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manual: Explain nix-env -E without -f #3208

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30 changes: 30 additions & 0 deletions doc/manual/command-ref/nix-env.xml
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Expand Up @@ -509,6 +509,36 @@ selecting the <literal>subversionWithJava</literal> attribute from the
set returned by calling the function defined in
<filename>./foo.nix</filename>.</para>

<para>To install an expression from a channel:

<screen>
$ nix-env -i -E 'channels: (channels.nixpkgs {}).git'</screen>

Here, <literal>-f</literal> is not given, so the expression is passed
as an argument the "default expressions" that
are setup by <command>nix-channel</command> by default in
<filename>~/.nix-defexpr</filename>.
That means we can access the channel from it (using the one called
<literal>nixpkgs</literal> in the example, as it is named in
<command>nix-channel --list</command>).</para>

<para>This also allows you to override packages:

<screen>
$ nix-env -i -E 'channels: (channels.nixpkgs {}).git.override { pythonSupport = false; }'</screen>

However, this is not recommended, because the overrides will not be remembered,
so the next update via <command>nix-env --upgrade</command> will undo them.
It is better to define such overrides in <filename>~/.config/nixpkgs/config.nix</filename>.</para>

<para>It is possible to install multiple derivations with an expression
by making it evaluate to an attribute set instead of a single derivation:

<screen>
$ nix-env -i -E 'channels: with channels.nixpkgs {}; { inherit git vim; }'</screen>

</para>

<para>A dry-run tells you which paths will be downloaded or built from
source:

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