/proc/config.gz

This file shows you the compile-time configuration settings for the kernel (gzip compressed, use zcat or zless to see its contents). It is available only if you enable it using CONFIG_IKCONFIG_PROC when you compile.

Say you want to upgrade to the next available kernel. Your current kernel works fine, so you'd like to use the same parameters, but you accidentally lost your original .config configuration file. Simply zcat /proc/config.gz > /usr/src/linux/.config and you're ready to go.

Example output:

#
# Automatically generated make config: don't edit
# Linux kernel version: 2.6.17
# Sat Jul 15 17:01:24 2006
#
CONFIG_X86_32=y
CONFIG_SEMAPHORE_SLEEPERS=y
CONFIG_X86=y
CONFIG_MMU=y
CONFIG_GENERIC_ISA_DMA=y
CONFIG_GENERIC_IOMAP=y
CONFIG_GENERIC_HWEIGHT=y
CONFIG_ARCH_MAY_HAVE_PC_FDC=y
CONFIG_DMI=y

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Extra step

After bringing in a config from some previous kernel, it's a VERY
good idea to do "make oldconfig", which will detect any new config
options which have appeared, and give you a chance to select
their settings.