mathbb{Q[x]}^{n \times n}$ if and only if they have the same invariant factors.  Since the characteristic polynomial is a product of the invariant factors it follows that $\det(xI-A) = \det(xI-B)$.        True or False:   let $A,B \in \mathbb{Q}^{n \times n }$. If $\det(xI-A) = \det(xI-B)$ (in $\mathbb{Q}[x]$) then $xI-A$ and $xI-B$ are equivalent  in $\mathbb{Q}[x]^{n \times n}$   I think this is false because of the fact that equality of determinants is not strong enough to guarantee all invariant factors are equal but I do not have an example.