我們知道在我們snap應用中，我們可以通過定義plug來訪問我們所需要的資源．在一個snap包中，我們也可以定義許多的應用，每個應用可以分別定義自己的plug．假如一個Snap包有一個plug是共同的，那麼，我們有上面辦法來定義一個全局的plug，這樣所有的在同一個包中的所有的snap應用都可以同時擁有這個plug．這個到底怎麼做呢？
關於snap中的interface及plug概念，請參閱我之前的文章＂安裝snap應用到Ubuntu 16.4桌面系統＂．
最近，我讀了一篇關於snap interface的文章．很受啟發．文章可以在地址找到．其中有一段非常有意思的一段話：
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們在我們的snapcraft.yaml中定義一個plug，但是它不被任何的應用所應用，那麼它隱含地就是所有的應用都有這個plug．
我們拿我們的例程https://github.com/liu-xiao-guo/helloworld-plug為例，我們定義如下：
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，我們寫了如下的句子：
plugs:
home:
interface: home
由於在我們的任何一個應用中都沒有引用home plug，所有這個plug將被定義為包級的plug，也就是說所有的app都享有這個plug．我們可以做一個簡單的測試．我們安裝好我們的hello snap後，執行如下的命令：
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
我們的createtohome腳本如下：
#!/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"
顯然它向home裡寫入一個叫做test.txt的文件．我們的寫入操作是成功的．
從我們的文件目錄中，我們可以看出來我們剛剛創建的文件test.txt．細心的開發者也可以出去上面定義的plug，在試一試是否成功？
更多Ubuntu相關信息見Ubuntu 專題頁面 http://www.linuxidc.com/topicnews.aspx?tid=2
