the_dao_of_minimal_linux_live.txt

   
   The Dao of Minimal Linux Live (27-Dec-2017)
   
   Author:    Ivan Davidov
   
   Website:   http://minimal.idzona.com
              http://skamilinux.hu/minimal
              http://ivandavidov.github.io/minimal
              http://minimal.linux-bg.org
   
   GitHub:    http://github.com/ivandavidov/minimal
              http://github.com/ivandavidov/minimal-linux-script

   Email:     davidov (dot) i {at} gmail [dot] com
   
   Redistributed by: <put your name and contact details somewhere here>
   
   ###   ###   ###
   
   Contents
   
   1. Preface
   2. Boot Process
   3. Inside The Shell Scripts
   4. Possible Improvements
   5. Next Steps
   6. Epilogue
   
   ###   ###   ###
   
   1. Preface
   
   Please feel free to redistribute this document in any form you see fit. However, I'd like to
   ask of you to respect my efforts and mention me as the original author - that's my only demand.
   
   Note that the explanations regarding the script details are based on the "28-Jul-2014"
   version of "Minimal Linux Live". Future versions of the scripts may already include some
   of the stuff discussed in the section "Possible Improvements".
   
   ###   ###   ###
   
   2. Boot Process
   
   The overall boot process is quite complex, but we need to know these details as we are 
   talking about operating systems and, precisely, Linux based operating systems.
   
   You can find some general information here:
   
      http://en.wikipedia.org/wiki/Linux_startup_process
   
   When it comes to live Linux OS, the overall boot process is the following:
   
      1) BIOS gives the execution control to the boot loader present on the boot media such
         as CD, DVD, USB flash or something else.
      2) In most cases, the boot loader is based on Syslinux. On our boot media, we should have
         two special files - the kernel file and the initramfs file.
      3) The boot media also contains a special configuration file, which points to the location
         of the kernel/initramfs files.
      4) Now that Syslinux knows where the kernel file is, it loads it in the RAM and passes
         the execution control to it.
      5) The kernel detects the available hardware, loads necessary drivers and then it passes
         the execution control to the initramfs.
      6) The initramfs file is an archive, automatically unpacked by the kernel in the
         RAM. The actual execution control is passed to the shell script file /init, which must
         be present in the initramfs file.
      7) Since we are talking about minimal live CD, in theory, /init is not supposed to do much.
         All we need from /init is to prepare the environment and provide the user with shell.
         In most live Linux distributions, the /init script is supposed to do much more than
         this, but for the time being, it's enough to know that the point at which we take 
         control is exactly the /init script.
     
   ###   ###   ###
   
   3. Inside The Shell Scripts
   
   So far, we have learnt that we need several pieces in order to build a live Linux OS:
   
      1) boot loader - to make our media bootable.
      2) kernel file - to take care of the initial system bootstrap process.
      3) initramfs file - to prepare the necessary environment for the user.
   
   This document focuses on parts (2) and (3). The reason why we won't discuss (1) in details is
   because the build process that we are going to rely on later, takes care of the boot loader
   part automatically.
   
   The ZIP, which you have downloaded, contains the following files:
   
      .config
      0_prepare.sh
      1_get_kernel.sh
      2_build_kernel.sh
      3_get_busybox.sh
      4_build_busybox.sh
      5_generate_rootfs.sh
      6_pack_rootfs.sh
      7_generate_iso.sh
      build_minimal_linux_live.sh
      
   If you have followed the explanation posted on http://minimal.idzona.com , then you should already
   know you need to execute the script "build_minimal_linux_live.sh". If you open this script
   with a text editor, you will find out that everything it does is to execute all other
   scripts one by one.
   
   From this point onwards, I encourage you to execute the script files one by one. We will
   start with "0_prepare.sh". If you take a look at the script's contents, you will find out that
   it only creates an empty folder named "work". We will store our temporary
   work files there.
   
   OK, let's continue with the kernel. Somehow, we need to end up with a fully functional kernel, which
   is able to perform the system initialization for us. We need to download the kernel sources, 
   compile them and finally pack the kernel.
   
   The script "1_get_kernel.sh" downloads the kernel sources and unpacks them. Here is the full
   source code of the script:
   
      #/bin/sh
      
      DOWNLOAD_URL=$(grep -i KERNEL_SOURCE_URL .config | cut -f2 -d'=')
      ARCHIVE_FILE=${DOWNLOAD_URL##*/}
      
      cd work
      rm -f $ARCHIVE_FILE
      wget $DOWNLOAD_URL
      rm -rf kernel
      mkdir kernel
      tar -xvf $ARCHIVE_FILE -C kernel
      cd ..   
   
   Firstly, we have to read the ".config" file in order to find the URL for the kernel sources,
   which is stored in the "DOWNLOAD_URL" variable. Then, we get the actual name of the archive 
   file and we store it in the variable "ARCHIVE_FILE".
   
   The important part of the script is the line where we get the kernel sources:
   
      wget $DOWNLOAD_URL
   
   The next important part of the script is the line where we unpack the kernel sources:
   
      tar -xvf $ARCHIVE_FILE -C kernel
   
   At this point, we have downloaded and unpacked the kernel sources. So far so good. Now, we need
   to compile them. We move to the next script in the chain called "2_build_kernel.sh". Here is
   the full source code that we are going to analyse:
   
      #/bin/sh
      
      cd work/kernel
      cd $(ls -d *)
      make clean
      make defconfig
      sed -i "s/.*CONFIG_DEFAULT_HOSTNAME.*/CONFIG_DEFAULT_HOSTNAME=\"minimal-linux-live\"/" .config
      make vmlinux
      cd ../../..   
   
   First we navigate to the kernel source folder. Then, we execute the following commands:
   
      make clean
     
   The above command cleans the output from our previous kernel builds. Obviously, if this is the
   first time we go through the build process, we have nothing to clean.
   
      make defconfig
   
   The abovementioned command creates new ".config" file in the current folder, which contains all
   default configuration parameters needed to build our new kernel. Note that the defaults
   will be either 32 or 64 bit specific, depending on your Linux environment.
   
      sed -i "s/.*CONFIG_DEFAULT_HOSTNAME.*/CONFIG_DEFAULT_HOSTNAME=\"minimal-linux-live\"/" .config
   
   The command searches for a specific line in the ".config" file, containing the string
   "CONFIG_DEFAULT_HOSTNAME" and replaces this whole line with the following text:
   
      CONFIG_DEFAULT_HOSTNAME="minimal-linux-live"
   
   Later, when you boot up your live Linux OS, you can execute the command "uname -a" and you will
   see the string "minimal-linux-live" in the output. All we did was to replace the default value
   "(none)" with our custom value.
   
   Now you have a sample command, which you can use to search for specific configurations in the
   file ".config" and replace these configurations with ones of your own. Obviously, you can skip
   this replacement - and everything will work just fine. Like I said, the only difference will be
   that the default host name will be set to "minimal-linux-live" instead of "(none)".
   
   Now we come to the most important and also slowest part in the whole process: we need to
   actually create the kernel.
   
      make vmlinux
   
   The abovementioned command will compile the Linux kernel. This usually takes quite a while, so I 
   guess it's high time for a short break. The final kernel file is located here:
   
      arch/x86/boot/bzImage
   
   OK, at this point we should have a kernel file compiled with default options, where we have
   changed the default host name with some other value of our own. Now, we move to the next
   part.
   
   The kernel itself is far from enough - we also need some kind of environment with basic
   command line tools like ls, cat, mkdir, etc. This environment is called "initramfs" which
   stands for "initial RAM file system". This is what we are going to do:
   
      1) We are going to download & compile Busybox. This will give us the basic command line
         tools that we need for normal work in console mode.
      2) We will use Busybox to provide us with some default directory/file structure, which
         we are going to modify into our own initramfs file.
   
   One by one - we need to download and unpack the Busybox sources. Let's take a look at the
   script file "3_get_busybox.sh":
   
      #/bin/sh
   
      DOWNLOAD_URL=$(grep -i BUSYBOX_SOURCE_URL .config | cut -f2 -d'=')
      ARCHIVE_FILE=${DOWNLOAD_URL##*/}
   
      cd work
      rm -f $ARCHIVE_FILE
      wget $DOWNLOAD_URL
      rm -rf busybox
      mkdir busybox
      tar -xvf $ARCHIVE_FILE -C busybox
      cd ..   
     
   The contents are almost identical to the one in "1_get_kernel.sh", which we already
   explained earlier. Long story short - we get the source archive with "wget" and then we
   unpack the sources with "tar". That's it. Nothing complex at all.
   
   Now that we have Busybox sources, we need to compile them. Let's take a look at the next
   script "4_build_busybox.sh" which we need to execute:
   
      #/bin/sh
   
      cd work/busybox
      cd $(ls -d *)
      make clean
      make defconfig
      sed -i "s/.*CONFIG_STATIC.*/CONFIG_STATIC=y/" .config
      make busybox
      make install
      cd ../../..
   
   You have probably already noticed it - we are going to execute several "make" commands
   and meanwhile, we will change one property in the ".config" file. Let's go
   through all of these commands:
   
      make clean
   
   The command above ensures that we will get rid of all build artefacts, assuming this
   is not the first time we go through the Busybox build process.
   
      make defconfig
   
   This command creates new ".config" file, which contains the defaults for proper
   Busybox build process. Again, nothing extraordinary here.
   
      sed -i "s/.*CONFIG_STATIC.*/CONFIG_STATIC=y/" .config
   
   The command above is very important, because we inform the build process to build
   a static version of Busybox, meaning that the Busybox executable file will not
   be dependent on any external library. We cannot skip this step, otherwise our OS will
   fail with "kernel panic" when we try to boot.
   
      make busybox
   
   The command above compiles Busybox. Nothing interesting here. The build should be
   significantly faster compared to the kernel build.
   
      make install
   
   The command above creates new "_install" folder and installs Busybox in it. We
   will use this folder as a base for our new initramfs file.
   
   At this point, we should have kernel compiled with default settings (yes, we changed
   one of them, but it's no big deal) and we should also have Busybox compiled with
   static option enabled and all other default settings.
   
   Now we come to the part where we create our own initramfs folder, which later on we
   will pack into a file. The steps that we are going to follow are these:
   
      1) Use the already created "_install" folder as base for initramfs.
      2) Create some new folders in the initramfs folder.
      3) Create some configuration files in the initramfs folder.
   
   Now we come to the script "5_generate_rootfs.sh" which is responsible for generating
   proper initramfs folder with all necessary files and sub-folders in it.
   
      #/bin/sh
   
      cd work
      rm -rf rootfs
      cd busybox
      cd $(ls -d *)
      cp -R _install ../../rootfs
      cd ../../rootfs
      rm -f linuxrc
      mkdir dev
      mkdir etc
      mkdir proc
      mkdir src
      mkdir sys
      mkdir tmp
      cd etc
      touch welcome.txt
      echo >> welcome.txt
      echo '  #####################################' >> welcome.txt
      echo '  #                                   #' >> welcome.txt
      echo '  #  Welcome to "Minimal Linux Live"  #' >> welcome.txt
      echo '  #                                   #' >> welcome.txt
      echo '  #####################################' >> welcome.txt
      echo >> welcome.txt
      cd ..
      touch init
      echo '#!/bin/sh' >> init
      echo 'dmesg -n 1' >> init
      echo 'mount -t devtmpfs none /dev' >> init
      echo 'mount -t proc none /proc' >> init
      echo 'mount -t sysfs none /sys' >> init
      echo 'cat /etc/welcome.txt' >> init
      echo 'while true' >> init
      echo 'do' >> init
      echo '  setsid cttyhack /bin/sh' >> init
      echo 'done' >> init
      echo >> init
      chmod +x init
      cp ../../*.sh src
      cp ../../.config src
      cd ../..
   
   As you can see, this script is much longer than the others. Let's take a look at the
   important parts one by one.
   
      cd work
      rm -rf rootfs
      cd busybox
      cd $(ls -d *)
      cp -R _install ../../rootfs
      cd ../../rootfs
   
   The code snippet above removes the old initramfs folder called "rootfs", then
   copies the "_install" folder (the one created by Busybox) with a new name - "rootfs".
   This folder is going to be our new initramfs focal point.
   
      rm -f linuxrc
   
   The folder "rootfs" contains file "linuxrc" which we don't need since we are going
   to use initramfs boot scheme. Take a look at the following Wikipedia article for
   more details:
   
      http://en.wikipedia.org/wiki/Initrd
   
   OK, lets go on.
   
      mkdir dev
      mkdir etc
      mkdir proc
      mkdir src
      mkdir sys
      mkdir tmp
   
   The code snippet above creates some basic folders which we are going to use later.
   
      cd etc
      touch welcome.txt
      echo >> welcome.txt
      echo '  #####################################' >> welcome.txt
      echo '  #                                   #' >> welcome.txt
      echo '  #  Welcome to "Minimal Linux Live"  #' >> welcome.txt
      echo '  #                                   #' >> welcome.txt
      echo '  #####################################' >> welcome.txt
      echo >> welcome.txt
      cd ..
   
   The code snippet above creates the file "/etc/welcome.txt" and fills it with the message
   which will be displayed every time we boot up the system.
   
      touch init
      echo '#!/bin/sh' >> init
      echo 'dmesg -n 1' >> init
      echo 'mount -t devtmpfs none /dev' >> init
      echo 'mount -t proc none /proc' >> init
      echo 'mount -t sysfs none /sys' >> init
      echo 'cat /etc/welcome.txt' >> init
      echo 'while true' >> init
      echo 'do' >> init
      echo '  setsid cttyhack /bin/sh' >> init
      echo 'done' >> init
      echo >> init
   
   The code snippet above creates the file "/init", which is going to be invoked by our kernel
   when we boot up the system. In fact, we create new shell script file. This is what our new
   "/init" script does:
   
      dmesg -n 1                     - We hide all kernel messages. We don't want them in our
                                       console. Only kernel panic messages will be displayed.
      mount -t devtmpfs none /dev    - With this command, we politely ask the kernel to
                                       populate the /dev folder with all necessary system
                                       devices like "console", "tty", etc. We also have nice
                                       names for the hardware devices like "sr0", "sda", etc.
      mount -t proc none /proc       - The kernel populates the /proc folder.
      mount -t sysfs none /sys       - The kernel populates the /sys folder.
      cat /etc/welcome.txt           - Now we display the welcome message.
      while true                     - This code snippet starts shell on "tty" device and
        setsid cttyhack /bin/sh        we rest assured that when we execute the "exit"
      done                             command new shell will be started automatically.
   
   As you can see, what the "/init" script file does isn't too much, however, it's really essential.
   We have only one more bit of code relevant to the "/init" file:
   
      chmod +x init
   
   The command above ensures that our "/init" script is executable.
   
      cp ../../*.sh src
      cp ../../.config src
   
   The last thing we do is to copy all "Minimal Linux Live" shell scripts (including the
   configuration file) in the "/src" folder. The scripts won't do anything there. This step
   simply ensures that the live Linux ISO file, which we are going to create later, contains
   the build sources, just for reference.
   
   At this point we have "rootfs" folder which contains all necessary files and
   configurations for our initramfs file. The next step is to pack the "rootfs" folder
   into an actual initramfs file. Let's take a look at "6_pack_rootfs.sh":
   
      #!/bin/sh
   
      cd work
      rm -f rootfs.cpio.gz
      cd rootfs
      find . | cpio -H newc -o | gzip > ../rootfs.cpio.gz
      cd ../..
   
   The abovementioned script removes the already existing initramfs file called "rootfs.cpio.gz",
   then creates a new file called "rootfs.cpio.gz" based on the contents of the
   "rootfs" folder.
   
   We are almost finished. At this point, we have already compiled the Linux kernel and we
   have also created initramfs file based on Busybox and our own set of configurations.
   The last thing to do is to generate a bootable ISO file. Let's take a look at the last
   script "7_generate_iso.sh":
   
      #/bin/sh
   
      rm -f minimal_linux_live.iso
      cd work/kernel
      cd $(ls -d *)
      make isoimage FDINITRD=../../rootfs.cpio.gz
      cp arch/x86/boot/image.iso ../../../minimal_linux_live.iso
      cd ../../..
   
   As usual, firstly we remove the already generated ISO file, assuming this is not the first
   time we go through this process. Then we navigate to the kernel's source folder and
   we execute the following command:
   
      make isoimage FDINITRD=../../rootfs.cpio.gz
   
   This command generates bootable ISO based on the already compiled Linux kernel and
   it uses the initramfs file which we pass to the "make" command as additional parameter.
   The generated ISO file is then copied to the folder where we are executing our scripts.
   
   UPDATE (02-Oct-2016): The "make isoimage" command relies on Syslinux older than 5.0.
   The issue: Syslinux versions after 5.0 require ldlinux.c32 which is not handled
   correctly by the kernel's Makefile. You need to appply a patch to the Makefile in order
   to make it work correctly. I find it easier to edit the Makefile and apply the patch
   changes manually:
   
   https://github.com/ivandavidov/minimal/issues/10
   
   http://github.com/mhiramat/boot2minc/blob/master/src/patches/kernel/x86-copy-linux-c32-for-newer.patch
   
   if [ -f /usr/$$i/syslinux/isolinux.bin ] ; then \                 # old line
     cp /usr/$$i/syslinux/isolinux.bin $(obj)/isoimage ; \           # old line
     if [ -f /usr/$$i/syslinux/ldlinux.c32 ] ; then \                # new line (patch)
       cp /usr/$$i/syslinux/ldlinux.c32 $(obj)/isoimage ; \          # new line (patch)
     fi ; \                                                          # new line (patch)
     break ; \                                                       # old line
   fi ; \                                                            # old line
   
   Another possible solution is to manually generate the ISO image by executing the
   command "genisoimage" with the appropriate arguments. You can take a look at the
   project "Minimal Linux Script" which follows the "genisoimage" approach:
   
   http://github.com/ivandavidov/minimal-linux-script
   
   ###   ###   ###
   
   4. Possible Improvements   
   
   Now that you have played around with your shiny new live Linux OS, you have probably
   noticed that you are logged in automatically and you have no restrictions. However,
   it only looks like you are automatically logged in. Yes, you get shell console and
   this console allows you to perform "root" operations, but this doesn't mean that you
   are logged in as "root" or any other user. In fact there are no users and groups in
   the system.
   
   I'll show you the proper way to add some users and groups from within your system
   while it is still running. You can examine the final result for yourself and modify
   the "5_generate_rootfs.sh" script according to your needs.
   
   OK, let's try with the "root" account!
   
      touch /etc/group
   
   The command above will create empty file "/etc/group" where we store the information
   for all groups.
   
      addgroup -g 0 root
   
   This one will create a group "root" with group identification number "0".
   It is important to provide 0 (zero) as group identifier (gid) because this is the
   expected gid for "root".
   
      touch /etc/passwd
   
   This command will create empty file "/etc/passwd" where we store the login
   information for our users.
   
      adduser -h /root -G root -u 0 root
   
   This command will create user "root" with home folder "/root", assign the new
   user to the "root" group and set user identifier (uid) "0". It is important to
   provide 0 (zero) as uid because this is the expected uid for the "root" user. Add
   whatever password you want when asked. Try to remember it.
   
      login
   
   The above command will initiate the login process. Now you should be able to log in
   as "root" with the password which you have provided.
   
   ---   ---   ---
   
   Another possible improvement is to set appropriate "global" rights (including the 't'
   sticky flag) for the /tmp folder. You can do that by executing the following command:
   
      chmod 1777 /tmp
   
   I haven't included this in the /init script, as in this particular situation
   it doesn't really matter. By default, we get uid=0 shell console and we have
   full and unrestricted access to the /tmp folder. However, if you decide to add more
   users to the system (not just the "root" user), you may also include the above command
   in the /init script file.
   
   ---   ---   ---
   
   Probably the easiest update you can do is to re-compile the kernel with other options 
   instead of the default ones. This is how we do it for the kernel. First of all we need 
   to know what possible options we have. Navigate to the kernel's source folder and execute
   the following command:
   
      make help
   
   We have several options for interactive ".config" generation. Let's go with this one:
   
      make menuconfig
   
   You might need to resolve one more dependency ("ncurses") if you want the above command
   to work. Once this is done, you can choose any kernel configuration options you like. The
   provided menu is quite complex, but we are not in a hurry. Take your time and enable or
   disable any options you want. Don't forget to save your configuration. Now all we need
   is to compile the new kernel.
   
      make vmlinux
   
   Depending on your configuration, this time the build might take longer. Obviously, if
   you decide to go through manual kernel ".config" configuration, you can no longer use
   the script "2_build_kernel.sh" because it will override your new configuration with
   the default one.
   
   ---   ---   ---
   
   Even though we haven't discussed this topic, you might have already noticed that the
   file ".config", which comes along with the other build scripts, contains the URL
   locations for the kernel sources and the Busybox sources. You can easily build live
   Linux OS based on different kernel/Busybox versions if you provide different
   URLs in the ".config" file. Here is the content of the default ".config" file:
   
      # You can find the latest Linux kernel source bundles here:
      #
      # http://kernel.org
      #
      KERNEL_SOURCE_URL=https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.15.6.tar.xz
   
      # You can find the latest Busybox source bundles here:
      #
      # http://busybox.net
      #
      BUSYBOX_SOURCE_URL=http://busybox.net/downloads/busybox-1.22.1.tar.bz2
   
   Of course, once you do that, you also need to go through the scripts  "1_get_kernel.sh"
   and "3_get_busybox.sh" or execute the relevant commands from these scripts manually.
   
   ---   ---   ---
   
   Another very interesting improvement is to execute /sbin/init and rely on further
   configuration provided in /etc/inittab. Here is the modified version of the shell
   script "5_generate_rootfs.sh":
   
      #/bin/sh
   
      cd work
      rm -rf rootfs
      cd busybox
      cd $(ls -d *)
      cp -R _install ../../rootfs
      cd ../../rootfs
      rm -f linuxrc
      mkdir dev
      mkdir etc
      mkdir proc
      mkdir src
      mkdir sys
      mkdir tmp
      cd etc
      touch bootscript.sh
      echo '#!/bin/sh' >> bootscript.sh
      echo 'dmesg -n 1' >> bootscript.sh
      echo 'mount -t devtmpfs none /dev' >> bootscript.sh
      echo 'mount -t proc none /proc' >> bootscript.sh
      echo 'mount -t sysfs none /sys' >> bootscript.sh
      echo >> bootscript.sh
      chmod +x bootscript.sh
      touch welcome.txt
      echo >> welcome.txt
      echo '  #####################################' >> welcome.txt
      echo '  #                                   #' >> welcome.txt
      echo '  #  Welcome to "Minimal Linux Live"  #' >> welcome.txt
      echo '  #                                   #' >> welcome.txt
      echo '  #####################################' >> welcome.txt
      echo >> welcome.txt
      touch inittab
      echo '::sysinit:/etc/bootscript.sh' >> inittab
      echo '::restart:/sbin/init' >> inittab
      echo '::ctrlaltdel:/sbin/reboot' >> inittab
      echo '::once:cat /etc/welcome.txt' >> inittab
      echo '::respawn:/bin/cttyhack /bin/sh' >> inittab
      echo 'tty2::once:cat /etc/welcome.txt' >> inittab
      echo 'tty2::respawn:/bin/sh' >> inittab
      echo 'tty3::once:cat /etc/welcome.txt' >> inittab
      echo 'tty3::respawn:/bin/sh' >> inittab
      echo 'tty4::once:cat /etc/welcome.txt' >> inittab
      echo 'tty4::respawn:/bin/sh' >> inittab
      echo >> inittab
      cd ..
      touch init
      echo '#!/bin/sh' >> init
      echo 'exec /sbin/init' >> init
      echo >> init
      chmod +x init
      cp ../../*.sh src
      cp ../../.config src
      cd ../..
   
   This script creates very minimal /init which executes /sbin/init. Then, the
   new init process (that is /sbin/init with PID 1) reads the file /etc/inittab and
   executes all commands provided there. The script /etc/bootscript.sh takes care of
   the initial "mount" stuff. We also have 4 terminals (that you can switch between
   with "Alt + F1" to "Alt + F4") and the contents of the file /etc/welcome.txt are
   displayed before we access the shell for each of these terminals.
   
   You can find more information about the supported /etc/inittab commands here:
   
      http://git.busybox.net/busybox/tree/examples/inittab
   
   Note that the details above are specific to Busybox. Usually, the "init" process
   supports runlevels, but that's not the case here.
   
   ---   ---   ---
   
   You are most probably using a normal user (i.e. not "root") when you execute the 
   scripts. One side effect of this is, the generated initrtamfs will keep the original 
   ownership of all files and folders. However, this leads to some interesting 
   discrepancies when you run the system. Some of the files/folders will have "root" 
   ownership (uid=0, gid=0), but most of the files/folders will have the same uid/gid
   as the user which you used in order to build the system. This has no implications
   at all since we have unrestricted shell console but if you'd like to "fix" this,
   you will have to either execute "5_generate_rootfs.sh" as "root" or manually
   change the ownership of the initramfs folder (i.e. "work/rootfs") before you
   execute the script "6_pack_rootfs.sh" with this command:
   
      chown -R root:root work/rootfs
   
   Note that the above command requires "root" permissions, so there is no way to
   escape from the abovementioned discrepancies unless you have "root" access.
   
   Also note that you may need to take care of the permissions for the script files
   in the "work/rootfs/src" folder. To do so, either delete the "src" folder before you
   run "6_pack_rootfs.sh" or make sure that all files have global "read" permissions.
   
   ###   ###   ###
   
   5. Next Steps
   
   OK, now we know how to build and run a basic Linux system. But that's not enough.
   Now we would like to build a more complex system, probably one which supports "pacman",
   "apt-get", or any other package manager.
   
   Building such a system requires a lot more work. A lot more! I mean it!
   
   For starters, "Minimal Linux Live" lives entirely in the "initramfs" space. Simply
   said - this is the RAM space. We never really get out of there. Basically this is
   what you need if you want a more functional live Linux system:
   
      1) Compressed file system (SquashFS)- where all the real stuff would be.
   
      2) GNU Coreutils - these are the real shell command line utilities (ls, cd, etc.).
         You need them as later on, we would "escape" from "initramfs" and use the
         actual file system available in the compressed file. Alternatively, you can
         overlay the whole "Busybox" environment on top of the compressed file system but
         that's not really a professional approach.
   
      3) You need to "switch_root" to the actual file system (the compressed one) which
         you first need to locate, then mount and finally make it "writable" by overlaying
         virtual RAM files/folders via UnionFS/AuFS (or something similar) in case the bootable
         media is not already writable. Remember that you are mounting compressed FS, which
         means that if you make any changes on files located in the original FS, they will
         not be persisted between reboots. Probably, you would like to handle this edge case.
      
      4) You need to prepare the stuff that you want to be present in the compressed file
         system in advance. Assuming that we are building a useful live Linux OS, the
         absolute minimum is "GNU Coreutils", network management utilities and probably
         some kind of package manager, e.g. "pacman" or "apt-get". Don't forget that most
         of this additional stuff very likely relies on different kernel options that
         also need to be enabled/configured beforehand.
      
      5) You need to rewrite /init and make it execute all necessary steps to fulfil point 3
         from the above. Different live Linux systems use different approaches here.
      
   All of the above is definitely *not* minimal but it's definitely doable if you put some
   additional effort. It will take time, but as a result you would have a fully functional live
   Linux OS, which you would have created entirely by yourself.
   
   ###   ###   ###
   
   6. Epilogue
   
   That's all folks! I hope you find this tutorial useful. And remember, we are talking
   about very minimal live Linux OS here. If you want to create something bigger, I
   suggest you take a look at "Linux From Scratch" (google it, you'll find it).
   
   At some point, you may also find it useful to play around with other live Linux
   distributions. Don't forget that the initramfs file is compressed and in order to
   get to the actual /init script you will need to uncompress it. Then you can see
   the actual file structure of the initial root file system and examine the /init
   script file. This will give you more insight on the boot process for the
   particular live Linux distributions that you are learning from.
   
   One interesting live Linux distribution is "Slax". Just download it and take a
   look at the /init script file (remember, you can uncompress the initramfs file).
   There is also a very good document which explains the internal root file system
   and the OS boot process for Slax:
   
      http://slax.org/en/documentation.php#internals
   
   The above URL may have changed but it is valid at the time I write this document.