what is the difference between apt-get and apt-upgrade

`apt-get` is a command-line tool used to install, remove, and manage packages on Ubuntu and other Debian-based Linux distributions. It is used to install new packages and upgrade existing ones.

`apt-upgrade` is a sub-command of `apt-get` used to upgrade the installed packages on the system to their latest versions. It is used to update the package index and upgrade all installed packages to their latest available versions.

So, the main difference between `apt-get` and `apt-upgrade` is that `apt-get` is used to perform various package management operations, while `apt-upgrade` is used specifically to upgrade installed packages to their latest versions.

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what is the difference between apt-get update and apt-get upgrade

`apt-get update` is used to update the package lists for upgrades and new packages that are available in the repositories. It downloads the package lists from the repositories and "updates" them to get information on the newest versions of packages and their dependencies.

`apt-get upgrade`, on the other hand, is used to actually upgrade the installed packages on the system. It installs newer versions of packages that are already installed on the system. It also installs any required dependencies for these packages.

In summary, `apt-get update` updates the package lists, while `apt-get upgrade` installs newer versions of the already installed packages.

apt-get update apt-get upgrade

apt-get update命令用于同步/etc/apt/sources.list和/etc/apt/sources.list.d中列出的源的索引，以获取最新的软件包。这个命令会从指定的位置（如Debian存档）检索和扫描Packages.gz文件，以便获取有关新的和更新的软件包的信息。在执行升级或dist-upgrade之前，应始终执行更新操作。请注意，由于无法提前知道软件包文件的大小，因此总体进度条可能不准确。[1]

apt-get upgrade命令用于安装系统上当前安装的所有软件包的最新版本，这些软件包的来源在/etc/apt/sources.list中列出。它会检索并升级当前已安装的具有新版本的软件包，绝不会删除当前已安装的软件包，也不会检索和安装尚未安装的软件包。无法在不改变其他软件包的安装状态的情况下升级的当前已安装软件包的新版本将保持在其当前版本。在执行upgrade之前必须先执行update，以便apt-get知道有新版本的软件包可用。[3]

因此，要更新系统上的软件包，应先执行apt-get update命令来同步软件包索引，然后再执行apt-get upgrade命令来安装最新版本的软件包。