This minimal nix install in a docker serves to provide a simple and harmeless local playground for learning intial nix mechanics.
docker build -t hello .
$ docker run --rm --entrypoint bash -it hello
$ nix profile install nixpkgs#cowsay
...
$ cowsay moo
_____
< moo >
-----
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||
What happened there? We asked nix to install, into our current
profile, the package cowsay from the nixpkgs flake.
What version was installed?
Let's take a look:
user@40a4bcde2962:/$ cowsay -h
.cowsay-wrapped version 3.7.0
...
The version installed in determined by the version of the nixpkgs
flake used. In this case, the nixpkgs flake, referenced on the
above command line, is resolved to a repo using the nix registry and
installed from there. We'll cover some of the details of that
installation later. For now lets make a small adjustment to the
registry and reinstall the tool.
# remove the 1st package from our profile
$ nix profile remove 1
removing 'flake:nixpkgs#legacyPackages.aarch64-linux.cowsay'
# update the registry to point to an older nix version
$ nix registry add nixpkgs github:NixOS/nixpkgs/22.05
# reinstall cowsay, having updated the registry
$ nix profile install nixpkgs#cowsay
# run the usage again, note the different version
$ cowsay -h
cow{say,think} version 3.03, (c) 1999 Tony Monroe
Let's take a look at the current history of our profile:
$ nix profile history
Version 1 (2024-01-15):
nix: ∅ -> 2.19.2
Version 2 (2024-01-16) <- 1:
flake:nixpkgs#legacyPackages.aarch64-linux.cowsay: ∅ -> 3.7.0, 3.7.0-man
Version 3 (2024-01-16) <- 2:
flake:nixpkgs#legacyPackages.aarch64-linux.cowsay: 3.7.0, 3.7.0-man -> ∅
Version 4 (2024-01-16) <- 3:
flake:nixpkgs#legacyPackages.aarch64-linux.cowsay: ∅ -> 3.04, 3.04-man
We can see that the record of different states over time matches our behaviour in this tutorial. Moreover, it is possible to jump to any of those points in history, e.g.
$ nix profile rollback --to 2
switching profile from version 4 to 2
$ cowsay -h
.cowsay-wrapped version 3.7.0
Notice that switching between increments in time is super fast. All the necessary data is already stored locally in the nix store. Switching is just a case of re-ppointing the latest profile link to a different version.
Let's take a look at what the profile is:
$ ls -l ~/.nix-profile
lrwxrwxrwx 1 user nogroup 44 Jan 16 13:46 /home/user/.nix-profile -> /home/user/.local/state/nix/profiles/profile
$ ls -l /home/user/.local/state/nix/profiles/profile
lrwxrwxrwx 1 user user 14 Jan 16 14:38 /home/user/.local/state/nix/profiles/profile -> profile-2-link
$ ls -l /home/user/.local/state/nix/profiles/profile-2-link
lrwxrwxrwx 1 user user 51 Jan 16 14:38 /home/user/.local/state/nix/profiles/profile-2-link -> /nix/store/nwz02v3d71383p0ja5hkizbm92pg4myb-profile
$ ls -l /nix/store/nwz02v3d71383p0ja5hkizbm92pg4myb-profile
total 16
dr-xr-xr-x 2 user user 4096 Jan 1 1970 bin
lrwxrwxrwx 1 user user 58 Jan 1 1970 etc -> /nix/store/761hq0abn07nrydrf6mls61bscx2vz2i-nix-2.19.2/etc
lrwxrwxrwx 1 user user 58 Jan 1 1970 lib -> /nix/store/761hq0abn07nrydrf6mls61bscx2vz2i-nix-2.19.2/lib
lrwxrwxrwx 1 user user 62 Jan 1 1970 libexec -> /nix/store/761hq0abn07nrydrf6mls61bscx2vz2i-nix-2.19.2/libexec
-r--r--r-- 1 user user 457 Jan 1 1970 manifest.json
dr-xr-xr-x 3 user user 4096 Jan 1 1970 share
As can be seen above. The profile is essentially a tree formed with
symbolic links and, eventually, some actual files located in the
directory tree under /nix/store.
Let's take a look at cowsay:
$ ls -l $(which cowsay)
lrwxrwxrwx 1 user user 67 Jan 1 1970 /home/user/.nix-profile/bin/cowsay -> /nix/store/fsana1m00f7j3prz6w24zj2gfr7f6slw-cowsay-3.7.0/bin/cowsay
$ ls -l /nix/store/fsana1m00f7j3prz6w24zj2gfr7f6slw-cowsay-3.7.0/bin/cowsay
-r-xr-xr-x 1 user user 479 Jan 1 1970 /nix/store/fsana1m00f7j3prz6w24zj2gfr7f6slw-cowsay-3.7.0/bin/cowsay
The cowsay found on the path (in ~/.nix-profile/bin) is also a
symbolic link to a file found in the nix store.
The data in the nix store is the output of derivations, which are one
of the key ideas underlying nix. A derivation describes how to build
something; it can be composed of other derivations, has inputs and
outputs. We can examine the cowsay derivation like this:
$ nix derivation show nixpkgs#cowsay
{
"/nix/store/gncy6jhm8kv4v3d9xindj9n8v6xjdg86-cowsay-3.04.drv": {
"args": [
"-e",
"/nix/store/9krlzvny65gdc8s7kpb6lkx8cd02c25b-default-builder.sh"
...
The beady-eyed observer will note that this refers to version 3.04 of
cowsay and not 3.7.0. This is because our registry is still
pointing us at the nixpkgs version 22.05. We could examine the
local-2 version using, for example, the following command:
$ nix derivation show /home/user/.local/state/nix/profiles/profile-2-link/bin/cowsay
{
"/nix/store/jsf28v3hg3bywhb6yawlml4c9hjcd5dv-cowsay-3.7.0.drv": {
"args": [
"-e",
"/nix/store/v6x3cs394jgqfbi0a42pam708flxaphh-default-builder.sh"
],
"builder": "/nix/store/rapsfvcigspigw56szsb9gdl2iv6007x-bash-5.2-p21/bin/bash",
"env": {
"__structuredAttrs": "",
"buildInputs": "/nix/store/711bj8pn6mjzqldyp6inalzmzw2y4b4g-perl-5.38.2",
"builder": "/nix/store/rapsfvcigspigw56szsb9gdl2iv6007x-bash-5.2-p21/bin/bash",
"cmakeFlags": "",
"configureFlags": "",
"depsBuildBuild": "",
"depsBuildBuildPropagated": "",
"depsBuildTarget": "",
"depsBuildTargetPropagated": "",
"depsHostHost": "",
"depsHostHostPropagated": "",
"depsTargetTarget": "",
"depsTargetTargetPropagated": "",
"doCheck": "",
"doInstallCheck": "",
"makeFlags": "prefix=/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9",
"man": "/nix/store/hsa9qfla6946d4yl71bi1pkxbb3kjc1j-cowsay-3.7.0-man",
"mesonFlags": "",
"name": "cowsay-3.7.0",
"nativeBuildInputs": "/nix/store/bfw3j7pl0hdx34c8zd5mf5s27pyhbp09-make-shell-wrapper-hook",
"out": "/nix/store/fsana1m00f7j3prz6w24zj2gfr7f6slw-cowsay-3.7.0",
"outputs": "out man",
"patches": "/nix/store/2ph6g2p2r82ha64axqpw0pbclg5rq68m-9e129fa0933cf1837672c97f5ae5ad4a1a10ec11.patch",
"pname": "cowsay",
"postInstall": "wrapProgram $out/bin/cowsay \\\n --suffix COWPATH : $out/share/cowsay/cows\n",
"propagatedBuildInputs": "",
"propagatedNativeBuildInputs": "",
"src": "/nix/store/njxi0sclvggsdw89kr861q1bd8w7l902-source",
"stdenv": "/nix/store/m1c1464albqrb8wphbv2g76wi1vk6482-stdenv-linux",
"strictDeps": "",
"system": "aarch64-linux",
"version": "3.7.0"
},
"inputDrvs": {
"/nix/store/6iaydg7sw66arfhddq5ypk418yphljy0-source.drv": {
"dynamicOutputs": {},
"outputs": [
"out"
]
},
"/nix/store/dzgqcj3aqdf8663gcslkcqs26f2y40k2-9e129fa0933cf1837672c97f5ae5ad4a1a10ec11.patch.drv": {
"dynamicOutputs": {},
"outputs": [
"out"
]
},
"/nix/store/igrw9h6x8c0g86qvgljc5k5rs3x1zmgd-stdenv-linux.drv": {
"dynamicOutputs": {},
"outputs": [
"out"
]
},
"/nix/store/is1amsny8ja64knxvzrynliylyy9yd57-make-shell-wrapper-hook.drv": {
"dynamicOutputs": {},
"outputs": [
"out"
]
},
"/nix/store/mwliv79nidfji7cacwyfqv11vhkni8zn-perl-5.38.2.drv": {
"dynamicOutputs": {},
"outputs": [
"out"
]
},
"/nix/store/qkc79fsjcp3acn70hsr84f8r4jqxwjxy-bash-5.2-p21.drv": {
"dynamicOutputs": {},
"outputs": [
"out"
]
}
},
"inputSrcs": [
"/nix/store/v6x3cs394jgqfbi0a42pam708flxaphh-default-builder.sh"
],
"name": "cowsay-3.7.0",
"outputs": {
"man": {
"path": "/nix/store/hsa9qfla6946d4yl71bi1pkxbb3kjc1j-cowsay-3.7.0-man"
},
"out": {
"path": "/nix/store/fsana1m00f7j3prz6w24zj2gfr7f6slw-cowsay-3.7.0"
}
},
"system": "aarch64-linux"
}
}
Examining the above, we can see that the references output in the last
few lines of the derivation description matches the location of the
actual cowsay script that we uncovered by following the symlinks.
We'll come to derivations in more detail in a later section.