`brew list` shows many things I did not install. Why? If something installed depends on them, how do I know?

Question

On a new machine, I used brew to install only four things: git, node, sqlite and less. Later on, when I ran brew update I was told many other things that I didn't install were "outdated". I ran brew list and got the following:

brotli              less               pcre2
c-ares              libnghttp2         python@3.9
ca-certificates     libuv              readline
gdbm                mpdecimal          sqlite
gettext             ncurses            xz
git                 node
icu4c               openssl@1.1

Where did these come from? If they were installed because git, node or less needs them, how do I find out?

This matters because I only want to consider updating packages that I specifically installed, and leave decisions about updating dependencies to the packages that depend on them.

Answer 1

Yes, Homebrew (aka brew), like all package managers, automatically installs dependencies, things needed for something else to work. This is a recursive process: If the dependency itself has dependencies, brew will install them too, and so on. brew list shows you everything installed on your system, including all these dependencies.

To see just the list of what you yourself installed, see the end of this answer.

brew deps

You can see the dependency tree for anything you installed using brew deps:

> brew deps node --tree

node
├── brotli
├── c-ares
├── icu4c
├── libnghttp2
├── libuv
├── openssl@1.1
│   └── ca-certificates
└── python@3.9
    ├── gdbm
    ├── mpdecimal
    ├── openssl@1.1
    │   └── ca-certificates
    ├── readline
    ├── sqlite
    │   └── readline
    └── xz

You can see in this tree many of the things you saw in brew list.

brew uses

But to answer part of your question, you can go in the opposite direction using brew uses:

> brew uses readline --installed

node            python@3.9            sqlite

The --installed flag is important, because without it the above command will list everything that uses readline, whether it is installed on your system or not. Notice also how brew uses is recursive (in the opposite direction as brew deps), showing both python@3.9 which uses readline directly, and node, which uses it indirectly.

brew desc

Now if you're curious as to what any of these installed dependencies do, use brew desc:

> brew desc pcre2

pcre2: Perl compatible regular expressions library with a new API

brew list --installed-on-request

Finally, to just get a list of what you specifically installed, just add --installed-on-request to brew list:

> brew list --installed-on-request

git
less
node
rar
sqlite

🌶️ brew desc $(brew list --installed-on-request)

If you forget why you installed many things, this command will show the same list as above, but with the brew desc for each.

🌈 You can make it easier to read with column:

brew desc $(brew list --installed-on-request) | column -t -s ":"

Create an alias so you don't have to memorize it all that!