我遇到了一个看起来有点奇怪的东西。
基础机器是 Ubuntu 18.04。我正在尝试创建一个自定义 initramfs + init 脚本，以用于与 qemu 实例一起使用的自定义编译内核。
从我用作 initramfs 基础的目录中：
[~/initramfs] $ find .
.
./proc
./root
./dev
./dev/console
./dev/sda1
./dev/null
./dev/tty
./sbin
./init
./etc
./lib64
./mnt
./mnt/root
./lib
./bin
./bin/busybox
./sys
只是现在需要的基础知识。busybox 二进制文件来自busybox-static 包，我已经确认它是静态编译的：
[~/initramfs]$ ldd bin/busybox
not a dynamic executable
在初始化脚本中，我有：
#!/bin/busybox sh
mount -t proc none /proc
mount -t sysfs none /sys
echo "Hi there"
umount /sys
umount /proc
poweroff
从那里，创建一个 initramfs.gz：
find . -print0 | cpio --null --create --verbose --format=newc | pigz --best > ~/initramfs.gz
当我将其设置为 qemu 的目标 initrd 时，内核按预期启动，然后：
[ 0.777443] Run /init as init process
/init: line 3: mount: not found
/init: line 4: mount: not found
Hi there
/init: line 8: umount: not found
/init: line 9: umount: not found
/init: line 11: poweroff: not found
mount 是busybox 的一部分。所以这很奇怪。
如果我修改 init 脚本并将/bin/busybox sh其作为要执行的第一个命令输入，那么我会如您所愿地进入一个 busybox 外壳。
[ 0.789949] Run /init as init process
BusyBox v1.27.2 (Ubuntu 1:1.27.2-2ubuntu3.2) built-in shell (ash)
Enter 'help' for a list of built-in commands.
sh: can't access tty; job control turned off
/ # [ 1.364618] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3
[ 1.386482] tsc: Refined TSC clocksource calibration: 3392.105 MHz
[ 1.388387] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x30e52cb7a6c, max_idle_ns: 440795310382 ns
[ 1.391965] clocksource: Switched to clocksource tsc
/ #
然后帮助显示：
/ # help
Built-in commands:
------------------
. : [ [[ alias bg break cd chdir command continue echo eval exec[ 71.772009] random: fast init done
exit export false fg getopts hash help history jobs kill let
local printf pwd read readonly return set shift source test times
trap true type ulimit umask unalias unset wait [ [[ acpid adjtimex
ar arp arping ash awk basename blkdiscard blockdev brctl bunzip2
bzcat bzip2 cal cat chgrp chmod chown chpasswd chroot chvt clear
cmp cp cpio crond crontab cttyhack cut date dc dd deallocvt depmod
devmem df diff dirname dmesg dnsdomainname dos2unix dpkg dpkg-deb
du dumpkmap dumpleases echo ed egrep env expand expr factor fallocate
false fatattr fdisk fgrep find fold free freeramdisk fsfreeze
fstrim ftpget ftpput getopt getty grep groups gunzip gzip halt
head hexdump hostid hostname httpd hwclock i2cdetect i2cdump
i2cget i2cset id ifconfig ifdown ifup init insmod ionice ip ipcalc
ipneigh kill killall klogd last less link linux32 linux64 linuxrc
ln loadfont loadkmap logger login logname logread losetup ls
lsmod lsscsi lzcat lzma lzop md5sum mdev microcom mkdir mkdosfs
mke2fs mkfifo mknod mkpasswd mkswap mktemp modinfo modprobe more
mount mt mv nameif nc netstat nl nproc nsenter nslookup od openvt
partprobe passwd paste patch pidof ping ping6 pivot_root poweroff
printf ps pwd rdate readlink realpath reboot renice reset rev
rm rmdir rmmod route rpm rpm2cpio run-parts sed seq setkeycodes
setpriv setsid sh sha1sum sha256sum sha512sum shred shuf sleep
sort ssl_client start-stop-daemon stat static-sh strings stty
su sulogin svc swapoff swapon switch_root sync sysctl syslogd
tac tail tar taskset tee telnet telnetd test tftp time timeout
top touch tr traceroute traceroute6 true truncate tty tunctl
ubirename udhcpc udhcpd uevent umount uname uncompress unexpand
uniq unix2dos unlink unlzma unshare unxz unzip uptime usleep
uudecode uuencode vconfig vi w watch watchdog wc wget which who
whoami xargs xxd xz xzcat yes zcat
所以我去寻找坐骑，发现也没有找到。哦，但如果我在它前面加上 /bin/busybox 直接调用它，它就可以工作......：
/ # type mount
mount is mount
/ # which mount
sh: which: not found
/ # /bin/busybox which mount
/ #
如果我将 /bin/busybox 添加到它们，我可以成功执行命令：
/ # /bin/busybox mount -t proc none /proc
/ #
似乎真的很随机，什么可以从busybox工作，什么不能工作，什么可以找到，什么不可以找到，例如find很好：
/ # find
.
./test
./sys
./bin
./bin/busybox
./lib
./mnt
./mnt/root
./lib64
./etc
./init
./sbin
./proc
./root
./dev
./dev/tty
./dev/null
./dev/sda1
./dev/console
我可以通过在 init 文件中的每个命令前面加上 来解决这个问题/bin/busybox，但如果我不必这样做，我真的宁愿不这样做！