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.config

.config is the default kernel configuration file.
This file is an ASCII file that stores the values of the config symbols (which determine, among others, which drivers are built, and if so if they're built into the kernel or as loadable module).

Creation of the .config file

This file is not part of the git source tree, it must be created before compiling the kernel, for example with 
$ make menuconfig
Alternatively, the default configuration can be produced with 
$ make defconfig
After its creation, it's located in the top directory of the kernel source tree.

Using a non-default config file

When building the Kernel, a non-default config file can be specified using the environment variable KCONFIG_CONFIG: 
$ make KCONFIG_CONFIG=config-lean

Scripted manipulation of .config values

.config values can be manipulated from shell scripts or the command line with scripts/config.

See also

The environment variable KCONFIG_CONFIG specifies the path to an alternate kernel config file (if set).
The environment variable KCONFIG_DEFCONFIG_LIST specifies a list of config files which can be used as basis to create .config if .config does not exist.
The elements in this list are separated by spaces.
The environment variable KCONFIG_OVERWRITECONFIG can be set to prevent Kconfig from breaking symlinked .config files.
include/config/auto.conf mirrors the content of `.config.
scripts/config allows to manipulate options in a .config file from the command line.
/proc/config.gz, .config.old

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