kumquat-buildroot/docs/manual/customize-post-image.txt

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=== Customization _after_ the images have been created

While post-build scripts (xref:rootfs-custom[]) are run _before_
building the filesystem image, kernel and bootloader, *post-image
scripts* can be used to perform some specific actions _after_ all images
have been created.

Post-image scripts can for example be used to automatically extract your
root filesystem tarball in a location exported by your NFS server, or
to create a special firmware image that bundles your root filesystem and
kernel image, or any other custom action required for your project.

To enable this feature, specify a space-separated list of post-image
scripts in config option +BR2_ROOTFS_POST_IMAGE_SCRIPT+ (in the +System
configuration+ menu). If you specify a relative path, it will be
relative to the root of the Buildroot tree.

Just like post-build scripts, post-image scripts are run with the main
Buildroot tree as current working directory. The path to the +images+
output directory is passed as the first argument to each script. If the
config option +BR2_ROOTFS_POST_SCRIPT_ARGS+ is not empty, these
arguments will be passed to the script too. All the scripts will be
passed the exact same set of arguments, it is not possible to pass
different sets of arguments to each script.

Again just like for the post-build scripts, the scripts have access to
the environment variables +BR2_CONFIG+, +HOST_DIR+, +STAGING_DIR+,
+TARGET_DIR+, +BUILD_DIR+, +BINARIES_DIR+, +CONFIG_DIR+ and
+BASE_DIR+.

The post-image scripts will be executed as the user that executes
Buildroot, which should normally _not_ be the root user. Therefore, any
action requiring root permissions in one of these scripts will require
special handling (usage of fakeroot or sudo), which is left to the
script developer.