Busybox is a multi-purpose binary, i.e. a single binary that is capable of doing a lot of things. This is the description that can be found in the website:

BusyBox combines tiny versions of many common UNIX utilities into a single small executable. It provides replacements for most of the utilities you usually find in GNU fileutils, shellutils, etc. The utilities in BusyBox generally have fewer options than their full-featured GNU cousins; however, the options that are included provide the expected functionality and behave very much like their GNU counterparts. BusyBox provides a fairly complete environment for any small or embedded system.

As of this writing, it is capable of providing most functionalities of at least the following programs:

Currently defined functions:
    [, [[, acpid, adjtimex, ar, arch, arp, arping, ash, awk,
    basename, bc, blkdiscard, blockdev, brctl, bunzip2, busybox,
    bzcat, bzip2, cal, cat, chgrp, chmod, chown, 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, nologin, nproc, nsenter, nslookup, nuke, od,
    openvt, partprobe, passwd, paste, patch, pidof, ping, ping6,
    pivot_root, poweroff, printf, ps, pwd, rdate, readlink, realpath,
    reboot, renice, reset, resume, rev, rm, rmdir, rmmod, route, rpm,
    rpm2cpio, run-init, run-parts, sed, seq, setkeycodes, setpriv,
    setsid, sh, sha1sum, sha256sum, sha512sum, shred, shuf, sleep,
    sort, ssl_client, start-stop-daemon, stat, strings, stty, su,
    sulogin, svc, svok, swapoff, swapon, switch_root, sync, sysctl,
    syslogd, tac, tail, tar, taskset, tc, 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

which probably makes it the item you want in a #toolbox.

Using busybox

There are several ways to invoke the tools that are contained in Busybox. The most basic one is to provide the command and its parameters as comand-line parameters to busybox itself, e.g. the following example does what you would expect:

$ busybox grep -i whatever *.txt

If you plan on using the tools interactively many times, typing busybox over and over can be annoying, so you have two options. The least invasive is to enter the shell provided by busybox, which is no bash but it’s honest. From there, all tools are just available by magic:

host-shell$ busybox sh
busybox-shell$ grep -i whatever *.txt | nl

If you don’t like Busybox’s shell (e.g. because you have a different one, or want to invoke commands from somewhere else) then it’s possible to install the sub-commands as symbolic links to the busybox executable itself. In fact, when it’s run with a different name, busybox becomes the program with that name. If this is your way, option --install will be helpful.

Why busybox In The #toolbox?

You might wonder why you would ever want to include Busybox in your #toolbox, provided that it includes pretty basic tools.

One consideration is that some of those tools are above basic level and might lack in a typical Linux installation. Two notable examples are ncat (as nc) and xxd, the former useful to set up quick TCP connections towards a destination (and check if it’s alive, how it works, etc.), the former to get a better idea of the contents of a (binary) file.

The shell itself, along with the “usual” Linux tooling, can come very handy anyway. As an example, you might run into a very stripped down Docker image where there is a single “business logic” executable only, and all tools are lacking; in this case, injecting Busybox in the container’s filesystem is an invaluable help for troubleshooting.

Last, but not least, Busybox is indeed also released as a statically compiled executable, just look at the binaries directory.