Having been directed to initramfs by an answer to my earlier question (thanks!), I've been working on getting initramfs working. I can now boot the kernel and drop to a shell prompt, where I can execute busybox commands, which is awesome.

Here's where I'm stuck-- there are (at least) two methods of generating initramfs images:

  1. By passing the kernel a path to a prebuilt directory hierarchy to be compressed
  2. By passing the kernel the name of a file that lists the files to be included.

The second method seemed a little cleaner, so I've been using that.

Just for reference, here's my file list so far:

dir /dev 755 0 0
nod /dev/console 644 0 0 c 5 1
nod /dev/loop0 644 0 0 b 7 0
dir /bin 755 1000 1000
slink /bin/sh busybox 777 0 0
file /bin/busybox /home/brandon/rascal-initramfs/bin/busybox 755 0 0
dir /proc 755 0 0
dir /sys 755 0 0
dir /mnt 755 0 0
file /init /home/brandon/rascal-initramfs/init.sh 755 0 0

Unfortunately, I have learned that busybox requires a long list of links to serve as aliases to all of its different commands. Is there a way to generate the list of all these commands so I can add it to my file list?

Alternatively, I could switch to method 1, using the prebuilt directory hierarchy, but I'm not sure how to make the /dev nodes in that case.

Both of these paths seem messy. Is there an elegant solution to this?

  • Have you considered user6113's answer? It seems to be a good answer to your question. Commented Jul 1, 2015 at 8:40

4 Answers 4


The first few lines of the initscript in my initramfs are simply:

busybox --install -s

Creates the symlinks for you.. Only takes an unmeasurably small amount of time on my 500Mhz board, possibly longer on very low hardware, but likely manageable. Saves a bunch of issues remembering to create all the right links when you update BB...


It's not the kernel that's generating the initramfs, it's cpio. So what you're really looking for is a way to build a cpio archive that contains devices, symbolic links, etc.

Your method 2 uses usr/gen_init_cpio in the kernel source tree to build the cpio archive during the kernel build. That's indeed a good way of building a cpio archive without having to populate the local filesystem first (which would require being root to create all the devices, or using fakeroot or a FUSE filesystem which I'm not sure has been written already).

All you're missing is generating the input file to gen_init_cpio as a build step. E.g. in shell:

exec >initramfs_source.txt
echo "dir /bin 755 0 0"
echo "file /bin/busybox $INITRAMFS_SOURCE_DIR/bin/busybox 755 0 0"
for x in sh ls cp …; do echo "slink /bin/$x busybox 777 0 0" done
# etc …

If you want to reflect the symbolic links to busybox that are present in your build tree, here's a way (I assume you're building on Linux):

  for x in *; do
    if [ "$(readlink "$x")" = busybox ]; then
      echo "slink /bin/$x busybox 777 0 0"
  done )

Here's a way to copy all your symbolic links:

find "$INITRAMFS_SOURCE_DIR" -type l -printf 'slink %p %l 777 0 0\n'

For busybox, maybe your build tree doesn't have the symlinks, and instead you want to create one for every utility that you've compiled in. The simplest way I can think of is to look through your busybox build tree for .*.o.cmd files: there's one per generated command.

find /path/to/busybox/build/tree -name '.*.cmd' -exec sh -c '
    for x; do
      echo "slink /bin/${x%%.*} busybox 777 0 0"
' _ {} +
  • Actually isn't you're missing step posted? I.e. just after 'Just for reference, here's my file list so far:'? Commented Dec 9, 2010 at 22:29
  • 1
    @Maciej: Good point, I hadn't thought of that way of interpreting the question, but matching the capabilities of the busybox executable with the initramfs generation is a good idea anyway. Commented Dec 9, 2010 at 23:28

If you are in the busybox shell (ash) you don't need to worry about aliases as they will be run as commands by default IIRC. Anyway busybox --help gives list of supported commands. In my case they are:

% busybox --help
BusyBox v1.17.4 (2010-11-25 12:49:55 GMT) multi-call binary.
Copyright (C) 1998-2009 Erik Andersen, Rob Landley, Denys Vlasenko
and others. Licensed under GPLv2.
See source distribution for full notice.

Usage: busybox [function] [arguments]...
   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,
    basename, bb, bbconfig, bbsh, beep, blkid, bootchartd, brctl, bunzip2,
    bzcat, bzip2, cal, cat, catv, chat, chattr, chgrp, chmod, chown,
    chpasswd, chpst, chroot, chrt, chvt, cksum, clear, cmp, comm, conspy,
    cp, cpio, crond, cryptpw, cttyhack, cut, date, dd, deallocvt, delgroup,
    deluser, depmod, devmem, df, dhcprelay, diff, dirname, dmesg,
    dnsdomainname, dos2unix, dpkg-deb, du, dumpkmap, dumpleases, echo, ed,
    egrep, eject, env, envdir, envuidgid, ether-wake, expand, expr, false,
    fbset, fdflush, fdformat, fdisk, fgconsole, fgrep, find, findfs,
    flash_eraseall, flash_lock, flash_unlock, flashcp, flock, free,
    freeramdisk, fsck, fsck.minix, fsync, ftpd, fuser, getopt, getty, grep,
    gunzip, gzip, halt, hd, hdparm, head, hexdump, hostname, httpd,
    hwclock, id, ifconfig, ifdown, ifenslave, ifplugd, ifup, init, insmod,
    install, ionice, ip, ipaddr, ipcrm, ipcs, iplink, iproute, iprule,
    iptunnel, kbd_mode, kill, killall, killall5, klogd, last, length, less,
    linux32, linux64, linuxrc, ln, loadfont, loadkmap, logger, login,
    logread, losetup, lpq, lpr, ls, lsattr, lsmod, lspci, lsusb, lzcat,
    lzma, lzop, lzopcat, makedevs, makemime, man, md5sum, mdev, mesg,
    microcom, mkdir, mkdosfs, mke2fs, mkfifo, mkfs.ext2, mkfs.minix,
    mkfs.reiser, mkfs.vfat, mknod, mkpasswd, mkswap, mktemp, modinfo,
    modprobe, more, mount, mountpoint, mt, mv, nameif, nc, netstat, nice,
    nmeter, nohup, nslookup, ntpd, openvt, passwd, patch, pgrep, pidof,
    ping, ping6, pipe_progress, pivot_root, pkill, popmaildir, poweroff,
    printenv, printf, ps, pscan, pwd, raidautorun, rdate, rdev, readahead,
    readlink, readprofile, realpath, reboot, reformime, renice, reset,
    resize, rev, rm, rmdir, rmmod, route, rtcwake, run-parts, runlevel,
    runsv, runsvdir, rx, script, scriptreplay, sed, sendmail, seq, setarch,
    setconsole, setfont, setkeycodes, setlogcons, setsid, setuidgid, sh,
    sha1sum, sha256sum, sha512sum, showkey, sleep, smemcap, softlimit,
    sort, split, start-stop-daemon, stat, strings, stty, su, sum, sv,
    svlogd, swapoff, swapon, switch_root, sync, sysctl, syslogd, tac, tail,
    tar, tee, telnet, telnetd, test, tftp, tftpd, time, timeout, top,
    touch, tr, traceroute, traceroute6, true, tty, ttysize, tunctl,
    tune2fs, ubiattach, ubidetach, udhcpc, udhcpd, umount, uname,
    uncompress, unexpand, uniq, unix2dos, unlzma, unlzop, unxz, unzip,
    uptime, usleep, vconfig, vi, vlock, volname, wall, watch, watchdog, wc,
    wget, which, who, whoami, xargs, xz, xzcat, yes, zcat, zcip

In case of first method you create by mknod(1) command. For example:

# mknod /my/dir/with/initrd/dev/console -m 644 c 5 0
  • In the OP's case, busybox --help is likely to only result in busybox: cannot execute binary file... Most embedded development is cross-compiling, we know pingswept is compiling for arm, and he's probably compiling on i386 or amd64. Commented Dec 9, 2010 at 23:29
  • Yes, that is actually true. I can run busybox --help once I boot the kernel on the ARM board. That's useful, but it doesn't help so much with generating the initramfs image.
    – pingswept
    Commented Dec 9, 2010 at 23:41
  • @Gilles: I assumed that someone who cross-compiles kernel did think about and, for example, run in the emulator. @pingswept: Possibly above list will help. Commented Dec 10, 2010 at 1:17
  • At work, our automated build environment doesn't have access to an emulator. I think this the norm in embedded development. Commented Dec 10, 2010 at 8:26

Current busybox (1.36.0 as an example) generates a busybox.links file when you use make install. This could be used with option 2 to generate the symlinks. All you'd need to do is loop:

make install
cat "busybox.links" | while read s; do
    echo "slink $s /bin/busybox 777 0 0" >> initramfs.conf; 
cat initramfs.conf

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