一、包管理基本原理

Linux系統具有強大的包管理能力，能夠自動從互聯網的軟件倉庫中搜索、安裝、升級、卸載軟件或操作系統。其基本原理為：

1. 廠商或開源組織將軟件包放至服務器中，構成源或軟件倉庫，以供用戶下載使用；
2. 軟件倉庫有很多，國內常用的源有阿里源、清華源等等。Ubuntu源信息記錄在/etc/apt/sources.list中

root@linux:/var/lib/apt/lists$ cat /etc/apt/sources.list
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb [arch=amd64] http://mirrors.aliyun.com/docker-ce/linux/ubuntu bionic stable
# deb-src [arch=amd64] http://mirrors.aliyun.com/docker-ce/linux/ubuntu bionic stable

3. 執行apt-get update命令，則會根據/etc/apt/sources.list記錄的源站信息，掃描源服務器，并建立軟件包的索引文件，存放在/var/lib/apt/lists/中

root@linux:/var/lib/apt/lists# ll
total 222044
drwxr-xr-x 4 root root    12288 Feb  7 10:17 ./
drwxr-xr-x 5 root root     4096 Nov 13  2020 ../
drwxr-xr-x 2 _apt root     4096 Nov 16  2020 auxfiles/
-rw-r----- 1 root root        0 Nov 16  2020 lock
-rw-r--r-- 1 root root    74562 Feb  7 07:11 mirrors.aliyun.com_ubuntu_dists_bionic-backports_InRelease
-rw-r--r-- 1 root root    59629 Dec 16 00:17 mirrors.aliyun.com_ubuntu_dists_bionic-backports_main_binary-amd64_Packages
-rw-r--r-- 1 root root    12910 Dec 16 00:17 mirrors.aliyun.com_ubuntu_dists_bionic-backports_main_i18n_Translation-en
-rw-r--r-- 1 root root    17863 Dec 16 00:17 mirrors.aliyun.com_ubuntu_dists_bionic-backports_main_source_Sources
-rw-r--r-- 1 root root    49624 Dec 16 02:14 mirrors.aliyun.com_ubuntu_dists_bionic-backports_universe_binary-amd64_Packages
-rw-r--r-- 1 root root    15723 Dec 16 02:14 mirrors.aliyun.com_ubuntu_dists_bionic-backports_universe_i18n_Translation-en
-rw-r--r-- 1 root root    20023 Dec 16 02:14 mirrors.aliyun.com_ubuntu_dists_bionic-backports_universe_source_Sources
-rw-r--r-- 1 root root   242048 Apr 27  2018 mirrors.aliyun.com_ubuntu_dists_bionic_InRelease
......

進一步，查看文件的信息，則：

root@linux:/var/lib/apt/lists# cat mirrors.aliyun.com_ubuntu_dists_bionic_InRelease | more
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512


Origin: Ubuntu
Label: Ubuntu
Suite: bionic
Version: 18.04
Codename: bionic
Date: Thu, 26 Apr 2018 23:37:48 UTC
Architectures: amd64 arm64 armhf i386 ppc64el s390x
Components: main restricted universe multiverse
Description: Ubuntu Bionic 18.04
MD5Sum:
 32a92a5c20f378d42dd2d2f4f28f6637        628836439 Contents-amd64
 53c6a594819b51a5755f88b45d1eff7f         37766986 Contents-arm64.gz
 cd7bf6d50403da4348ee48138eace986        585939706 Contents-ppc64el
 af42d07307c5d1398c75e28986b36509        616261664 Contents-i386
 98f43960ac7029b4948a13614038593c        584794633 Contents-armhf
 31848d4faedf11946ff1af9c281f4e6b         39528051 Contents-amd64.gz

4. 執行apt-get install XXX，則會根據軟件包的索引文件，找到最新版本的軟件包及其依賴關系，下載(或更新)、解壓、安裝和配置軟件包。

root@linux:/var/lib/apt/lists# apt-get install git
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  grub-pc-bin
Use 'apt autoremove' to remove it.
Suggested packages:
  git-daemon-run | git-daemon-sysvinit git-doc git-el git-email git-gui gitk gitweb git-cvs git-mediawiki git-svn
The following packages will be upgraded:
  git
1 upgraded, 0 newly installed, 0 to remove and 315 not upgraded.

5. 執行apt-get upgrade，則可以將所有軟件包一次性升級到最新版本。

root@linux:/var/lib/apt/lists# apt-get upgrade
......
Updating certificates in /etc/ssl/certs...
0 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d...


done.
done.
Processing triggers for initramfs-tools (0.130ubuntu3.13) ...
update-initramfs: Generating /boot/initrd.img-4.15.0-122-generic

二、工具apt-get和apt區別：

1. Ubuntu 16.04版本前，使用apt-get、apt-cache和apt-config進行包的管理；存在的問題：命令過于分散，并且apt-cache和apt-config用得比較少。
2. Ubuntu 16.04版本發布時，引入apt命令以解決命令分散的問題；存在的問題：apt并未完全取代apt-get命令。

apt命令              原有命令             命令的功能
apt install       apt-get install        安裝軟件包
apt remove        apt-get remove         移除軟件包
apt purge         apt-get purge          移除軟件包及配置文件
apt update        apt-get update         刷新存儲庫索引
apt upgrade       apt-get upgrade        升級所有可升級的軟件包
apt autoremove    apt-get autoremove     自動刪除不需要的包
apt full-upgrade  apt-get dist-upgrade   在升級軟件包時自動處理依賴關系
apt search        apt-cache search       搜索應用程序
apt show          apt-cache show         顯示安裝細節