NixOS Configuration File
The 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, ... }:
{ = true;
= "alice@example.org";
= "/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:
Strings
Strings are enclosed in double quotes, e.g.
= "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.
=
''
127.0.0.2 other-localhost
10.0.0.1 server
'';
The main difference is that it strips from each line a number of spaces
equal to the minimal indentation of the string as a whole (disregarding
the indentation of empty lines), and that characters like
" and \ are not special (making it
more convenient for including things like shell code). See more info
about this in the Nix manual
here.
Booleans
These can be true or false, e.g.
= true;
= false;
Integers
For example,
."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.)
Sets
Sets were introduced above. They are name/value pairs enclosed in braces,
as in the option definition
."/boot" =
{ device = "/dev/sda1";
fsType = "ext4";
options = [ "rw" "data=ordered" "relatime" ];
};
Lists
The important thing to note about lists is that list elements are
separated by whitespace, like this:
= [ "fuse" "kvm-intel" "coretemp" ];
List elements can be any other type, e.g. sets:
swapDevices = [ { device = "/dev/disk/by-label/swap"; } ];
Packages
Usually, 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:
=
[ pkgs.thunderbird
pkgs.emacs
];
= pkgs.postgresql_10;
The latter option definition changes the default PostgreSQL package used
by NixOS’s PostgreSQL service to 10.x. For more information on
packages, including how to add new ones, see
.