Note for Yocto Project Reference Manual v2.4 - ch8.141 update-alternatives.bbclass

當有多個recipes提供相同的命令時, update-alternatives.bbclass可以幫你做替代("alternative"), 例如，可以從busybox，binutils和elfutils中使用ar命令。 update-alternatives.bbclass會幫你重新命名，以便安裝多個package而不會發生衝突。無論安裝或隨後刪除哪些package，ar命令仍然有效。
要使用update-alternatives.bbclass，你需要為這個package定義一些變量：
ALTERNATIVE_ = "name1 name2 name3 ..."
列出所有這個package的alternatives指令

設定name這個命令的link的路徑
ALTERNATIVE_LINK_NAME[name] = "target"

設定該package的預設priority，數值越大，priority越高
ALTERNATIVE_PRIORITY

針對特定命令另外設定priority，數值越大，priority越高
ALTERNATIVE_PRIORITY[name] = "20"

範例

brook這個package的檔案結構

brook@vista:~/oe-core/meta/recipes-devtools/brook$ tree
.
|-- brook-1.0.0
|   |-- brookbox
|   |-- copyright
|   `-- insmod
`-- brook_1.0.0.bb

1 directory, 4 files

brookbox類似busybox，後面會有多個檔案link到該檔案，而insmod是一個獨立檔案

brook_1.0.0.bb

SUMMARY = "Brook demo update-alternatives"
DESCRIPTION = "This package is used for Brook to demo update-alternatives"
LICENSE = "GPLv2+"
LIC_FILES_CHKSUM = "file://${WORKDIR}/copyright;md5=afcc69d729fbf1d0a2af28ce44a23991 \
"
SRC_URI = "file://insmod \
        file://copyright \
        file://brookbox \
"
inherit update-alternatives

ALTERNATIVE_${PN} = "insmod route reboot shutdown poweroff"
ALTERNATIVE_PRIORITY = "100"

ALTERNATIVE_LINK_NAME[insmod] = "${base_sbindir}/insmod"
ALTERNATIVE_PRIORITY[insmod] = "50"

ALTERNATIVE_LINK_NAME[route] = "${base_sbindir}/route"
ALTERNATIVE_PRIORITY[route] = "50"

ALTERNATIVE_LINK_NAME[reboot] = "${base_sbindir}/reboot"

ALTERNATIVE_LINK_NAME[shutdown] = "${base_sbindir}/shutdown"
ALTERNATIVE_PRIORITY[shutdown] = "10"

ALTERNATIVE_LINK_NAME[poweroff] = "${base_sbindir}/poweroff"
ALTERNATIVE_PRIORITY[poweroff] = "500"

do_install () {
        install -d                              ${D}${base_sbindir}
        install -m 0755    ${WORKDIR}/insmod    ${D}${base_sbindir}/insmod
        install -m 0755    ${WORKDIR}/brookbox  ${D}${base_sbindir}/brook
        ln -s ${base_sbindir}/brook ${D}${base_sbindir}/route
        ln -s ${base_sbindir}/brook ${D}${base_sbindir}/reboot
        ln -s ${base_sbindir}/brook ${D}${base_sbindir}/shutdown
        ln -s ${base_sbindir}/brook ${D}${base_sbindir}/poweroff
}

sysvinit也同時註冊了reboot，shutdown與poweroff，其priority都是200，而這裡的priority分別為100，10，500，所以最後結果如下

brook@vista:~/oe-core/build/tmp-glibc/work/oe-linux-gnueabi/image/1.0-r0/rootfs$ ls -al sbin/|grep brook
-rwxr-xr-x  1 jenkins jenkins      6 Dec  9 22:17 brook
lrwxrwxrwx  1 jenkins jenkins     18 Dec  9 22:17 insmod -> /sbin/insmod.brook
-rwxr-xr-x  1 jenkins jenkins      6 Dec  9 22:17 insmod.brook
lrwxrwxrwx  1 jenkins jenkins     20 Dec  9 22:17 poweroff -> /sbin/poweroff.brook
lrwxrwxrwx  1 jenkins jenkins     11 Dec  9 22:17 poweroff.brook -> /sbin/brook
lrwxrwxrwx  1 jenkins jenkins     11 Dec  9 22:17 reboot.brook -> /sbin/brook
lrwxrwxrwx  1 jenkins jenkins     17 Dec  9 22:17 route -> /sbin/route.brook
lrwxrwxrwx  1 jenkins jenkins     11 Dec  9 22:17 route.brook -> /sbin/brook
lrwxrwxrwx  1 jenkins jenkins     11 Dec  9 22:17 shutdown.brook -> /sbin/brook

reboot與shutdown都小於sysvinit，所以不適用brook，poweroff高於sysvinit，所以使用brook這個package提供的命令

參考資料:
• Yocto Project Reference Manual 2.4