Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/source/command-ref/nix-env.md

@@ -0,0 +1,134 @@
+# Name
+
+`nix-env` - manipulate or query Nix user environments
+
+# Synopsis
+
+`nix-env` *operation* [*options*] [*arguments…*]
+  [`--option` *name* *value*]
+  [`--arg` *name* *value*]
+  [`--argstr` *name* *value*]
+  [{`--file` | `-f`} *path*]
+  [{`--profile` | `-p`} *path*]
+  [`--system-filter` *system*]
+  [`--dry-run`]
+
+# Description
+
+The command `nix-env` is used to manipulate Nix user environments. User
+environments are sets of software packages available to a user at some
+point in time. In other words, they are a synthesised view of the
+programs available in the Nix store. There may be many user
+environments: different users can have different environments, and
+individual users can switch between different environments.
+
+`nix-env` takes exactly one *operation* flag which indicates the
+subcommand to be performed. The following operations are available:
+
+- [`--install`](./nix-env/install.md)
+- [`--upgrade`](./nix-env/upgrade.md)
+- [`--uninstall`](./nix-env/uninstall.md)
+- [`--set`](./nix-env/set.md)
+- [`--set-flag`](./nix-env/set-flag.md)
+- [`--query`](./nix-env/query.md)
+- [`--switch-profile`](./nix-env/switch-profile.md)
+- [`--list-generations`](./nix-env/list-generations.md)
+- [`--delete-generations`](./nix-env/delete-generations.md)
+- [`--switch-generation`](./nix-env/switch-generation.md)
+- [`--rollback`](./nix-env/rollback.md)
+
+These pages can be viewed offline:
+
+- `man nix-env-<operation>`.
+
+  Example: `man nix-env-install`
+
+- `nix-env --help --<operation>`
+
+  Example: `nix-env --help --install`
+
+# Package sources
+
+`nix-env` can obtain packages from multiple sources:
+
+- An attribute set of derivations from:
+  - The [default Nix expression](@docroot@/command-ref/files/default-nix-expression.md) (by default)
+  - A Nix file, specified via `--file`
+  - A [profile](@docroot@/command-ref/files/profiles.md), specified via `--from-profile`
+  - A Nix expression that is a function which takes default expression as argument, specified via `--from-expression`
+- A [store path](@docroot@/store/store-path.md)
+
+# Selectors
+
+Several operations, such as [`nix-env --query`](./nix-env/query.md) and [`nix-env --install`](./nix-env/install.md), take a list of *arguments* that specify the packages on which to operate.
+
+Packages are identified based on a `name` part and a `version` part of a [symbolic derivation name](@docroot@/language/derivations.md#attr-name):
+
+- `name`: Everything up to but not including the first dash (`-`) that is *not* followed by a letter.
+- `version`: The rest, excluding the separating dash.
+
+> **Example**
+>
+> `nix-env` parses the symbolic derivation name `apache-httpd-2.0.48` as:
+>
+> ```json
+> {
+>   "name": "apache-httpd",
+>   "version": "2.0.48"
+> }
+> ```
+
+> **Example**
+>
+> `nix-env` parses the symbolic derivation name `firefox.*` as:
+>
+> ```json
+> {
+>   "name": "firefox.*",
+>   "version": ""
+> }
+> ```
+
+The `name` parts of the *arguments* to `nix-env` are treated as extended regular expressions and matched against the `name` parts of derivation names in the package source.
+The match is case-sensitive.
+The regular expression can optionally be followed by a dash (`-`) and a version number; if omitted, any version of the package will match.
+For details on regular expressions, see [**regex**(7)](https://linux.die.net/man/7/regex).
+
+> **Example**
+>
+> Common patterns for finding package names with `nix-env`:
+>
+> - `firefox`
+>
+>   Matches the package name `firefox` and any version.
+>
+> - `firefox-32.0`
+>
+>   Matches the package name `firefox` and version `32.0`.
+>
+> - `gtk\\+`
+>
+>   Matches the package name `gtk+`.
+>   The `+` character must be escaped using a backslash (`\`) to prevent it from being interpreted as a quantifier, and the backslash must be escaped in turn with another backslash to ensure that the shell passes it on.
+>
+> - `.\*`
+>
+>   Matches any package name.
+>   This is the default for most commands.
+>
+> - `'.*zip.*'`
+>
+>   Matches any package name containing the string `zip`.
+>   Note the dots: `'*zip*'` does not work, because in a regular expression, the character `*` is interpreted as a quantifier.
+>
+> - `'.*(firefox|chromium).*'`
+>
+>   Matches any package name containing the strings `firefox` or `chromium`.
+
+# Files
+
+`nix-env` operates on the following files.
+
+{{#include ./files/default-nix-expression.md}}
+
+{{#include ./files/profiles.md}}