Terms and Definitions in Nix Project

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Term	Context	Meaning	Related Links
User Environment	Nix	A set of "active" applications. These applications usually exist in the Nix store. A single Nix user may have multiple user environments. Profiles and generations are closely related.	Nix Manual - Basic Package Management chapter Nix Manual - Profiles chapter Nix Manual - nix-env
(User) Profile	Nix	Profiles simplify managing and switching between user environments, and thus control which applications and system configurations are in active use. Generally, a profile is a link to a generation, and the corresponding profiles folder collects a list of generations. A standalone Nix installation (i.e. on a Linux distro that is not NixOS) operates mainly on user profiles. In NixOS, there is also a system profile that manages the system-wide configuration (e.g. /etc, the kernel, initrd, systemd). Other tools like Home Manager may also have their own profiles. By default, a user's active profile is stored at ~/.nix-profile: $ ls -l ~/.nix-profile lrwxrwxrwx ... /home/username/.nix-profile -> /nix/var/nix/profiles/per-user/username/profile	Nix Manual: Profiles Nix Pills - The first profile
Generation	Nix	An instance of a user environment. When a user makes any change to their environment, such as installing or removing packages, a new generation of the environment is created instead of modifying the environment in-place. This ensures that updates are atomic and the user can easily roll-back to any previous generation if something goes wrong. The current generation is a user's currently active user profile.	Nix Manual: Profiles
Derivation	Nix	A Nix expression which describes a build action. Derivations are analogous to package definitions in other package managers. High-level derivations, such as the ones describing packages in Nixpkgs, get evaluated into low-level derivations (called store derivations), for instance by using the nix-instantiate command. nix-store --realise runs the build commands described in the low-level derivation, producing one or more output paths. nix-build is a user-friendly wrapper for the previous two commands.	Nix Manual: Glossary - Derivation Nix Manual: Derivation
Output path	Nix	A store path produced by a derivation. These are generally analogous to built packages, or pieces of them. $ ls -ld /nix/store/*-firefox-9*/ dr-xr-xr-x ... /nix/store/v4b8...3d0w-firefox-92.0/	Nix Manual: Derivation
rec { }	Nix expressions	The { } block contains "mutually recursive" attributes, which means they can refer to each other.	Nix Manual: Syntax and semantics: Recursive sets
expression evaluator	Nix	The part of the Nix program which reads and evaluates a Nix expression.	Nix Manual: Common Options --arg Nix Manual: Built-in Functions
stdenv	Nix expressions	An attribute which contains things expected in the most basic Unix environment. (e.g. Bash shell, gcc, cp, tar, grep, etc.)	all-packages.nix: stdenv =]
config.nix or nixpkgs-config.nix	NixOS Wiki	A Nix expression retrieved by and applied to the all-packages.nix Nix expression. This file enables an end-user to customize the Nix expressions contained in the community-owned NixPkgs list or to define entirely new Nix expressions to use with Nix commands. This file's path can be overridden by the NIXPKGS_CONFIG environment variable.	all-packages.nix: config NixPkgs Release Notes
attribute path	nix-env takes this if you pass the `-A` flag [1]	an unambiguous identifier for a package
symbolic package name	[2]	This string represents what you commonly think of as a package. There can be multiple packages with the symbolic name "hello".
selector	this term is used in nix-env error messages [3], it seems to be actually a DrvName struct [4] (a derivation name)	see "symbolic package name"
selection path	nix-shell error message [5]	see "attribute path"[6]
derivation name	manual[7], source code [8]	see "symbolic package name"
package name	IRC[9]	see "symbolic package name"
attribute selection path	source[10]	see "attribute path"