apt vs apt-get

Every Debian system has both, most tutorials use them interchangeably, and the advice you get is usually "apt is the new one". That is close enough to be unhelpful: they are not different generations of a tool, they are two interfaces to the same library, aimed at two different readers.

They are the same package

Not a rewrite, not a replacement, not a fork:

dpkg -S "$(command -v apt)" "$(command -v apt-get)"

apt: /usr/bin/apt
apt: /usr/bin/apt-get

Both binaries come out of the apt package, built from the same source, linked against the same libapt-pkg; see apt for what that front end can do. Anything apt can do, it does by calling the same code apt-get calls, and dpkg -S is what the query above is doing.

Who each one is written for

apt is the end-user interface. It defaults to the things that help a person at a terminal (a progress bar, colour, a summary of what will change) and its command set is a curated subset of the most-used operations, gathered from apt-get and apt-cache into one place.

apt-get is the stable interface. Its command set is complete rather than curated, its output is dull on purpose, and its behaviour is held still between releases so that scripts written years ago keep working.

That is the whole distinction, and apt is unusually direct about it. Piped into anything other than a terminal, apt says so itself:

apt list --installed 2>&1 | head -4

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Listing...

It prints that warning only when it detects that stdout is not a terminal, which is to say only when it thinks you might be scripting it. apt-get never prints it, because for apt-get that is not a mistake. The manual puts it plainly under "Script usage and differences from other APT tools":

The apt(8) commandline is designed as an end-user tool and it may change behavior between versions. While it tries not to break backward compatibility this is not guaranteed either if a change seems beneficial for interactive use.

The command names differ where the behaviour does

The overlap is large but not total, and the two upgrade commands are where people get caught:

apt --help | grep -E "^  (upgrade|full-upgrade)"

  upgrade - upgrade the system by installing/upgrading packages
  full-upgrade - upgrade the system by removing/installing/upgrading packages

apt-get --help | grep -E "^  (upgrade|dist-upgrade)"

  upgrade - Perform an upgrade
  dist-upgrade - Distribution upgrade, see apt-get(8)

apt full-upgrade and apt-get dist-upgrade are the same operation under two names: upgrade everything, and remove packages if that is what it takes. The plain upgrade in each tool is where they genuinely differ: apt upgrade will install new packages when an upgrade needs them, and apt-get upgrade will not, holding those packages back instead. That single difference is behind most of the "why is apt-get not upgrading this" confusion, and behind packages appearing to be kept back.

apt also folds in commands that were never apt-get's at all: apt search, apt show and apt list are apt-cache operations wearing a friendlier name.

Which to use

At a terminal, use apt. It is nicer to read, it needs less typing, and the instability the warning refers to is a change in output formatting between Debian releases, which matters to a script and not to you.

In a script, a cron job, a systemd unit or a Dockerfile, use apt-get (and apt-cache for queries). Not because apt will break today, but because nothing promises it won't, and the failure mode is a script that parses output fine for two years and then silently matches nothing after an upgrade. Pass -y and set DEBIAN_FRONTEND=noninteractive while you are there.

Ignore advice to prefer one for correctness. They resolve dependencies identically, because it is the same resolver. Anything claiming apt installs different packages than apt-get is describing the upgrade difference above, not a difference in the underlying decision.

Neither replaces dpkg, which sits underneath both and does not know anything about repositories or dependency resolution.