apt

The difference between apt update and apt upgrade is the following:

• update
  is just fetching the available updates for already installed packages, but doesn't change anything to these installed packages

• upgrade
  is effectively upgrading the update candidates to the latest versions for the currently used distro.  For this reason, your permission is explicitly required to do the upgrade.  The permission is not asked for the update since that doesn't change anything yet to your environment.

To know if a package is already installed, there's two options.  Let's take the package sshfs as a guinea pig.

• Option one, the simple option: dpkg -s sshfs
  This will return the following output:
  
  

geertvc@Win10:~$ dpkg -s sshfs
Package: sshfs
Status: install ok installed
Priority: optional
Section: utils
Installed-Size: 122
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Architecture: amd64
Source: sshfs-fuse
Version: 3.7.3-1.1build3
Depends: libc6 (>= 2.38), libfuse3-3 (>= 3.2.3), libglib2.0-0t64 (>= 2.24.0), fuse3, openssh-client
Breaks: fuse (<< 3)
Description: filesystem client based on SSH File Transfer Protocol
  sshfs is a filesystem client based on the SSH File Transfer Protocol.
  Since most SSH servers already support this protocol it is very easy
  to set up: i.e. on the server side there's nothing to do.  On the
  client side mounting the filesystem is as easy as logging into the
  server with ssh.
  .
  sshfs is FUSE (Filesystem in USErspace).
Homepage: https://github.com/libfuse/sshfs
Original-Maintainer: Bartosz Fenski <fenio@debian.org>

• Option 2: dpkg-query -l sshfs
  This will return the following output:
  
  

geertvc@Win10:~$ dpkg-query -l sshfs
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version         Architecture Description
+++-==============-===============-============-=====================================================
ii  sshfs          3.7.3-1.1build3 amd64        filesystem client based on SSH File Transfer Protocol

To me, this is less informative, though...