NixOS Configuration FileThe NixOS configuration file generally looks like this:
{ config, pkgs, ... }:
{ option definitions
}
The first line ({ config, pkgs, ... }:) denotes
that this is actually a function that takes at least the two arguments
config and pkgs. (These are
explained later.) The function returns a set of
option definitions ({ ... }). These definitions have the
form name =
value, where
name is the name of an option and
value is its value. For example,
{ config, pkgs, ... }:
{ services.httpd.enable = true;
services.httpd.adminAddr = "alice@example.org";
services.httpd.documentRoot = "/webroot";
}
defines a configuration with three option definitions that together
enable the Apache HTTP Server with /webroot as
the document root.Sets can be nested, and in fact dots in option names are
shorthand for defining a set containing another set. For instance,
defines a set named
services that contains a set named
httpd, which in turn contains an option definition
named enable with value true.
This means that the example above can also be written as:
{ config, pkgs, ... }:
{ services = {
httpd = {
enable = true;
adminAddr = "alice@example.org";
documentRoot = "/webroot";
};
};
}
which may be more convenient if you have lots of option definitions
that share the same prefix (such as
services.httpd).NixOS checks your option definitions for correctness. For
instance, if you try to define an option that doesn’t exist (that is,
doesn’t have a corresponding option declaration),
nixos-rebuild will give an error like:
The option `services.httpd.enable' defined in `/etc/nixos/configuration.nix' does not exist.
Likewise, values in option definitions must have a correct type. For
instance, must be a Boolean
(true or false). Trying to give
it a value of another type, such as a string, will cause an error:
The option value `services.httpd.enable' in `/etc/nixos/configuration.nix' is not a boolean.
Options have various types of values. The most important are:
StringsStrings are enclosed in double quotes, e.g.
networking.hostName = "dexter";
Special characters can be escaped by prefixing them with a
backslash (e.g. \").Multi-line strings can be enclosed in double
single quotes, e.g.
networking.extraHosts =
''
127.0.0.2 other-localhost
10.0.0.1 server
'';
The main difference is that preceding whitespace is
automatically stripped from each line, and that characters like
" and \ are not special
(making it more convenient for including things like shell
code).BooleansThese can be true or
false, e.g.
networking.firewall.enable = true;
networking.firewall.allowPing = false;
IntegersFor example,
boot.kernel.sysctl."net.ipv4.tcp_keepalive_time" = 60;
(Note that here the attribute name
net.ipv4.tcp_keepalive_time is enclosed in
quotes to prevent it from being interpreted as a set named
net containing a set named
ipv4, and so on. This is because it’s not a
NixOS option but the literal name of a Linux kernel
setting.)SetsSets were introduced above. They are name/value pairs
enclosed in braces, as in the option definition
fileSystems."/boot" =
{ device = "/dev/sda1";
fsType = "ext4";
options = [ "rw" "data=ordered" "relatime" ];
};
ListsThe important thing to note about lists is that list
elements are separated by whitespace, like this:
boot.kernelModules = [ "fuse" "kvm-intel" "coretemp" ];
List elements can be any other type, e.g. sets:
swapDevices = [ { device = "/dev/disk/by-label/swap"; } ];
PackagesUsually, the packages you need are already part of the Nix
Packages collection, which is a set that can be accessed through
the function argument pkgs. Typical uses:
environment.systemPackages =
[ pkgs.thunderbird
pkgs.emacs
];
postgresql.package = pkgs.postgresql90;
The latter option definition changes the default PostgreSQL
package used by NixOS’s PostgreSQL service to 9.0. For more
information on packages, including how to add new ones, see
.