如何在Snap包中定义全局的plug

我们知道在我们snap应用中，我们可以通过定义plug来访问我们所需要的资源．在一个snap包中，我们也可以定义许多的应用，每个应用可以分别定义自己的plug．假如一个Snap包有一个plug是共同的，那么，我们有上面办法来定义一个全局的plug，这样所有的在同一个包中的所有的snap应用都可以同时拥有这个plug．这个到底怎么做呢？

关于snap中的interface及plug概念，请参阅我之前的文章＂安装snap应用到Ubuntu 16.4桌面系统＂．

最近，我读了一篇关于snap interface的文章．很受启发．文章可以在地址找到．其中有一段非常有意思的一段话：

Note that only the links app
refers to plugs, the bookmarks app does not. If a plug or slot is declared in a snap, but not associated with a specific application they will be implicitly bound to all apps. When a plug or slot is specified by one or more apps, as in the above example, it
will be bound only to those applications. Compare that to the following code:

它的意思就是如果我们在我们的snapcraft.yaml中定义一个plug，但是它不被任何的应用所应用，那么它隐含地就是所有的应用都有这个plug．

我们拿我们的例程https://github.com/liu-xiao-guo/helloworld-plug为例，我们定义如下：

name: hello-xiaoguo
version: "1.0"
architectures: [ all ]
summary: The 'hello-world' of snaps
description: |
This is a simple snap example that includes a few interesting binaries
to demonstrate snaps and their confinement.
* hello-world.env  - dump the env of commands run inside app sandbox
* hello-world.evil - show how snappy sandboxes binaries
* hello-world.sh   - enter interactive shell that runs in app sandbox
* hello-world      - simply output text
confinement: strict
type: app  #it can be gadget or framework

apps:
env:
command: bin/env
evil:
command: bin/evil
sh:
command: bin/sh
hello-world:
command: bin/echo
createfile:
command: bin/createfile
createfiletohome:
command: bin/createfiletohome
writetocommon:
command: bin/writetocommon

plugs:
home:
interface: home

parts:
hello:
plugin: copy
files:
./bin: bin

在上面的snapcraft.yaml文件中，我们写了如下的句子：

plugs:
home:
interface: home

由于在我们的任何一个应用中都没有引用home plug，所有这个plug将被定义为包级的plug，也就是说所有的app都享有这个plug．我们可以做一个简单的测试．我们安装好我们的hello snap后，执行如下的命令：

liuxg@liuxg:~$ hello-xiaoguo.createfiletohome
Hello a nice World!
This example demonstrates the app confinement
This app tries to write to its own user directory
Succeeded! Please find a file created at /home/liuxg/snap/hello-xiaoguo/x1/test.txt
If do not see this, please file a bug

我们的createtohome脚本如下：

#!/bin/sh

set -e
echo "Hello a nice World!"

echo "This example demonstrates the app confinement"
echo "This app tries to write to its own user directory"

echo "Haha" > /home/$USER/test.txt

echo "Succeeded! Please find a file created at $HOME/test.txt"
echo "If do not see this, please file a bug"

显然它向home里写入一个叫做test.txt的文件．我们的写入操作是成功的．



从我们的文件目录中，我们可以看出来我们刚刚创建的文件test.txt．细心的开发者也可以出去上面定义的plug，在试一试是否成功？