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<xi:include href="preface.chapter.xml" />
<part>
<title>Using Nixpkgs</title>
<xi:include href="using/configuration.xml" />
<xi:include href="using/overlays.xml" />
<xi:include href="using/overrides.xml" />
<xi:include href="using/configuration.chapter.xml" />
<xi:include href="using/overlays.chapter.xml" />
<xi:include href="using/overrides.chapter.xml" />
<xi:include href="functions.xml" />
</part>
<part>

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# Global configuration {#chap-packageconfig}
Nix comes with certain defaults about what packages can and cannot be installed, based on a package's metadata. By default, Nix will prevent installation if any of the following criteria are true:
- The package is thought to be broken, and has had its `meta.broken` set to `true`.
- The package isn't intended to run on the given system, as none of its `meta.platforms` match the given system.
- The package's `meta.license` is set to a license which is considered to be unfree.
- The package has known security vulnerabilities but has not or can not be updated for some reason, and a list of issues has been entered in to the package's `meta.knownVulnerabilities`.
Note that all this is checked during evaluation already, and the check includes any package that is evaluated. In particular, all build-time dependencies are checked. `nix-env -qa` will (attempt to) hide any packages that would be refused.
Each of these criteria can be altered in the nixpkgs configuration.
The nixpkgs configuration for a NixOS system is set in the `configuration.nix`, as in the following example:
```nix
{
nixpkgs.config = {
allowUnfree = true;
};
}
```
However, this does not allow unfree software for individual users. Their configurations are managed separately.
A user's nixpkgs configuration is stored in a user-specific configuration file located at `~/.config/nixpkgs/config.nix`. For example:
```nix
{
allowUnfree = true;
}
```
Note that we are not able to test or build unfree software on Hydra due to policy. Most unfree licenses prohibit us from either executing or distributing the software.
## Installing broken packages {#sec-allow-broken}
There are two ways to try compiling a package which has been marked as broken.
- For allowing the build of a broken package once, you can use an environment variable for a single invocation of the nix tools:
```shellSession
$ export NIXPKGS_ALLOW_BROKEN=1
```
- For permanently allowing broken packages to be built, you may add `allowBroken = true;` to your user's configuration file, like this:
```nix
{
allowBroken = true;
}
```
## Installing packages on unsupported systems {#sec-allow-unsupported-system}
There are also two ways to try compiling a package which has been marked as unsupported for the given system.
- For allowing the build of an unsupported package once, you can use an environment variable for a single invocation of the nix tools:
```shellSession
$ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1
```
- For permanently allowing unsupported packages to be built, you may add `allowUnsupportedSystem = true;` to your user's configuration file, like this:
```nix
{
allowUnsupportedSystem = true;
}
```
The difference between a package being unsupported on some system and being broken is admittedly a bit fuzzy. If a program *ought* to work on a certain platform, but doesn't, the platform should be included in `meta.platforms`, but marked as broken with e.g. `meta.broken = !hostPlatform.isWindows`. Of course, this begs the question of what \"ought\" means exactly. That is left to the package maintainer.
## Installing unfree packages {#sec-allow-unfree}
There are several ways to tweak how Nix handles a package which has been marked as unfree.
- To temporarily allow all unfree packages, you can use an environment variable for a single invocation of the nix tools:
```shellSession
$ export NIXPKGS_ALLOW_UNFREE=1
```
- It is possible to permanently allow individual unfree packages, while still blocking unfree packages by default using the `allowUnfreePredicate` configuration option in the user configuration file.
This option is a function which accepts a package as a parameter, and returns a boolean. The following example configuration accepts a package and always returns false:
```nix
{
allowUnfreePredicate = (pkg: false);
}
```
For a more useful example, try the following. This configuration only allows unfree packages named roon-server and visual studio code:
```nix
{
allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
"roon-server"
"vscode"
];
}
```
- It is also possible to allow and block licenses that are specifically acceptable or not acceptable, using `allowlistedLicenses` and `blocklistedLicenses`, respectively.
The following example configuration allowlists the licenses `amd` and `wtfpl`:
```nix
{
allowlistedLicenses = with lib.licenses; [ amd wtfpl ];
}
```
The following example configuration blocklists the `gpl3Only` and `agpl3Only` licenses:
```nix
{
blocklistedLicenses = with lib.licenses; [ agpl3Only gpl3Only ];
}
```
Note that `allowlistedLicenses` only applies to unfree licenses unless `allowUnfree` is enabled. It is not a generic allowlist for all types of licenses. `blocklistedLicenses` applies to all licenses.
A complete list of licenses can be found in the file `lib/licenses.nix` of the nixpkgs tree.
## Installing insecure packages {#sec-allow-insecure}
There are several ways to tweak how Nix handles a package which has been marked as insecure.
- To temporarily allow all insecure packages, you can use an environment variable for a single invocation of the nix tools:
```shellSession
$ export NIXPKGS_ALLOW_INSECURE=1
```
- It is possible to permanently allow individual insecure packages, while still blocking other insecure packages by default using the `permittedInsecurePackages` configuration option in the user configuration file.
The following example configuration permits the installation of the hypothetically insecure package `hello`, version `1.2.3`:
```nix
{
permittedInsecurePackages = [
"hello-1.2.3"
];
}
```
- It is also possible to create a custom policy around which insecure packages to allow and deny, by overriding the `allowInsecurePredicate` configuration option.
The `allowInsecurePredicate` option is a function which accepts a package and returns a boolean, much like `allowUnfreePredicate`.
The following configuration example only allows insecure packages with very short names:
```nix
{
allowInsecurePredicate = pkg: builtins.stringLength (lib.getName pkg) <= 5;
}
```
Note that `permittedInsecurePackages` is only checked if `allowInsecurePredicate` is not specified.
## Modify packages via `packageOverrides` {#sec-modify-via-packageOverrides}
You can define a function called `packageOverrides` in your local `~/.config/nixpkgs/config.nix` to override Nix packages. It must be a function that takes pkgs as an argument and returns a modified set of packages.
```nix
{
packageOverrides = pkgs: rec {
foo = pkgs.foo.override { ... };
};
}
```
## Declarative Package Management {#sec-declarative-package-management}
### Build an environment {#sec-building-environment}
Using `packageOverrides`, it is possible to manage packages declaratively. This means that we can list all of our desired packages within a declarative Nix expression. For example, to have `aspell`, `bc`, `ffmpeg`, `coreutils`, `gdb`, `nixUnstable`, `emscripten`, `jq`, `nox`, and `silver-searcher`, we could use the following in `~/.config/nixpkgs/config.nix`:
```nix
{
packageOverrides = pkgs: with pkgs; {
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
aspell
bc
coreutils
gdb
ffmpeg
nixUnstable
emscripten
jq
nox
silver-searcher
];
};
};
}
```
To install it into our environment, you can just run `nix-env -iA nixpkgs.myPackages`. If you want to load the packages to be built from a working copy of `nixpkgs` you just run `nix-env -f. -iA myPackages`. To explore what's been installed, just look through `~/.nix-profile/`. You can see that a lot of stuff has been installed. Some of this stuff is useful some of it isn't. Let's tell Nixpkgs to only link the stuff that we want:
```nix
{
packageOverrides = pkgs: with pkgs; {
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
aspell
bc
coreutils
gdb
ffmpeg
nixUnstable
emscripten
jq
nox
silver-searcher
];
pathsToLink = [ "/share" "/bin" ];
};
};
}
```
`pathsToLink` tells Nixpkgs to only link the paths listed which gets rid of the extra stuff in the profile. `/bin` and `/share` are good defaults for a user environment, getting rid of the clutter. If you are running on Nix on MacOS, you may want to add another path as well, `/Applications`, that makes GUI apps available.
### Getting documentation {#sec-getting-documentation}
After building that new environment, look through `~/.nix-profile` to make sure everything is there that we wanted. Discerning readers will note that some files are missing. Look inside `~/.nix-profile/share/man/man1/` to verify this. There are no man pages for any of the Nix tools! This is because some packages like Nix have multiple outputs for things like documentation (see section 4). Let's make Nix install those as well.
```nix
{
packageOverrides = pkgs: with pkgs; {
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
aspell
bc
coreutils
ffmpeg
nixUnstable
emscripten
jq
nox
silver-searcher
];
pathsToLink = [ "/share/man" "/share/doc" "/bin" ];
extraOutputsToInstall = [ "man" "doc" ];
};
};
}
```
This provides us with some useful documentation for using our packages. However, if we actually want those manpages to be detected by man, we need to set up our environment. This can also be managed within Nix expressions.
```nix
{
packageOverrides = pkgs: with pkgs; rec {
myProfile = writeText "my-profile" ''
export PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/sbin:/bin:/usr/sbin:/usr/bin
export MANPATH=$HOME/.nix-profile/share/man:/nix/var/nix/profiles/default/share/man:/usr/share/man
'';
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
(runCommand "profile" {} ''
mkdir -p $out/etc/profile.d
cp ${myProfile} $out/etc/profile.d/my-profile.sh
'')
aspell
bc
coreutils
ffmpeg
man
nixUnstable
emscripten
jq
nox
silver-searcher
];
pathsToLink = [ "/share/man" "/share/doc" "/bin" "/etc" ];
extraOutputsToInstall = [ "man" "doc" ];
};
};
}
```
For this to work fully, you must also have this script sourced when you are logged in. Try adding something like this to your `~/.profile` file:
```shellSession
#!/bin/sh
if [ -d $HOME/.nix-profile/etc/profile.d ]; then
for i in $HOME/.nix-profile/etc/profile.d/*.sh; do
if [ -r $i ]; then
. $i
fi
done
fi
```
Now just run `source $HOME/.profile` and you can starting loading man pages from your environment.
### GNU info setup {#sec-gnu-info-setup}
Configuring GNU info is a little bit trickier than man pages. To work correctly, info needs a database to be generated. This can be done with some small modifications to our environment scripts.
```nix
{
packageOverrides = pkgs: with pkgs; rec {
myProfile = writeText "my-profile" ''
export PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/sbin:/bin:/usr/sbin:/usr/bin
export MANPATH=$HOME/.nix-profile/share/man:/nix/var/nix/profiles/default/share/man:/usr/share/man
export INFOPATH=$HOME/.nix-profile/share/info:/nix/var/nix/profiles/default/share/info:/usr/share/info
'';
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
(runCommand "profile" {} ''
mkdir -p $out/etc/profile.d
cp ${myProfile} $out/etc/profile.d/my-profile.sh
'')
aspell
bc
coreutils
ffmpeg
man
nixUnstable
emscripten
jq
nox
silver-searcher
texinfoInteractive
];
pathsToLink = [ "/share/man" "/share/doc" "/share/info" "/bin" "/etc" ];
extraOutputsToInstall = [ "man" "doc" "info" ];
postBuild = ''
if [ -x $out/bin/install-info -a -w $out/share/info ]; then
shopt -s nullglob
for i in $out/share/info/*.info $out/share/info/*.info.gz; do
$out/bin/install-info $i $out/share/info/dir
done
fi
'';
};
};
}
```
`postBuild` tells Nixpkgs to run a command after building the environment. In this case, `install-info` adds the installed info pages to `dir` which is GNU info's default root node. Note that `texinfoInteractive` is added to the environment to give the `install-info` command.

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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-packageconfig">
<title>Global configuration</title>
<para>
Nix comes with certain defaults about what packages can and cannot be installed, based on a package's metadata. By default, Nix will prevent installation if any of the following criteria are true:
</para>
<itemizedlist>
<listitem>
<para>
The package is thought to be broken, and has had its <literal>meta.broken</literal> set to <literal>true</literal>.
</para>
</listitem>
<listitem>
<para>
The package isn't intended to run on the given system, as none of its <literal>meta.platforms</literal> match the given system.
</para>
</listitem>
<listitem>
<para>
The package's <literal>meta.license</literal> is set to a license which is considered to be unfree.
</para>
</listitem>
<listitem>
<para>
The package has known security vulnerabilities but has not or can not be updated for some reason, and a list of issues has been entered in to the package's <literal>meta.knownVulnerabilities</literal>.
</para>
</listitem>
</itemizedlist>
<para>
Note that all this is checked during evaluation already, and the check includes any package that is evaluated. In particular, all build-time dependencies are checked. <literal>nix-env -qa</literal> will (attempt to) hide any packages that would be refused.
</para>
<para>
Each of these criteria can be altered in the nixpkgs configuration.
</para>
<para>
The nixpkgs configuration for a NixOS system is set in the <literal>configuration.nix</literal>, as in the following example:
<programlisting>
{
nixpkgs.config = {
allowUnfree = true;
};
}
</programlisting>
However, this does not allow unfree software for individual users. Their configurations are managed separately.
</para>
<para>
A user's nixpkgs configuration is stored in a user-specific configuration file located at <filename>~/.config/nixpkgs/config.nix</filename>. For example:
<programlisting>
{
allowUnfree = true;
}
</programlisting>
</para>
<para>
Note that we are not able to test or build unfree software on Hydra due to policy. Most unfree licenses prohibit us from either executing or distributing the software.
</para>
<section xml:id="sec-allow-broken">
<title>Installing broken packages</title>
<para>
There are two ways to try compiling a package which has been marked as broken.
</para>
<itemizedlist>
<listitem>
<para>
For allowing the build of a broken package once, you can use an environment variable for a single invocation of the nix tools:
<screen><prompt>$ </prompt>export NIXPKGS_ALLOW_BROKEN=1</screen>
</para>
</listitem>
<listitem>
<para>
For permanently allowing broken packages to be built, you may add <literal>allowBroken = true;</literal> to your user's configuration file, like this:
<programlisting>
{
allowBroken = true;
}
</programlisting>
</para>
</listitem>
</itemizedlist>
</section>
<section xml:id="sec-allow-unsupported-system">
<title>Installing packages on unsupported systems</title>
<para>
There are also two ways to try compiling a package which has been marked as unsupported for the given system.
</para>
<itemizedlist>
<listitem>
<para>
For allowing the build of an unsupported package once, you can use an environment variable for a single invocation of the nix tools:
<screen><prompt>$ </prompt>export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1</screen>
</para>
</listitem>
<listitem>
<para>
For permanently allowing unsupported packages to be built, you may add <literal>allowUnsupportedSystem = true;</literal> to your user's configuration file, like this:
<programlisting>
{
allowUnsupportedSystem = true;
}
</programlisting>
</para>
</listitem>
</itemizedlist>
<para>
The difference between a package being unsupported on some system and being broken is admittedly a bit fuzzy. If a program <emphasis>ought</emphasis> to work on a certain platform, but doesn't, the platform should be included in <literal>meta.platforms</literal>, but marked as broken with e.g. <literal>meta.broken = !hostPlatform.isWindows</literal>. Of course, this begs the question of what "ought" means exactly. That is left to the package maintainer.
</para>
</section>
<section xml:id="sec-allow-unfree">
<title>Installing unfree packages</title>
<para>
There are several ways to tweak how Nix handles a package which has been marked as unfree.
</para>
<itemizedlist>
<listitem>
<para>
To temporarily allow all unfree packages, you can use an environment variable for a single invocation of the nix tools:
<screen><prompt>$ </prompt>export NIXPKGS_ALLOW_UNFREE=1</screen>
</para>
</listitem>
<listitem>
<para>
It is possible to permanently allow individual unfree packages, while still blocking unfree packages by default using the <literal>allowUnfreePredicate</literal> configuration option in the user configuration file.
</para>
<para>
This option is a function which accepts a package as a parameter, and returns a boolean. The following example configuration accepts a package and always returns false:
<programlisting>
{
allowUnfreePredicate = (pkg: false);
}
</programlisting>
</para>
<para>
For a more useful example, try the following. This configuration only allows unfree packages named roon-server and visual studio code:
<programlisting>
{
allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
"roon-server"
"vscode"
];
}
</programlisting>
</para>
</listitem>
<listitem>
<para>
It is also possible to allow and block licenses that are specifically acceptable or not acceptable, using <literal>allowlistedLicenses</literal> and <literal>blocklistedLicenses</literal>, respectively.
</para>
<para>
The following example configuration allowlists the licenses <literal>amd</literal> and <literal>wtfpl</literal>:
<programlisting>
{
allowlistedLicenses = with lib.licenses; [ amd wtfpl ];
}
</programlisting>
</para>
<para>
The following example configuration blocklists the <literal>gpl3Only</literal> and <literal>agpl3Only</literal> licenses:
<programlisting>
{
blocklistedLicenses = with lib.licenses; [ agpl3Only gpl3Only ];
}
</programlisting>
</para>
<para>
Note that <literal>allowlistedLicenses</literal> only applies to unfree licenses unless <literal>allowUnfree</literal> is enabled. It is not a generic allowlist for all types of licenses. <literal>blocklistedLicenses</literal> applies to all licenses.
</para>
</listitem>
</itemizedlist>
<para>
A complete list of licenses can be found in the file <filename>lib/licenses.nix</filename> of the nixpkgs tree.
</para>
</section>
<section xml:id="sec-allow-insecure">
<title>Installing insecure packages</title>
<para>
There are several ways to tweak how Nix handles a package which has been marked as insecure.
</para>
<itemizedlist>
<listitem>
<para>
To temporarily allow all insecure packages, you can use an environment variable for a single invocation of the nix tools:
<screen><prompt>$ </prompt>export NIXPKGS_ALLOW_INSECURE=1</screen>
</para>
</listitem>
<listitem>
<para>
It is possible to permanently allow individual insecure packages, while still blocking other insecure packages by default using the <literal>permittedInsecurePackages</literal> configuration option in the user configuration file.
</para>
<para>
The following example configuration permits the installation of the hypothetically insecure package <literal>hello</literal>, version <literal>1.2.3</literal>:
<programlisting>
{
permittedInsecurePackages = [
"hello-1.2.3"
];
}
</programlisting>
</para>
</listitem>
<listitem>
<para>
It is also possible to create a custom policy around which insecure packages to allow and deny, by overriding the <literal>allowInsecurePredicate</literal> configuration option.
</para>
<para>
The <literal>allowInsecurePredicate</literal> option is a function which accepts a package and returns a boolean, much like <literal>allowUnfreePredicate</literal>.
</para>
<para>
The following configuration example only allows insecure packages with very short names:
<programlisting>
{
allowInsecurePredicate = pkg: builtins.stringLength (lib.getName pkg) &lt;= 5;
}
</programlisting>
</para>
<para>
Note that <literal>permittedInsecurePackages</literal> is only checked if <literal>allowInsecurePredicate</literal> is not specified.
</para>
</listitem>
</itemizedlist>
</section>
<!--============================================================-->
<section xml:id="sec-modify-via-packageOverrides">
<title>Modify packages via <literal>packageOverrides</literal></title>
<para>
You can define a function called <varname>packageOverrides</varname> in your local <filename>~/.config/nixpkgs/config.nix</filename> to override Nix packages. It must be a function that takes pkgs as an argument and returns a modified set of packages.
<programlisting>
{
packageOverrides = pkgs: rec {
foo = pkgs.foo.override { ... };
};
}
</programlisting>
</para>
</section>
<section xml:id="sec-declarative-package-management">
<title>Declarative Package Management</title>
<section xml:id="sec-building-environment">
<title>Build an environment</title>
<para>
Using <literal>packageOverrides</literal>, it is possible to manage packages declaratively. This means that we can list all of our desired packages within a declarative Nix expression. For example, to have <literal>aspell</literal>, <literal>bc</literal>, <literal>ffmpeg</literal>, <literal>coreutils</literal>, <literal>gdb</literal>, <literal>nixUnstable</literal>, <literal>emscripten</literal>, <literal>jq</literal>, <literal>nox</literal>, and <literal>silver-searcher</literal>, we could use the following in <filename>~/.config/nixpkgs/config.nix</filename>:
</para>
<screen>
{
packageOverrides = pkgs: with pkgs; {
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
aspell
bc
coreutils
gdb
ffmpeg
nixUnstable
emscripten
jq
nox
silver-searcher
];
};
};
}
</screen>
<para>
To install it into our environment, you can just run <literal>nix-env -iA nixpkgs.myPackages</literal>. If you want to load the packages to be built from a working copy of <literal>nixpkgs</literal> you just run <literal>nix-env -f. -iA myPackages</literal>. To explore what's been installed, just look through <filename>~/.nix-profile/</filename>. You can see that a lot of stuff has been installed. Some of this stuff is useful some of it isn't. Let's tell Nixpkgs to only link the stuff that we want:
</para>
<screen>
{
packageOverrides = pkgs: with pkgs; {
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
aspell
bc
coreutils
gdb
ffmpeg
nixUnstable
emscripten
jq
nox
silver-searcher
];
pathsToLink = [ "/share" "/bin" ];
};
};
}
</screen>
<para>
<literal>pathsToLink</literal> tells Nixpkgs to only link the paths listed which gets rid of the extra stuff in the profile. <filename>/bin</filename> and <filename>/share</filename> are good defaults for a user environment, getting rid of the clutter. If you are running on Nix on MacOS, you may want to add another path as well, <filename>/Applications</filename>, that makes GUI apps available.
</para>
</section>
<section xml:id="sec-getting-documentation">
<title>Getting documentation</title>
<para>
After building that new environment, look through <filename>~/.nix-profile</filename> to make sure everything is there that we wanted. Discerning readers will note that some files are missing. Look inside <filename>~/.nix-profile/share/man/man1/</filename> to verify this. There are no man pages for any of the Nix tools! This is because some packages like Nix have multiple outputs for things like documentation (see section 4). Let's make Nix install those as well.
</para>
<screen>
{
packageOverrides = pkgs: with pkgs; {
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
aspell
bc
coreutils
ffmpeg
nixUnstable
emscripten
jq
nox
silver-searcher
];
pathsToLink = [ "/share/man" "/share/doc" "/bin" ];
extraOutputsToInstall = [ "man" "doc" ];
};
};
}
</screen>
<para>
This provides us with some useful documentation for using our packages. However, if we actually want those manpages to be detected by man, we need to set up our environment. This can also be managed within Nix expressions.
</para>
<screen>
{
packageOverrides = pkgs: with pkgs; rec {
myProfile = writeText "my-profile" ''
export PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/sbin:/bin:/usr/sbin:/usr/bin
export MANPATH=$HOME/.nix-profile/share/man:/nix/var/nix/profiles/default/share/man:/usr/share/man
'';
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
(runCommand "profile" {} ''
mkdir -p $out/etc/profile.d
cp ${myProfile} $out/etc/profile.d/my-profile.sh
'')
aspell
bc
coreutils
ffmpeg
man
nixUnstable
emscripten
jq
nox
silver-searcher
];
pathsToLink = [ "/share/man" "/share/doc" "/bin" "/etc" ];
extraOutputsToInstall = [ "man" "doc" ];
};
};
}
</screen>
<para>
For this to work fully, you must also have this script sourced when you are logged in. Try adding something like this to your <filename>~/.profile</filename> file:
</para>
<screen>
#!/bin/sh
if [ -d $HOME/.nix-profile/etc/profile.d ]; then
for i in $HOME/.nix-profile/etc/profile.d/*.sh; do
if [ -r $i ]; then
. $i
fi
done
fi
</screen>
<para>
Now just run <literal>source $HOME/.profile</literal> and you can starting loading man pages from your environment.
</para>
</section>
<section xml:id="sec-gnu-info-setup">
<title>GNU info setup</title>
<para>
Configuring GNU info is a little bit trickier than man pages. To work correctly, info needs a database to be generated. This can be done with some small modifications to our environment scripts.
</para>
<screen>
{
packageOverrides = pkgs: with pkgs; rec {
myProfile = writeText "my-profile" ''
export PATH=$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin:/sbin:/bin:/usr/sbin:/usr/bin
export MANPATH=$HOME/.nix-profile/share/man:/nix/var/nix/profiles/default/share/man:/usr/share/man
export INFOPATH=$HOME/.nix-profile/share/info:/nix/var/nix/profiles/default/share/info:/usr/share/info
'';
myPackages = pkgs.buildEnv {
name = "my-packages";
paths = [
(runCommand "profile" {} ''
mkdir -p $out/etc/profile.d
cp ${myProfile} $out/etc/profile.d/my-profile.sh
'')
aspell
bc
coreutils
ffmpeg
man
nixUnstable
emscripten
jq
nox
silver-searcher
texinfoInteractive
];
pathsToLink = [ "/share/man" "/share/doc" "/share/info" "/bin" "/etc" ];
extraOutputsToInstall = [ "man" "doc" "info" ];
postBuild = ''
if [ -x $out/bin/install-info -a -w $out/share/info ]; then
shopt -s nullglob
for i in $out/share/info/*.info $out/share/info/*.info.gz; do
$out/bin/install-info $i $out/share/info/dir
done
fi
'';
};
};
}
</screen>
<para>
<literal>postBuild</literal> tells Nixpkgs to run a command after building the environment. In this case, <literal>install-info</literal> adds the installed info pages to <literal>dir</literal> which is GNU info's default root node. Note that <literal>texinfoInteractive</literal> is added to the environment to give the <literal>install-info</literal> command.
</para>
</section>
</section>
</chapter>

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# Overlays {#chap-overlays}
This chapter describes how to extend and change Nixpkgs using overlays. Overlays are used to add layers in the fixed-point used by Nixpkgs to compose the set of all packages.
Nixpkgs can be configured with a list of overlays, which are applied in order. This means that the order of the overlays can be significant if multiple layers override the same package.
## Installing overlays {#sec-overlays-install}
The list of overlays can be set either explicitly in a Nix expression, or through `<nixpkgs-overlays>` or user configuration files.
### Set overlays in NixOS or Nix expressions {#sec-overlays-argument}
On a NixOS system the value of the `nixpkgs.overlays` option, if present, is passed to the system Nixpkgs directly as an argument. Note that this does not affect the overlays for non-NixOS operations (e.g. `nix-env`), which are [looked up](#sec-overlays-lookup) independently.
The list of overlays can be passed explicitly when importing nixpkgs, for example `import <nixpkgs> { overlays = [ overlay1 overlay2 ]; }`.
NOTE: DO NOT USE THIS in nixpkgs. Further overlays can be added by calling the `pkgs.extend` or `pkgs.appendOverlays`, although it is often preferable to avoid these functions, because they recompute the Nixpkgs fixpoint, which is somewhat expensive to do.
### Install overlays via configuration lookup {#sec-overlays-lookup}
The list of overlays is determined as follows.
1. First, if an [`overlays` argument](#sec-overlays-argument) to the Nixpkgs function itself is given, then that is used and no path lookup will be performed.
2. Otherwise, if the Nix path entry `<nixpkgs-overlays>` exists, we look for overlays at that path, as described below.
See the section on `NIX_PATH` in the Nix manual for more details on how to set a value for `<nixpkgs-overlays>.`
3. If one of `~/.config/nixpkgs/overlays.nix` and `~/.config/nixpkgs/overlays/` exists, then we look for overlays at that path, as described below. It is an error if both exist.
If we are looking for overlays at a path, then there are two cases:
- If the path is a file, then the file is imported as a Nix expression and used as the list of overlays.
- If the path is a directory, then we take the content of the directory, order it lexicographically, and attempt to interpret each as an overlay by:
- Importing the file, if it is a `.nix` file.
- Importing a top-level `default.nix` file, if it is a directory.
Because overlays that are set in NixOS configuration do not affect non-NixOS operations such as `nix-env`, the `overlays.nix` option provides a convenient way to use the same overlays for a NixOS system configuration and user configuration: the same file can be used as `overlays.nix` and imported as the value of `nixpkgs.overlays`.
## Defining overlays {#sec-overlays-definition}
Overlays are Nix functions which accept two arguments, conventionally called `self` and `super`, and return a set of packages. For example, the following is a valid overlay.
```nix
self: super:
{
boost = super.boost.override {
python = self.python3;
};
rr = super.callPackage ./pkgs/rr {
stdenv = self.stdenv_32bit;
};
}
```
The first argument (`self`) corresponds to the final package set. You should use this set for the dependencies of all packages specified in your overlay. For example, all the dependencies of `rr` in the example above come from `self`, as well as the overridden dependencies used in the `boost` override.
The second argument (`super`) corresponds to the result of the evaluation of the previous stages of Nixpkgs. It does not contain any of the packages added by the current overlay, nor any of the following overlays. This set should be used either to refer to packages you wish to override, or to access functions defined in Nixpkgs. For example, the original recipe of `boost` in the above example, comes from `super`, as well as the `callPackage` function.
The value returned by this function should be a set similar to `pkgs/top-level/all-packages.nix`, containing overridden and/or new packages.
Overlays are similar to other methods for customizing Nixpkgs, in particular the `packageOverrides` attribute described in <xref linkend="sec-modify-via-packageOverrides"/>. Indeed, `packageOverrides` acts as an overlay with only the `super` argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.
## Using overlays to configure alternatives {#sec-overlays-alternatives}
Certain software packages have different implementations of the same interface. Other distributions have functionality to switch between these. For example, Debian provides [DebianAlternatives](https://wiki.debian.org/DebianAlternatives). Nixpkgs has what we call `alternatives`, which are configured through overlays.
### BLAS/LAPACK {#sec-overlays-alternatives-blas-lapack}
In Nixpkgs, we have multiple implementations of the BLAS/LAPACK numerical linear algebra interfaces. They are:
- [OpenBLAS](https://www.openblas.net/)
The Nixpkgs attribute is `openblas` for ILP64 (integer width = 64 bits) and `openblasCompat` for LP64 (integer width = 32 bits). `openblasCompat` is the default.
- [LAPACK reference](http://www.netlib.org/lapack/) (also provides BLAS)
The Nixpkgs attribute is `lapack-reference`.
- [Intel MKL](https://software.intel.com/en-us/mkl) (only works on the x86_64 architecture, unfree)
The Nixpkgs attribute is `mkl`.
- [BLIS](https://github.com/flame/blis)
BLIS, available through the attribute `blis`, is a framework for linear algebra kernels. In addition, it implements the BLAS interface.
- [AMD BLIS/LIBFLAME](https://developer.amd.com/amd-aocl/blas-library/) (optimized for modern AMD x86_64 CPUs)
The AMD fork of the BLIS library, with attribute `amd-blis`, extends BLIS with optimizations for modern AMD CPUs. The changes are usually submitted to the upstream BLIS project after some time. However, AMD BLIS typically provides some performance improvements on AMD Zen CPUs. The complementary AMD LIBFLAME library, with attribute `amd-libflame`, provides a LAPACK implementation.
Introduced in [PR #83888](https://github.com/NixOS/nixpkgs/pull/83888), we are able to override the `blas` and `lapack` packages to use different implementations, through the `blasProvider` and `lapackProvider` argument. This can be used to select a different provider. BLAS providers will have symlinks in `$out/lib/libblas.so.3` and `$out/lib/libcblas.so.3` to their respective BLAS libraries. Likewise, LAPACK providers will have symlinks in `$out/lib/liblapack.so.3` and `$out/lib/liblapacke.so.3` to their respective LAPACK libraries. For example, Intel MKL is both a BLAS and LAPACK provider. An overlay can be created to use Intel MKL that looks like:
```nix
self: super:
{
blas = super.blas.override {
blasProvider = self.mkl;
};
lapack = super.lapack.override {
lapackProvider = self.mkl;
};
}
```
This overlay uses Intel's MKL library for both BLAS and LAPACK interfaces. Note that the same can be accomplished at runtime using `LD_LIBRARY_PATH` of `libblas.so.3` and `liblapack.so.3`. For instance:
```shellSession
$ LD_LIBRARY_PATH=$(nix-build -A mkl)/lib:$LD_LIBRARY_PATH nix-shell -p octave --run octave
```
Intel MKL requires an `openmp` implementation when running with multiple processors. By default, `mkl` will use Intel's `iomp` implementation if no other is specified, but this is a runtime-only dependency and binary compatible with the LLVM implementation. To use that one instead, Intel recommends users set it with `LD_PRELOAD`. Note that `mkl` is only available on `x86_64-linux` and `x86_64-darwin`. Moreover, Hydra is not building and distributing pre-compiled binaries using it.
For BLAS/LAPACK switching to work correctly, all packages must depend on `blas` or `lapack`. This ensures that only one BLAS/LAPACK library is used at one time. There are two versions of BLAS/LAPACK currently in the wild, `LP64` (integer size = 32 bits) and `ILP64` (integer size = 64 bits). Some software needs special flags or patches to work with `ILP64`. You can check if `ILP64` is used in Nixpkgs with `blas.isILP64` and `lapack.isILP64`. Some software does NOT work with `ILP64`, and derivations need to specify an assertion to prevent this. You can prevent `ILP64` from being used with the following:
```nix
{ stdenv, blas, lapack, ... }:
assert (!blas.isILP64) && (!lapack.isILP64);
stdenv.mkDerivation {
...
}
```
### Switching the MPI implementation {#sec-overlays-alternatives-mpi}
All programs that are built with [MPI](https://en.wikipedia.org/wiki/Message_Passing_Interface) support use the generic attribute `mpi` as an input. At the moment Nixpkgs natively provides two different MPI implementations:
- [Open MPI](https://www.open-mpi.org/) (default), attribute name
`openmpi`
- [MPICH](https://www.mpich.org/), attribute name `mpich`
To provide MPI enabled applications that use `MPICH`, instead of the default `Open MPI`, simply use the following overlay:
```nix
self: super:
{
mpi = self.mpich;
}
```

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@ -1,279 +0,0 @@
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-overlays">
<title>Overlays</title>
<para>
This chapter describes how to extend and change Nixpkgs using overlays. Overlays are used to add layers in the fixed-point used by Nixpkgs to compose the set of all packages.
</para>
<para>
Nixpkgs can be configured with a list of overlays, which are applied in order. This means that the order of the overlays can be significant if multiple layers override the same package.
</para>
<!--============================================================-->
<section xml:id="sec-overlays-install">
<title>Installing overlays</title>
<para>
The list of overlays can be set either explicitly in a Nix expression, or through <literal>&lt;nixpkgs-overlays></literal> or user configuration files.
</para>
<section xml:id="sec-overlays-argument">
<title>Set overlays in NixOS or Nix expressions</title>
<para>
On a NixOS system the value of the <literal>nixpkgs.overlays</literal> option, if present, is passed to the system Nixpkgs directly as an argument. Note that this does not affect the overlays for non-NixOS operations (e.g. <literal>nix-env</literal>), which are <link xlink:href="#sec-overlays-lookup">looked</link> up independently.
</para>
<para>
The list of overlays can be passed explicitly when importing nixpkgs, for example <literal>import &lt;nixpkgs> { overlays = [ overlay1 overlay2 ]; }</literal>.
</para>
<para>
NOTE: DO NOT USE THIS in nixpkgs. Further overlays can be added by calling the <literal>pkgs.extend</literal> or <literal>pkgs.appendOverlays</literal>, although it is often preferable to avoid these functions, because they recompute the Nixpkgs fixpoint, which is somewhat expensive to do.
</para>
</section>
<section xml:id="sec-overlays-lookup">
<title>Install overlays via configuration lookup</title>
<para>
The list of overlays is determined as follows.
</para>
<para>
<orderedlist>
<listitem>
<para>
First, if an <link xlink:href="#sec-overlays-argument"><varname>overlays</varname> argument</link> to the Nixpkgs function itself is given, then that is used and no path lookup will be performed.
</para>
</listitem>
<listitem>
<para>
Otherwise, if the Nix path entry <literal>&lt;nixpkgs-overlays></literal> exists, we look for overlays at that path, as described below.
</para>
<para>
See the section on <literal>NIX_PATH</literal> in the Nix manual for more details on how to set a value for <literal>&lt;nixpkgs-overlays>.</literal>
</para>
</listitem>
<listitem>
<para>
If one of <filename>~/.config/nixpkgs/overlays.nix</filename> and <filename>~/.config/nixpkgs/overlays/</filename> exists, then we look for overlays at that path, as described below. It is an error if both exist.
</para>
</listitem>
</orderedlist>
</para>
<para>
If we are looking for overlays at a path, then there are two cases:
<itemizedlist>
<listitem>
<para>
If the path is a file, then the file is imported as a Nix expression and used as the list of overlays.
</para>
</listitem>
<listitem>
<para>
If the path is a directory, then we take the content of the directory, order it lexicographically, and attempt to interpret each as an overlay by:
<itemizedlist>
<listitem>
<para>
Importing the file, if it is a <literal>.nix</literal> file.
</para>
</listitem>
<listitem>
<para>
Importing a top-level <filename>default.nix</filename> file, if it is a directory.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</itemizedlist>
</para>
<para>
Because overlays that are set in NixOS configuration do not affect non-NixOS operations such as <literal>nix-env</literal>, the <filename>overlays.nix</filename> option provides a convenient way to use the same overlays for a NixOS system configuration and user configuration: the same file can be used as <filename>overlays.nix</filename> and imported as the value of <literal>nixpkgs.overlays</literal>.
</para>
<!-- TODO: Example of sharing overlays between NixOS configuration
and configuration lookup. Also reference the example
from the sec-overlays-argument paragraph about NixOS.
-->
</section>
</section>
<!--============================================================-->
<section xml:id="sec-overlays-definition">
<title>Defining overlays</title>
<para>
Overlays are Nix functions which accept two arguments, conventionally called <varname>self</varname> and <varname>super</varname>, and return a set of packages. For example, the following is a valid overlay.
</para>
<programlisting>
self: super:
{
boost = super.boost.override {
python = self.python3;
};
rr = super.callPackage ./pkgs/rr {
stdenv = self.stdenv_32bit;
};
}
</programlisting>
<para>
The first argument (<varname>self</varname>) corresponds to the final package set. You should use this set for the dependencies of all packages specified in your overlay. For example, all the dependencies of <varname>rr</varname> in the example above come from <varname>self</varname>, as well as the overridden dependencies used in the <varname>boost</varname> override.
</para>
<para>
The second argument (<varname>super</varname>) corresponds to the result of the evaluation of the previous stages of Nixpkgs. It does not contain any of the packages added by the current overlay, nor any of the following overlays. This set should be used either to refer to packages you wish to override, or to access functions defined in Nixpkgs. For example, the original recipe of <varname>boost</varname> in the above example, comes from <varname>super</varname>, as well as the <varname>callPackage</varname> function.
</para>
<para>
The value returned by this function should be a set similar to <filename>pkgs/top-level/all-packages.nix</filename>, containing overridden and/or new packages.
</para>
<para>
Overlays are similar to other methods for customizing Nixpkgs, in particular the <literal>packageOverrides</literal> attribute described in <xref linkend="sec-modify-via-packageOverrides"/>. Indeed, <literal>packageOverrides</literal> acts as an overlay with only the <varname>super</varname> argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.
</para>
</section>
<section xml:id="sec-overlays-alternatives">
<title>Using overlays to configure alternatives</title>
<para>
Certain software packages have different implementations of the same interface. Other distributions have functionality to switch between these. For example, Debian provides <link
xlink:href="https://wiki.debian.org/DebianAlternatives">DebianAlternatives</link>. Nixpkgs has what we call <literal>alternatives</literal>, which are configured through overlays.
</para>
<section xml:id="sec-overlays-alternatives-blas-lapack">
<title>BLAS/LAPACK</title>
<para>
In Nixpkgs, we have multiple implementations of the BLAS/LAPACK numerical linear algebra interfaces. They are:
</para>
<itemizedlist>
<listitem>
<para>
<link xlink:href="https://www.openblas.net/">OpenBLAS</link>
</para>
<para>
The Nixpkgs attribute is <literal>openblas</literal> for ILP64 (integer width = 64 bits) and <literal>openblasCompat</literal> for LP64 (integer width = 32 bits). <literal>openblasCompat</literal> is the default.
</para>
</listitem>
<listitem>
<para>
<link xlink:href="http://www.netlib.org/lapack/">LAPACK reference</link> (also provides BLAS)
</para>
<para>
The Nixpkgs attribute is <literal>lapack-reference</literal>.
</para>
</listitem>
<listitem>
<para>
<link
xlink:href="https://software.intel.com/en-us/mkl">Intel MKL</link> (only works on the x86_64 architecture, unfree)
</para>
<para>
The Nixpkgs attribute is <literal>mkl</literal>.
</para>
</listitem>
<listitem>
<para>
<link
xlink:href="https://github.com/flame/blis">BLIS</link>
</para>
<para>
BLIS, available through the attribute <literal>blis</literal>, is a framework for linear algebra kernels. In addition, it implements the BLAS interface.
</para>
</listitem>
<listitem>
<para>
<link
xlink:href="https://developer.amd.com/amd-aocl/blas-library/">AMD BLIS/LIBFLAME</link> (optimized for modern AMD x86_64 CPUs)
</para>
<para>
The AMD fork of the BLIS library, with attribute <literal>amd-blis</literal>, extends BLIS with optimizations for modern AMD CPUs. The changes are usually submitted to the upstream BLIS project after some time. However, AMD BLIS typically provides some performance improvements on AMD Zen CPUs. The complementary AMD LIBFLAME library, with attribute <literal>amd-libflame</literal>, provides a LAPACK implementation.
</para>
</listitem>
</itemizedlist>
<para>
Introduced in <link
xlink:href="https://github.com/NixOS/nixpkgs/pull/83888">PR #83888</link>, we are able to override the <literal>blas</literal> and <literal>lapack</literal> packages to use different implementations, through the <literal>blasProvider</literal> and <literal>lapackProvider</literal> argument. This can be used to select a different provider. BLAS providers will have symlinks in <literal>$out/lib/libblas.so.3</literal> and <literal>$out/lib/libcblas.so.3</literal> to their respective BLAS libraries. Likewise, LAPACK providers will have symlinks in <literal>$out/lib/liblapack.so.3</literal> and <literal>$out/lib/liblapacke.so.3</literal> to their respective LAPACK libraries. For example, Intel MKL is both a BLAS and LAPACK provider. An overlay can be created to use Intel MKL that looks like:
</para>
<programlisting>
self: super:
{
blas = super.blas.override {
blasProvider = self.mkl;
};
lapack = super.lapack.override {
lapackProvider = self.mkl;
};
}
</programlisting>
<para>
This overlay uses Intel’s MKL library for both BLAS and LAPACK interfaces. Note that the same can be accomplished at runtime using <literal>LD_LIBRARY_PATH</literal> of <literal>libblas.so.3</literal> and <literal>liblapack.so.3</literal>. For instance:
</para>
<screen>
<prompt>$ </prompt>LD_LIBRARY_PATH=$(nix-build -A mkl)/lib:$LD_LIBRARY_PATH nix-shell -p octave --run octave
</screen>
<para>
Intel MKL requires an <literal>openmp</literal> implementation when running with multiple processors. By default, <literal>mkl</literal> will use Intel’s <literal>iomp</literal> implementation if no other is specified, but this is a runtime-only dependency and binary compatible with the LLVM implementation. To use that one instead, Intel recommends users set it with <literal>LD_PRELOAD</literal>. Note that <literal>mkl</literal> is only available on <literal>x86_64-linux</literal> and <literal>x86_64-darwin</literal>. Moreover, Hydra is not building and distributing pre-compiled binaries using it.
</para>
<para>
For BLAS/LAPACK switching to work correctly, all packages must depend on <literal>blas</literal> or <literal>lapack</literal>. This ensures that only one BLAS/LAPACK library is used at one time. There are two versions of BLAS/LAPACK currently in the wild, <literal>LP64</literal> (integer size = 32 bits) and <literal>ILP64</literal> (integer size = 64 bits). Some software needs special flags or patches to work with <literal>ILP64</literal>. You can check if <literal>ILP64</literal> is used in Nixpkgs with <varname>blas.isILP64</varname> and <varname>lapack.isILP64</varname>. Some software does NOT work with <literal>ILP64</literal>, and derivations need to specify an assertion to prevent this. You can prevent <literal>ILP64</literal> from being used with the following:
</para>
<programlisting>
{ stdenv, blas, lapack, ... }:
assert (!blas.isILP64) &amp;&amp; (!lapack.isILP64);
stdenv.mkDerivation {
...
}
</programlisting>
</section>
<section xml:id="sec-overlays-alternatives-mpi">
<title>Switching the MPI implementation</title>
<para>
All programs that are built with <link xlink:href="https://en.wikipedia.org/wiki/Message_Passing_Interface">MPI</link> support use the generic attribute <varname>mpi</varname> as an input. At the moment Nixpkgs natively provides two different MPI implementations:
<itemizedlist>
<listitem>
<para>
<link xlink:href="https://www.open-mpi.org/">Open MPI</link> (default), attribute name <varname>openmpi</varname>
</para>
</listitem>
<listitem>
<para>
<link xlink:href="https://www.mpich.org/">MPICH</link>, attribute name <varname>mpich</varname>
</para>
</listitem>
</itemizedlist>
</para>
<para>
To provide MPI enabled applications that use <literal>MPICH</literal>, instead of the default <literal>Open MPI</literal>, simply use the following overlay:
</para>
<programlisting>
self: super:
{
mpi = self.mpich;
}
</programlisting>
</section>
</section>
</chapter>

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# Overriding {#chap-overrides}
Sometimes one wants to override parts of `nixpkgs`, e.g. derivation attributes, the results of derivations.
These functions are used to make changes to packages, returning only single packages. [Overlays](#chap-overlays), on the other hand, can be used to combine the overridden packages across the entire package set of Nixpkgs.
## &lt;pkg&gt;.override {#sec-pkg-override}
The function `override` is usually available for all the derivations in the nixpkgs expression (`pkgs`).
It is used to override the arguments passed to a function.
Example usages:
```nix
pkgs.foo.override { arg1 = val1; arg2 = val2; ... }
```
<!-- TODO: move below programlisting to a new section about extending and overlays and reference it -->
```nix
import pkgs.path { overlays = [ (self: super: {
foo = super.foo.override { barSupport = true ; };
})]};
```
```nix
mypkg = pkgs.callPackage ./mypkg.nix {
mydep = pkgs.mydep.override { ... };
}
```
In the first example, `pkgs.foo` is the result of a function call with some default arguments, usually a derivation. Using `pkgs.foo.override` will call the same function with the given new arguments.
## &lt;pkg&gt;.overrideAttrs {#sec-pkg-overrideAttrs}
The function `overrideAttrs` allows overriding the attribute set passed to a `stdenv.mkDerivation` call, producing a new derivation based on the original one. This function is available on all derivations produced by the `stdenv.mkDerivation` function, which is most packages in the nixpkgs expression `pkgs`.
Example usage:
```nix
helloWithDebug = pkgs.hello.overrideAttrs (oldAttrs: rec {
separateDebugInfo = true;
});
```
In the above example, the `separateDebugInfo` attribute is overridden to be true, thus building debug info for `helloWithDebug`, while all other attributes will be retained from the original `hello` package.
The argument `oldAttrs` is conventionally used to refer to the attr set originally passed to `stdenv.mkDerivation`.
::: note
Note that `separateDebugInfo` is processed only by the `stdenv.mkDerivation` function, not the generated, raw Nix derivation. Thus, using `overrideDerivation` will not work in this case, as it overrides only the attributes of the final derivation. It is for this reason that `overrideAttrs` should be preferred in (almost) all cases to `overrideDerivation`, i.e. to allow using `stdenv.mkDerivation` to process input arguments, as well as the fact that it is easier to use (you can use the same attribute names you see in your Nix code, instead of the ones generated (e.g. `buildInputs` vs `nativeBuildInputs`), and it involves less typing).
:::
## &lt;pkg&gt;.overrideDerivation {#sec-pkg-overrideDerivation}
::: warning
You should prefer `overrideAttrs` in almost all cases, see its documentation for the reasons why. `overrideDerivation` is not deprecated and will continue to work, but is less nice to use and does not have as many abilities as `overrideAttrs`.
:::
::: warning
Do not use this function in Nixpkgs as it evaluates a Derivation before modifying it, which breaks package abstraction and removes error-checking of function arguments. In addition, this evaluation-per-function application incurs a performance penalty, which can become a problem if many overrides are used. It is only intended for ad-hoc customisation, such as in `~/.config/nixpkgs/config.nix`.
:::
The function `overrideDerivation` creates a new derivation based on an existing one by overriding the original's attributes with the attribute set produced by the specified function. This function is available on all derivations defined using the `makeOverridable` function. Most standard derivation-producing functions, such as `stdenv.mkDerivation`, are defined using this function, which means most packages in the nixpkgs expression, `pkgs`, have this function.
Example usage:
```nix
mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
name = "sed-4.2.2-pre";
src = fetchurl {
url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
};
patches = [];
});
```
In the above example, the `name`, `src`, and `patches` of the derivation will be overridden, while all other attributes will be retained from the original derivation.
The argument `oldAttrs` is used to refer to the attribute set of the original derivation.
::: note
A package's attributes are evaluated *before* being modified by the `overrideDerivation` function. For example, the `name` attribute reference in `url = "mirror://gnu/hello/${name}.tar.gz";` is filled-in *before* the `overrideDerivation` function modifies the attribute set. This means that overriding the `name` attribute, in this example, *will not* change the value of the `url` attribute. Instead, we need to override both the `name` *and* `url` attributes.
:::
## lib.makeOverridable {#sec-lib-makeOverridable}
The function `lib.makeOverridable` is used to make the result of a function easily customizable. This utility only makes sense for functions that accept an argument set and return an attribute set.
Example usage:
```nix
f = { a, b }: { result = a+b; };
c = lib.makeOverridable f { a = 1; b = 2; };
```
The variable `c` is the value of the `f` function applied with some default arguments. Hence the value of `c.result` is `3`, in this example.
The variable `c` however also has some additional functions, like
[c.override](#sec-pkg-override) which can be used to override the
default arguments. In this example the value of
`(c.override { a = 4; }).result` is 6.

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doc/using/overrides.xml
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@ -1,145 +0,0 @@
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
xml:id="chap-overrides">
<title>Overriding</title>
<para>
Sometimes one wants to override parts of <literal>nixpkgs</literal>, e.g. derivation attributes, the results of derivations.
</para>
<para>
These functions are used to make changes to packages, returning only single packages. <link xlink:href="#chap-overlays">Overlays</link>, on the other hand, can be used to combine the overridden packages across the entire package set of Nixpkgs.
</para>
<section xml:id="sec-pkg-override">
<title>&lt;pkg&gt;.override</title>
<para>
The function <varname>override</varname> is usually available for all the derivations in the nixpkgs expression (<varname>pkgs</varname>).
</para>
<para>
It is used to override the arguments passed to a function.
</para>
<para>
Example usages:
<programlisting>pkgs.foo.override { arg1 = val1; arg2 = val2; ... }</programlisting>
<!-- TODO: move below programlisting to a new section about extending and overlays
and reference it
-->
<programlisting>
import pkgs.path { overlays = [ (self: super: {
foo = super.foo.override { barSupport = true ; };
})]};
</programlisting>
<programlisting>
mypkg = pkgs.callPackage ./mypkg.nix {
mydep = pkgs.mydep.override { ... };
}
</programlisting>
</para>
<para>
In the first example, <varname>pkgs.foo</varname> is the result of a function call with some default arguments, usually a derivation. Using <varname>pkgs.foo.override</varname> will call the same function with the given new arguments.
</para>
</section>
<section xml:id="sec-pkg-overrideAttrs">
<title>&lt;pkg&gt;.overrideAttrs</title>
<para>
The function <varname>overrideAttrs</varname> allows overriding the attribute set passed to a <varname>stdenv.mkDerivation</varname> call, producing a new derivation based on the original one. This function is available on all derivations produced by the <varname>stdenv.mkDerivation</varname> function, which is most packages in the nixpkgs expression <varname>pkgs</varname>.
</para>
<para>
Example usage:
<programlisting>
helloWithDebug = pkgs.hello.overrideAttrs (oldAttrs: rec {
separateDebugInfo = true;
});
</programlisting>
</para>
<para>
In the above example, the <varname>separateDebugInfo</varname> attribute is overridden to be true, thus building debug info for <varname>helloWithDebug</varname>, while all other attributes will be retained from the original <varname>hello</varname> package.
</para>
<para>
The argument <varname>oldAttrs</varname> is conventionally used to refer to the attr set originally passed to <varname>stdenv.mkDerivation</varname>.
</para>
<note>
<para>
Note that <varname>separateDebugInfo</varname> is processed only by the <varname>stdenv.mkDerivation</varname> function, not the generated, raw Nix derivation. Thus, using <varname>overrideDerivation</varname> will not work in this case, as it overrides only the attributes of the final derivation. It is for this reason that <varname>overrideAttrs</varname> should be preferred in (almost) all cases to <varname>overrideDerivation</varname>, i.e. to allow using <varname>stdenv.mkDerivation</varname> to process input arguments, as well as the fact that it is easier to use (you can use the same attribute names you see in your Nix code, instead of the ones generated (e.g. <varname>buildInputs</varname> vs <varname>nativeBuildInputs</varname>), and it involves less typing).
</para>
</note>
</section>
<section xml:id="sec-pkg-overrideDerivation">
<title>&lt;pkg&gt;.overrideDerivation</title>
<warning>
<para>
You should prefer <varname>overrideAttrs</varname> in almost all cases, see its documentation for the reasons why. <varname>overrideDerivation</varname> is not deprecated and will continue to work, but is less nice to use and does not have as many abilities as <varname>overrideAttrs</varname>.
</para>
</warning>
<warning>
<para>
Do not use this function in Nixpkgs as it evaluates a Derivation before modifying it, which breaks package abstraction and removes error-checking of function arguments. In addition, this evaluation-per-function application incurs a performance penalty, which can become a problem if many overrides are used. It is only intended for ad-hoc customisation, such as in <filename>~/.config/nixpkgs/config.nix</filename>.
</para>
</warning>
<para>
The function <varname>overrideDerivation</varname> creates a new derivation based on an existing one by overriding the original's attributes with the attribute set produced by the specified function. This function is available on all derivations defined using the <varname>makeOverridable</varname> function. Most standard derivation-producing functions, such as <varname>stdenv.mkDerivation</varname>, are defined using this function, which means most packages in the nixpkgs expression, <varname>pkgs</varname>, have this function.
</para>
<para>
Example usage:
<programlisting>
mySed = pkgs.gnused.overrideDerivation (oldAttrs: {
name = "sed-4.2.2-pre";
src = fetchurl {
url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
};
patches = [];
});
</programlisting>
</para>
<para>
In the above example, the <varname>name</varname>, <varname>src</varname>, and <varname>patches</varname> of the derivation will be overridden, while all other attributes will be retained from the original derivation.
</para>
<para>
The argument <varname>oldAttrs</varname> is used to refer to the attribute set of the original derivation.
</para>
<note>
<para>
A package's attributes are evaluated *before* being modified by the <varname>overrideDerivation</varname> function. For example, the <varname>name</varname> attribute reference in <varname>url = "mirror://gnu/hello/${name}.tar.gz";</varname> is filled-in *before* the <varname>overrideDerivation</varname> function modifies the attribute set. This means that overriding the <varname>name</varname> attribute, in this example, *will not* change the value of the <varname>url</varname> attribute. Instead, we need to override both the <varname>name</varname> *and* <varname>url</varname> attributes.
</para>
</note>
</section>
<section xml:id="sec-lib-makeOverridable">
<title>lib.makeOverridable</title>
<para>
The function <varname>lib.makeOverridable</varname> is used to make the result of a function easily customizable. This utility only makes sense for functions that accept an argument set and return an attribute set.
</para>
<para>
Example usage:
<programlisting>
f = { a, b }: { result = a+b; };
c = lib.makeOverridable f { a = 1; b = 2; };
</programlisting>
</para>
<para>
The variable <varname>c</varname> is the value of the <varname>f</varname> function applied with some default arguments. Hence the value of <varname>c.result</varname> is <literal>3</literal>, in this example.
</para>
<para>
The variable <varname>c</varname> however also has some additional functions, like <link linkend="sec-pkg-override">c.override</link> which can be used to override the default arguments. In this example the value of <varname>(c.override { a = 4; }).result</varname> is 6.
</para>
</section>
</chapter>