Conf File Prefix

nix.conf - Nix configuration file

Description

Nix supports a variety of configuration settings, which are read from configuration files or taken as command line flags.

Configuration file

By default Nix reads settings from each of the following places. Settings are applied on top of one another, so later settings overwrite earlier entries.

The system-wide configuration file nix.conf in the configuration directory.

The configuration directory defaults to ${sysconfdir}/nix (i.e. /etc/nix on most Unix systems) or %PROGRAMDATA%\nix\conf on Windows, and can be overridden with NIX_CONF_DIR.

Values loaded in this file are not forwarded to the Nix daemon. The client assumes that the daemon has already loaded them.
If NIX_USER_CONF_FILES is set, then each path in the list will be loaded in reverse order. The list separator is : on Unix and ; on Windows.

Otherwise it will look for nix.conf in:
- The user configuration directory
- On Unix additionally, nix/nix.conf under each directory in XDG_CONFIG_DIRS
If NIX_CONFIG is set, its contents are treated as the contents of a configuration file. In particular, settings are separated by newlines, just as in the configuration file.

File format

Configuration files consist of name = value pairs, one per line. Comments start with a # character.

Example:

keep-outputs = true       # Nice for developers
keep-derivations = true   # Idem

Other files can be included with a line like include <path>, where <path> is interpreted relative to the current configuration file. A missing file is an error unless !include is used instead.

A configuration setting usually overrides any previous value. However, for settings that take a list of items, you can prefix the name of the setting by extra- to append to the previous value.

For instance,

substituters = a b
extra-substituters = c d

defines the substituters setting to be a b c d.

Unknown option names are not an error, and are simply ignored with a warning.

Command line flags

Configuration options can be set on the command line, overriding the values set in the configuration file:

Every configuration setting has corresponding command line flag (e.g. --max-jobs 16). Boolean settings do not need an argument, and can be explicitly disabled with the no- prefix (e.g. --keep-failed and --no-keep-failed).

Unknown option names are invalid flags (unless there is already a flag with that name), and are rejected with an error.
The flag --option <name> <value> is interpreted exactly like a <name> = <value> in a setting file.

Unknown option names are ignored with a warning.

The extra- prefix is supported for settings that take a list of items (e.g. --extra-trusted users alice or --option extra-trusted-users alice).

Integer settings