BusyBox - 人人都該備一個在身邊的執行檔

Introduction

BusyBox 這專案首次釋出於 1999 年 11 月 4 日, 目標是要為嵌入式環境提供一個迷你、單一的靜態連結執行檔, 程式撰寫的時候也一直會考量到資源的限制, 現在 BusyBox 已經發展成包含數百個常見指令的單一執行檔。

BusyBox 是一個典型的 Multicall Binary 範例, Multicall Binary 指的是一個 binary 檔同時包含好幾隻不同的程式, 依照執行這 binary 的方式而決定要跑裡面的哪種程式, 這技巧的知名例子就是 BusyBox, 把撰寫的許多 command 編在一起, 共用許多部份, 藉此達到省空間的目的 (BusyBox 的目標是跑在資源相對有限的嵌入式系統), 達到這效果的方式則是利用 argv[0] 來做判斷, 如果名稱是內部程式之一的話, 就去執行該程式的部份, 因此可以看到諸多個指令都只是 BusyBox 的 symbolic link, 但執行起來效果卻不同。

例如這個樣子:

-rwxr-xr-x 1 root root 931664 Aug  9 13:19 busybox*
lrwxrwxrwx 1 root root      7 Aug  9 13:20 ls -> busybox*
lrwxrwxrwx 1 root root      7 Aug  9 13:20 pwd -> busybox*

安裝 & 使用

$ sudo pacman -S busybox
resolving dependencies...
looking for conflicting packages...

Package (1)        New Version  Net Change

community/busybox  1.23.2-1       0.90 MiB

Total Installed Size:  0.90 MiB

:: Proceed with installation? [Y/n]
(1/1) checking keys in keyring                       [##########################################] 100%
(1/1) checking package integrity                     [##########################################] 100%
(1/1) loading package files                          [##########################################] 100%
(1/1) checking for file conflicts                    [##########################################] 100%
(1/1) checking available disk space                  [##########################################] 100%
(1/1) installing busybox                             [##########################################] 100%
You may want to do setuid on /usr/bin/busybox
chmod 4555 /usr/bin/busybox
$ pacman -Ql busybox
busybox /usr/
busybox /usr/bin/
busybox /usr/bin/busybox
$ ls -l /usr/bin/busybox
-rwxr-xr-x 1 root root 931664 Mar 23 17:43 /usr/bin/busybox
$ file /usr/bin/busybox
/usr/bin/busybox: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped

裝起來就這麼個執行檔,靜態連結,沒有其他 dependency, 總共 931664 bytes (不到 1 MB), 裡面卻包含了眾多系統常見的基本功能:

$ busybox
BusyBox v1.23.2 (2015-03-23 12:42:42 MSK) multi-call binary.
BusyBox is copyrighted by many authors between 1998-2012.
Licensed under GPLv2. See source distribution for detailed
copyright notices.

Usage: busybox [function [arguments]...]
or: busybox --list[-full]
or: busybox --install [-s] [DIR]
or: function [arguments]...

    BusyBox is a multi-call binary that combines many common Unix
    utilities into a single executable.  Most people will create a
    link to busybox for each function they wish to use and BusyBox
    will act like whatever it was invoked as.

Currently defined functions:
    [, [[, acpid, addgroup, adduser, adjtimex, ar, arp, arping, ash, awk, base64, basename,
    bbconfig, beep, blkid, blockdev, bootchartd, brctl, bunzip2, bzcat, bzip2, cal, cat, catv,
    chat, chattr, chgrp, chmod, chown, chpasswd, chpst, chroot, chrt, chvt, cksum, clear, cmp,
    comm, cp, cpio, crond, crontab, cryptpw, cttyhack, cut, date, dc, dd, deallocvt, delgroup,
    deluser, depmod, df, dhcprelay, diff, dirname, dmesg, dnsd, dnsdomainname, dos2unix, du,
    dumpkmap, dumpleases, echo, ed, egrep, eject, env, envdir, envuidgid, ether-wake, expand,
    expr, fakeidentd, false, fatattr, fbset, fbsplash, fdflush, fdformat, fdisk, fgconsole,
    fgrep, find, findfs, flock, fold, free, freeramdisk, fsck, fsck.minix, fstrim, fsync,
    ftpd, ftpget, ftpput, fuser, getopt, getty, grep, groups, gunzip, gzip, halt, hd, hdparm,
    head, hexdump, hostid, hostname, httpd, hwclock, id, ifconfig, ifdown, ifenslave, ifplugd,
    ifup, inetd, init, inotifyd, insmod, install, ionice, iostat, ip, ipaddr, ipcalc, ipcrm,
    ipcs, iplink, iproute, iprule, iptunnel, kbd_mode, kill, killall, killall5, klogd, last,
    less, linux32, linux64, linuxrc, ln, loadfont, loadkmap, logger, login, logname, logread,
    losetup, lpd, lpq, lpr, ls, lsattr, lsmod, lsof, lspci, lsusb, lzcat, lzma, makedevs,
    makemime, man, md5sum, mdev, mesg, microcom, mkdir, mkdosfs, mke2fs, mkfifo, mkfs.ext2,
    mkfs.minix, mkfs.vfat, mknod, mkpasswd, mkswap, mktemp, modinfo, modprobe, more, mount,
    mountpoint, mpstat, mt, mv, nameif, nbd-client, nc, netstat, nice, nmeter, nohup,
    nslookup, ntpd, od, openvt, passwd, patch, pgrep, pidof, ping, ping6, pipe_progress,
    pivot_root, pkill, pmap, popmaildir, poweroff, powertop, printenv, printf, ps, pscan,
    pstree, pwd, pwdx, raidautorun, rdate, rdev, readahead, readlink, readprofile, realpath,
    reboot, reformime, renice, reset, resize, rev, rfkill, rm, rmdir, rmmod, route, rpm2cpio,
    rtcwake, run-parts, runlevel, runsv, runsvdir, rx, script, scriptreplay, sed, sendmail,
    seq, setarch, setconsole, setfont, setkeycodes, setlogcons, setserial, setsid, setuidgid,
    sh, sha1sum, sha256sum, sha3sum, sha512sum, showkey, shuf, slattach, sleep, smemcap,
    softlimit, sort, split, start-stop-daemon, stat, strings, stty, su, sulogin, sum, sv,
    svlogd, swapoff, swapon, switch_root, sync, sysctl, syslogd, tac, tail, tar, taskset,
    tcpsvd, tee, telnet, telnetd, test, tftp, tftpd, time, timeout, top, touch, tr,
    traceroute, traceroute6, true, tty, ttysize, tunctl, tune2fs, ubiattach, ubidetach,
    ubimkvol, ubirmvol, ubirsvol, ubiupdatevol, udhcpc, udhcpc6, udhcpd, udpsvd, umount,
    uname, uncompress, unexpand, uniq, unix2dos, unlink, unlzma, unxz, unzip, uptime, users,
    usleep, uudecode, uuencode, vconfig, vi, vlock, volname, wall, watch, watchdog, wc, wget,
    which, who, whoami, whois, xargs, xz, xzcat, yes, zcat, zcip

這麼多功能,這樣的檔案大小,當然不是像 GNU coreutils 那樣有一大堆的 options 支援, 但是作為基本的功能還是有的, 其中甚至有 httpd 可以當作小小的 SimpleHTTPServer 來使用。

使用方式可以把要用的指令接在 BusyBox 後頭 (例如 $ busybox ls), 或者用 symbolic link 來換成想要的指令 (例如 $ ln -s busybox ls; ./ls), 可以這麼做的原因是 BusyBox 會根據 argv[0] 來判斷要執行的 command, 如果 argv[0]busybox 的話就吃 argv[1] 來決定 command, 不然就直接用 argv[0] 來決定 command。

Build From Source

$ git clone git://git.busybox.net/busybox
$ cd busybox
$ make menuconfig   # 或 oldconfig/defconfig,可以選擇要編進去的功能
$ make -j4
$ ls -l busybox
-rwxr-xr-x 1 dv users 831440 Aug  9 12:34 busybox