nixpkgs/nixos/doc/manual/manpages/nixos-generate-config.8

.Dd January 1, 1980
.Dt nixos-generate-config 8
.Os
.Sh NAME
.Nm nixos-generate-config
.Nd generate NixOS configuration modules
.
.
.
.Sh SYNOPSIS
.Nm nixos-generate-config
.Op Fl -force
.Op Fl -root Ar root
.Op Fl -dir Ar dir
.
.
.
.Sh DESCRIPTION
This command writes two NixOS configuration modules:
.Bl -tag -width indent
.It Pa /etc/nixos/hardware-configuration.nix
This module sets NixOS configuration options based on your current hardware
configuration. In particular, it sets the
.Va fileSystem
option to reflect all currently mounted file systems, the
.Va swapDevices
option to reflect active swap devices, and the
.Va boot.initrd.*
options to ensure that the initial ramdisk contains any kernel modules necessary
for mounting the root file system.
.Pp
If this file already exists, it is overwritten. Thus, you should not modify it
manually. Rather, you should include it from your
.Pa /etc/nixos/configuration.nix Ns
, and re-run
.Nm
to update it whenever your hardware configuration changes.
.
.It Pa /etc/nixos/configuration.nix
This is the main NixOS system configuration module. If it already exists, it’s
left unchanged. Otherwise,
.Nm
will write a template for you to customise.
.El
.
.
.
.Sh OPTIONS
.Bl -tag -width indent
.It Fl -root Ar root
If this option is given, treat the directory
.Ar root
as the root of the file system. This means that configuration files will be written to
.Ql Ar root Ns /etc/nixos Ns
, and that any file systems outside of
.Ar root
are ignored for the purpose of generating the
.Va fileSystems
option.
.
.It Fl -dir Ar dir
If this option is given, write the configuration files to the directory
.Ar dir
instead of
.Pa /etc/nixos Ns
\&.
.
.It Fl -force
Overwrite
.Pa /etc/nixos/configuration.nix
if it already exists.
.
.It Fl -no-filesystems
Omit everything concerning file systems and swap devices from the hardware configuration.
.
.It Fl -show-hardware-config
Don't generate
.Pa configuration.nix
or
.Pa hardware-configuration.nix
and print the hardware configuration to stdout only.
.El
.
.
.
.Sh EXAMPLES
This command is typically used during NixOS installation to write initial
configuration modules. For example, if you created and mounted the target file
systems on
.Pa /mnt
and
.Pa /mnt/boot Ns
, you would run:
.Bd -literal -offset indent
$ nixos-generate-config --root /mnt
.Ed
.
.Pp
The resulting file
.Pa /mnt/etc/nixos/hardware-configuration.nix
might look like this:
.Bd -literal -offset indent
# Do not modify this file!  It was generated by 'nixos-generate-config'
# and may be overwritten by future invocations.  Please make changes
# to /etc/nixos/configuration.nix instead.
{ config, pkgs, ... }:

{
  imports =
    [ <nixos/modules/installer/scan/not-detected.nix>
    ];

  boot.initrd.availableKernelModules = [ "ehci_hcd" "ahci" ];
  boot.kernelModules = [ "kvm-intel" ];
  boot.extraModulePackages = [ ];

  fileSystems."/" =
    { device = "/dev/disk/by-label/nixos";
      fsType = "ext3";
      options = [ "rw" "data=ordered" "relatime" ];
    };

  fileSystems."/boot" =
    { device = "/dev/sda1";
      fsType = "ext3";
      options = [ "rw" "errors=continue" "user_xattr" "acl" "barrier=1" "data=writeback" "relatime" ];
    };

  swapDevices =
    [ { device = "/dev/sda2"; }
    ];

  nix.maxJobs = 8;
}
.Ed
.
.Pp
It will also create a basic
.Pa /mnt/etc/nixos/configuration.nix Ns
, which you should edit to customise the logical configuration of your system. \
This file includes the result of the hardware scan as follows:
.Bd -literal -offset indent
imports = [ ./hardware-configuration.nix ];
.Ed
.
.Pp
After installation, if your hardware configuration changes, you can run:
.Bd -literal -offset indent
$ nixos-generate-config
.Ed
.
.Pp
to update
.Pa /etc/nixos/hardware-configuration.nix Ns
\&. Your
.Pa /etc/nixos/configuration.nix
will
.Em not
be overwritten.
.
.Sh AUTHORS
.An -nosplit
.An Eelco Dolstra
and
.An the Nixpkgs/NixOS contributors