## tracing – Recovery of the limit points of the concave shell (or alpha form) in 3D

I am trying to find the limit points of a 3D concave region which is described by a list of points. If the region was convex, I would take the convex shell and simply extract the coordinates from it.

However, since the shape is concave, I have to use an alternative algorithm. For example, I can implement alpha forms as proposed in

DelaunayMesh in a specified closed region – creating a concave hull from a set of points

and

Find a concave shell.

I can successfully get a MeshRegion object that represents the shape I want. However, unlike the convex shell, required implementations such as Delauney triangulation or the code offered in https://mathematica.stackexchange.com/a/88769/45020 produces a MeshRegion which always contains all the points. It simply selects the triangles to include, but it retains the internal triangles. So, I cannot easily choose only the points which are at the border. I have tried to apply RegionBoundary which works for 2D shapes but when applied to 3D mesh it acts as an identity for some reason.

Examples of data:

``````pts = Join(RandomPoint(Cuboid({0, 0, 0}, {1, 1, 1}), 1000),
RandomPoint(Cuboid({1, 0, 0}, {2, 0.5, 0.5}), 1000));
``````

I want to get only those points at the limit of the form (reject the points inside).

## adb – The SHELL command works well as a standalone command, but fails when placed in a shell script?

Android 8.1 PIXEL 2 with root access.

I have already seen this question here, but the only solution did not help me at all. I will try to make this as simple as possible for the example.

If i run the following command in the terminal emulator or adb shell:

``````am force-stop com.myapp.frustrated
``````

It will exit the app very well. But when I run it in this shell script:

``````#!/bin/sh
am force-stop com.myapp.frustrated
``````

Nothing is happening. It's strange to me that I can do many other commands just fine in a script, but a handful of them don't work.

Any suggestions you have are greatly appreciated.

## shell – Write a script to add an eth0 interface to the S7 edge

can anyone take a look at this script? I want to add it to an sh file. When I connect my Ethernet adapter, I don't want to enter all the commands again. I just want to run it from the shell. Is it correct?

``````#!/bin/sh
ip link set eth0 up
sleep 1;
ip addr add 192.168.178.50/24 dev eth0
sleep 1;
ip route add default via 192.168.178.1 dev eth0
sleep 1;
ndc network create 1
sleep 1;
ndc network interface add 1 eth0
sleep 1;
ndc resolver setnetdns 1 8.8.8.8 8.8.4.4
sleep 1;
ndc network default set 1
``````

## Scheduling a LaunchAgent shell task on macOS Catalina

I have created a .plist file at:

``````/Users/_USER_/Library/LaunchAgents/com._APP_.plist
``````

Which contains:

``````

Label
com._APP_
ProgramArguments

sh
/Users/_USER_/Developer/script.sh

StartCalendarInterval

Hour
0
Minute
15

``````

This script works well if I use:
`sh /Users/_USER_/Developer/script.sh` from a terminal, however, it does not seem to work automatically at 15 m after midnight. How could I make it work?

## How to suppress XFS file system warning in mongodb shell?

Whenever I start a mongodb shell, I get this annoying warning:

``````Server has startup warnings:
2020-02-01T08:54:27.220+0100 I  STORAGE  [initandlisten]
2020-02-01T08:54:27.220+0100 I  STORAGE  [initandlisten] ** WARNING: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine
2020-02-01T08:54:27.220+0100 I  STORAGE  [initandlisten] **          See http://dochub.mongodb.org/core/prodnotes-filesystem
``````

I understand that in some environments this is useful. But in my particular case, I am using mongodb on a VPS server which has a preinstalled ext4 file system. There is no option to change this. And it's not necessary anyway – in this particular application, there are no performance issues and there never will be. The server is already under control for this application.

I was browsing the documentation to find a command line option to suppress this warning message, but there is none. Or maybe there is one, but I did not find it?

## python3 – How NOT to change the shell prompt when activating a python venv?

I created a venv like this:

``````python3 -m venv .venv
``````

When I activate it, the shell prompt is changed.

``````antkong@konga-mbp ~/dev/my-project (git-branch-name)
\$ source .venv/bin/activate
(.venv) konga-mbp:my-project antkong\$
``````

How can I keep the prompts the same?

## What is the convex shell of the quaternion symmetries of the three-dimensional cube?

It is well known that there are exactly five regular three-dimensional convex polyhedra, called Platonic solids.
In 1852 the Swiss mathematician Ludwig Schlafli discovered that there were exactly six regular 4 convex polytopes (the generalization of
4-dimensional polyhedra) and that, for dimensions 5 and above, there are only three!

The six 4 regular polytopes are:

``````NAME                    VERTEXES   EDGES   FACES  CELLS
Hypertetrahedron              5      10      10      5
Hypercube                    16      32      24      8
Hyperoctahedron               8      24      32     16
24-cell                      24      96      96     24
Hyperdodecahedron           600    1200     720    120
Hypericosahedron            120     720    1200    600
``````

The simplest to describe are the first two:
a model for the hypertetrahedron can be obtained as the convex shell of the canonical base in $$mathbb R ^ 5$$ (hence a
4-dimensional object), while a model for the hypercube is the Cartesian product $$(0, 1) times (0, 1) times (0, 1) times (0, 1)$$.

As in the case of 3 dimensions, the dual of a regular 4-polytope is also a regular 4-polytope and it turns out that the regular six
The 4-polytopes found by Schlafli are linked to each other via duality as follows.

``````Hypetetrahedron    <->   Itself
24-cell            <->   Itself
Hypercube          <->   Hyperoctahedron
Hyperdodecahedron  <->   Hypericosahedron
``````

This means that it is enough to describe the 24 cells and the hypericosahedron to be all known. In other words:

``````Hypertetrahedron    =  convex hull of the canonical basis in 5 dimensions
Hypercube           =  (0,1)x(0,1)x(0,1)x(0,1)
Hyperoctahedron     =  dual of the hypercube
Hyperdodecahedron   =  dual of the hypericosahedron
24-cell                ???
Hypericosahedron       ???
``````

The description of the last two 4-polytopes above can be obtained by considering the quaternions $$mathbb Q$$.
visualization $$mathbb R ^ 3$$ in $$mathbb Q$$ via the card
$$(x, y, z) mapsto xi + yj + zk,$$
it is well known that each quaternion
$$q$$, with $$Green q Green = 1$$, give a rotation $$R_q$$ sure $$mathbb R ^ 3$$ via the formula
$$R_q (v) = qvq ^ {- 1}, quad forall v in mathbb R ^ 3.$$

In fact the correspondence $$q mapsto R_q$$ is a double coating of $$N / A (3)$$ by the unitary sphere $$mathbb Q$$.

leasing $$P_ {20}$$ be the icosahedron $$mathbb R ^ 3$$, Take into account quaternionic symmetries of $$P_ {20}$$, which I will write as
$$mathbb {QS} (P_ {20})$$, defined as the
set of all unitary quaternions $$q$$ such as $$R_q$$ leaves $$P_ {20}$$
invariant. In symbols
$$mathbb {QS} (P_ {20}) = {q in mathbb Q: Vert q Vert = 1, R_q (P_ {20}) = P_ {20} }.$$
Well, the convex shell of $$mathbb {QS} (P_ {20})$$ in $$mathbb R ^ 4$$ proves to be a model for the hypericosahedron!

Since the symmetries of a regular polyhedron are the same as the symmetries of its dual, it is clear that the symmetries
of $$P_ {12}$$, the dodecahedron, is nothing new: the convex shell of $$mathbb {QS} (P_ {12})$$ is just another model for the
hypericosaèdre.

Go to the tetrahedron (double auto), call it $$P_4$$, the convex shell of $$mathbb {QS} (P_ {4})$$ give a model for the
4 remaining polytopes, namely the 24 cells, completing the description of the six 4 Schlafli polytopes.

Question: What is the convex shell of the quaternion symmetries of the 3-dimensional cube?

If I am not mistaken, this polytope 4 has 48 vertices and 144 edges, so it is not in Schlafli's list and therefore cannot be regular.

## root access – Individual commands work fine, but shell script generates errors

I am trying to create a shell script to configure iptables and add some routes to my device running Android 10. I have root access.

Here is the script. I called him `script.sh`:

``````#!/system/bin/sh
iptables -t filter -F FORWARD
iptables -t nat -F POSTROUTING
iptables -t filter -I FORWARD -j ACCEPT
iptables -t nat -I POSTROUTING -j MASQUERADE
ip rule add from 192.168.43.0/24 lookup 61
ip route add default dev tun0 scope link table 61
ip route add 192.168.43.0/24 dev wlan1 scope link table 61
ip route add broadcast 255.255.255.255 dev wlan1 scope link table 61

sysctl -w net.ipv6.conf.wlan1.disable_ipv6=1
``````

I have a root console open via ADB, and I can execute each individual command from the script manually in the console, as follows:

… etc.

But this happens when I run my script:

and I don't really understand why.

I have limited skills with Android and Linux, so I tried to follow and combine a few examples, but I was unlucky.
https://www.pctips4u.com/2016/03/writing-shell-scripts-in-android.html
https://gist.github.com/LouWii/9626480

## linux – Shell starts in edit mode only. No prompt appears, no command can be executed

My tcsh appears in a weird way. Whether as a login shell or in a terminal, it now always starts with a simple cursor. I can type text in the terminal but that's all I can do. No prompts appear, no linked keys work, nothing.

Help with this problem?

## How to block to open Web Shell in my CentOS 6

I am currently using mod_security in combination with maldec and blocking almost all attempts to download shells, which leads to error 406. However, I have found a "problem" if I download Shell via an ftp client, then I can use shell in the hosting. my question is how to block the opening of the shell even if it is downloaded via ftp? any mod_security rule to use and if so what? I also use cPanel