From Power Up To Bash Prompt Greg O'Keefe, gcokeefe@postoffice.utas.edu.au v0.7, April 2000 This is a brief description of what happens in a Linux system, from the time that you turn on the power, to the time that you log in and get a bash prompt. It is organised by package to make it easier for people who want to build a system from source code. Understanding this will be helpful when you need to solve problems or configure your sys­ tem. ______________________________________________________________________ Table of Contents 1. Introduction 2. Hardware 2.1 Configuration 2.2 Exercises 2.3 More Information 3. Lilo 3.1 Configuration 3.2 Exercises 3.3 More Information 4. The Linux Kernel 4.1 Configuration 4.2 Exercises 4.3 More Information 5. The GNU C Library 5.1 Configuration 5.2 Exercises 5.3 More Information 6. Init 6.1 Configuration 6.2 Exercises 6.3 More Information 7. The Filesystem 7.1 Configuration 7.2 Exercises 7.3 More Information 8. Kernel Daemons 8.1 Configuration 8.2 Exercises 8.3 More Information 9. System Logger 9.1 Configuration 9.2 Exercises 9.3 More Information 10. Getty and Login 10.1 Configuration 10.2 Exercises 11. Bash 11.1 Configuration 11.2 Exercises 11.3 More Information 12. Commands 13. Building A Minimal Linux System From Source 13.1 What You Will Need 13.2 The Filesystem 13.3 MAKEDEV 13.4 Kernel 13.5 Lilo 13.6 Glibc 13.7 SysVinit 13.8 Ncurses 13.9 Bash 13.10 Util-linux (getty and login) 13.11 Sh-utils 13.12 Towards Useability 13.13 Random Tips 13.14 More Information 14. Conclusion 15. Administrivia 15.1 Copyright 15.2 Homepage 15.3 Feedback 15.4 Acknowledgements 15.5 Change History 15.5.1 0.6 -> 0.7 15.5.2 0.5 -> 0.6 15.6 TODO ______________________________________________________________________ 1. Introduction I find it frustrating that many things happen inside my Linux machine that I do not understand. If, like me, you want to really understand your system rather than just knowing how to use it, this document should be a good place to start. This kind of background knowledge is also needed if you want to be a top notch Linux problem solver. I assume that you have a working Linux box, and understand some basic things about Unix and PC hardware. If not, an excellent place to start learning is Eric S. Raymond's The Unix and Internet Fundamentals HOWTO It is short, very readable and covers all the basics. The main thread in this document is how Linux starts itself up. But it also tries to be a more comprehensive learning resource. I have included exercises in each section. If you actually do some of these, you will learn much more than you could by just reading. There are also links to source code downloads. The reason for this is that I hope some readers will undertake the best Linux learning exercise that I know of, which is building a system from source code. Giambattista Vico, an Italian philosopher (1668-1744) said ``verum ipsum factum'', which means ``understanding arises through making''. Thanks to Alex (see ``Acknowledgements'') for this quote. If you want to ``roll your own'', you should also see Gerard Beekmans' Linux From Scratch HOWTO (LFS). LFS has detailed instructions on building a complete useable system from source code. On the LFS website, you will also find a mailing list for people building systems this way. What I have included in this document, is instructions (see ``Building a Minimal Linux System From Source'') for building a ``toy'' system, purely as a learning exercise. Packages are presented in the order in which they appear in the system startup process. This means that if you install the packages in this order you can reboot after each installation, and see the system get a little closer to giving you a bash prompt each time. There is a reassuring sense of progress in this. I recommend that you first read the main text of each section, skipping the exercises and references. Then decide how deep an understanding you want to develop, and how much effort you are prepared to put in. Then start at the beginning again, doing the exercises and additional reading as you go. 2. Hardware When you first turn on your computer it tests itself to make sure everything is in working order. This is called the ``Power on self test''. Then a program called the bootstrap loader, located in the ROM BIOS, looks for a boot sector. A boot sector is the first sector of a disk and has a small program that can load an operating system. Boot sectors are marked with a magic number 0xAA55 = 43603 at byte 0x1FE = 510. That's the last two bytes of the sector. This is how the hardware can tell whether the sector is a boot sector or not. The bootstrap loader has a list of places to look for a boot sector. My old machine looks in the primary floppy drive, then the primary hard drive. More modern machines can also look for a boot sector on a CD-ROM. If it finds a boot sector, it loads it into memory and passes control to the program that loads the operating system. On a typical Linux system, this program will be LILO's first stage boot loader. There are many different ways of setting your system up to boot though. See the LILO User's Guide for details. See section ``LILO'' for a URL. Obviously there is a lot more to say about what PC hardware does. But this is not the place to say it. See one of the many good books about PC hardware. 2.1. Configuration The machine stores some information about itself in its CMOS. This includes what disks and RAM are in the system. The machine's BIOS contains a program to let you modify these settings. Check the messages on your screen as the machine is turned on to see how to access it. On my machine, you press the delete key before it begins loading its operating system. 2.2. Exercises A good way to learn about PC hardware is to build a machine out of second hand parts. Get at least a 386 so you can easily run Linux on it. It won't cost much. Ask around, someone might give you some of the parts you need. Check out, download compile and make a boot disk for Unios . (They used to have a home page at , but it disappeared) This is just a bootable ``Hello World!'' program, consisting of just over 100 lines of assembler code. It would be good to see it converted to a format that the GNU assembler as can understand. Open the boot disk image for unios with a hex editor. This image is 512 bytes long, exactly one sector. Find the magic number 0xAA55. Do the same for the boot sector from a bootable floppy disk or your own computer. You can use the dd command to copy it to a file: dd if=/dev/fd0 of=boot.sector. Be very careful to get if (input file) and of (output file) the right way round! Check out the source code for LILO's boot loader. 2.3. More Information · The Unix and Internet Fundamentals HOWTO by Eric S. Raymond, especially section 3, What happens when you switch on a computer? · The first chapter of The LILO User's Guide gives an excellent explanation of PC disk partitions and booting. See section ``LILO'' for a URL. · The NEW Peter Norton Programmer's Guide to the IBM PC & PS/2, by Peter Norton and Richard Wilton, Microsoft Press 1988 There is a newer Norton book, which looks good, but I can't afford it right now! · One of the many books available on upgrading PC's 3. Lilo When the computer loads a boot sector on a normal Linux system, what it loads is actually a part of lilo, called the ``first stage boot loader''. This is a tiny program who's only job in life is to load and run the ``second stage boot loader''. The second stage loader gives you a prompt (if it was installed that way) and loads the operating system you choose. When your system is up and running, and you run lilo, what you are actually running is the ``map installer''. This reads the configuration file /etc/lilo.conf and writes the boot loaders, and information about the operating systems it can load, to the hard disk. There are lots of different ways to set your system up to boot. What I have just explained is the most obvious and ``normal'' way, at least for a system who's main operating system is Linux. The Lilo Users' Guide explains several examples of ``boot concepts''. It is worth reading these, and trying some of them out. 3.1. Configuration The configuration file for lilo is /etc/lilo.conf. There is a manual page for it: type man lilo.conf into a shell to see it. The main thing in lilo.conf is one entry for each thing that lilo is set up to boot. For a Linux entry, this includes where the kernel is, and what disk partition to mount as the root filesystem. For other operating systems, the main piece of information is which partition to boot from. 3.2. Exercises DANGER: take care with these exercises. It is easy enough to get something wrong and screw up your master boot record and make your system unuseable. Make sure you have a working rescue disk, and know how to use it to fix things up again. See below for a link to tomsrtbt, the rescue disk I use and recommend. The best precaution is to use a machine that doesn't matter. Set up lilo on a floppy disk. It doesn't matter if there is nothing other than a kernel on the floppy - you will get a ``kernel panic'' when the kernel is ready to load init, but at least you will know that lilo is working. If you like you can press on and see how much of a system you can get going on the floppy. This is probably the second best Linux learning activity around. See the Bootdisk HOWTO (url below), and tomsrtbt (url below) for clues. Get lilo to boot unios (see section ``hardware exercises'' for a URL). As an extra challenge, see if you can do this on a floppy disk. Make a boot-loop. Get lilo in the master boot record to boot lilo in one of the primary partition boot sectors, and have that boot lilo in the master boot record... Or perhaps use the master boot record and all four primary partitions to make a five point loop. Fun! 3.3. More Information · The lilo man page. · The Lilo package (see ``downloads'') contains the ``LILO User's Guide'' lilo-u-21.ps.gz (or a later version). You may already have this document though. Check /usr/doc/lilo or there abouts. The postscript version is better than the plain text, since it contains diagrams and tables. · tomsrtbt the coolest single floppy linux. Makes a great rescue disk. · The Bootdisk HOWTO 4. The Linux Kernel The kernel does quite a lot really. I think a fair way of summing it up is that it makes the hardware do what the programs want, fairly and efficiently. The processor can only execute one instruction at a time, but Linux systems appear to be running lots of things simultaneously. The kernel acheives this by switching from task to task really quickly. It makes the best use of the processor by keeping track of which processes are ready to go, and which ones are waiting for something like a record from a hard disk file, or some keyboard input. This kernel task is called scheduling. If a program isn't doing anything, then it doesn't need to be in RAM. Even a program that is doing something, might have parts that aren't doing anything. The address space of each process is divided into pages. The Kernel keeps track of which pages of which processes are being used the most. The pages that aren't used so much can be moved out to the swap partition. When they are needed again, another unused page can be paged out to make way for it. This is virtual memory management. If you have ever compiled your own Kernel, you will have noticed that there are many many options for specific devices. The kernel contains a lot of specific code to talk to diverse kinds of hardware, and present it all in a nice uniform way to the application programs. The Kernel also manages the filesystem, interprocess communication, and a lot of networking stuff. Once the kernel is loaded, the first thing it does is look for an init program to run. 4.1. Configuration Most of the configuration of the kernel is done when you build it, using make menuconfig, or make xconfig in /usr/src/linux/ (or wherever your Linux kernel source is). You can reset the default video mode, root filesystem, swap device and RAM disk size using rdev. These parameters and more can also be passed to the kernel from lilo. You can give lilo parameters to pass to the kernel either in lilo.conf, or at the lilo prompt. For example if you wanted to use hda3 as your root file system instead of hda2, you might type LILO: linux root=/dev/hda3 If you are building a system from source, you can make life a lot simpler by creating a ``monolithic'' kernel. That is one with no modules. Then you don't have to copy kernel modules to the target system. NOTE: The System.map file is used by the kernel logger to determine the module names generating messages. The program top also uses this information. When you copy the kernel to the target system, copy System.map too. 4.2. Exercises Think about this: /dev/hda3 is a special type of file that describes a hard disk partition. But it lives on a file system just like all other files. The kernel wants to know which partition to mount as the root filesystem - it doesn't have a file system yet. So how can it read /dev/hda3 to find out which partition to mount? If you haven't already: build your own kernel. Read all the help information for each option. See how small a kernel you can make that still works. You can learn a lot by leaving the wrong things out! Read ``The Linux Kernel'' (URL below) and as you do, find the parts of the source code that it refers to. The book (as I write) refers to kernel version 2.0.33, which is pretty out of date. It might be easier to follow if you download this old version and read the source there. Its amazing to find bits of C code called ``process'' and ``page''. Hack! See if you can make it spit out some extra messages or something. 4.3. More Information · /usr/src/linux/README and the contents of /usr/src/linux/Documentation/ (These may be in some other place on your system) · The Kernel HOWTO · The help available when you configure a kernel using make menuconfig or make xconfig · The Linux Kernel (and other LDP Guides) · Kernel source download see ``downloads'' 5. The GNU C Library The next thing that happens as your computer starts up is that init is loaded and run. However, init, like almost all programs, uses functions from libraries. You may have seen an example C program like this: main() { printf("Hello World!\n"); } The program contains no definition of printf, so where does it come from? It comes from the standard C libraries, on a GNU/Linux system, glibc. If you compile it under Visual C++, then it comes from a Microsoft implementation of the same standard functions. There are zillions of these standard functions, for math, string, dates/times memory allocation and so on. Everything in Unix (including Linux) is either written in C or has to try hard to pretend it is, so everything uses these functions. If you look in /lib on your linux system you will see lots of files called libsomething.so or libsomething.a etc. They are libraries of these functions. Glibc is just the GNU implementation of these functions. There are two ways programs can use these library functions. If you statically link a program, these library functions are copied into the executable that gets created. This is what the libsomething.a libraries are for. If you dynamically link a program (and this is the default), then when the program is running and needs the library code, it is called from the libsomething.so file. The command ldd is your friend when you want to work out which libraries are needed by a particular program. For example, here are the libraries that bash uses: [greg@Curry power2bash]$ ldd /bin/bash libtermcap.so.2 => /lib/libtermcap.so.2 (0x40019000) libc.so.6 => /lib/libc.so.6 (0x4001d000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000) 5.1. Configuration Some of the functions in the libraries depend on where you are. For example, in Australia we write dates as dd/mm/yy, but Americans write mm/dd/yy. There is a program that comes with the glibc distribution called localedef which enables you to set this up. 5.2. Exercises Use ldd to find out what libraries your favourite applications use. Use ldd to find out what libraries init uses. Make a toy library, with just one or two functions in it. The program ar is used to create them, the man page for ar might be a good place to start investigating how this is done. Write, compile and link a program that uses this library. 5.3. More Information · source code, see section ``downloads'' 6. Init I will only talk about the ``System V'' style of init that Linux systems mostly use. There are alternatives. In fact, you can put any program you like in /sbin/init, and the kernel will run it when it has finished loading. It is init's job to get everthing running the way it should be. It checks that the file systems are ok and mounts them. It starts up ``daemons'' to log system messages, do networking, serve web pages, listen to your mouse and so on. It also starts the getty processes that put the login prompts on your virtual terminals. There is a whole complicated story about switching ``run-levels'', but I'm going to mostly skip that, and just talk about system start up. Init reads the file /etc/inittab, which tells it what to do. Typically, the first thing it is told to do is to run an initialisation script. The program that executes (or interprets) this script is bash, the same program that gives you a command prompt. In Debian systems, the initialisation script is /etc/init.d/rcS, on Red Hat, /etc/rc.d/rc.sysinit. This is where the filesystems get checked and mounted, the clock set, swap space enabled, hostname gets set etc. Next, another script is called to take us into the default run-level. This just means a set of subsystems to start up. There is a set of directories /etc/rc.d/rc0.d, /etc/rc.d/rc1.d, ..., /etc/rc.d/rc6.d in Red Hat, or /etc/rc0.d, /etc/rc1.d, ..., /etc/rc6.d in Debian, which correspond to the run-levels. If we are going into runlevel 3 on a Debian system, then the script runs all the scripts in /etc/rc3.d that start with `S' (for start). These scripts are really just links to scripts in another directory usually called init.d. So our run-level script was called by init, and it is looking in a directory for scripts starting with `S'. It might find S10syslog first. The numbers tell the run-level script which order to run them in. So in this case S10syslog gets run first, since there were no scripts starting with S00 ... S09. But S10syslog is really a link to /etc/init.d/syslog which is a script to start and stop the system logger. Because the link starts with an `S', the run-level script knows to execute the syslog script with a ``start'' parameter. There are corresponding links starting with `K' (for kill), which specify what to shut down and in what order when leaving the run-level. To change what subsystems start up by default, you must set up these links in the rcN.d directory, where N is the default runlevel set in your inittab. The last important thing that init does is to start some getty's. These are ``respawned'' which means that if they stop, init just starts them again. Most distributions come with six virtual terminals. You may want less than this to save memory, or more so you can leave lots of things running and quickly flick to them as you need them. You may also want to run a getty for a text terminal or a dial in modem. In this case you will need to edit the inittab file. 6.1. Configuration /etc/inittab is the top level configuration file for init. The rcN.d directories, where N = 0, 1, ..., 6 determine what subsystems are started. Somewhere in one of the scripts invoked by init, the mount -a command will be issued. This means mount all the file systems that are supposed to be mounted. The file /etc/fstab defines what is supposed to be mounted. If you want to change what gets mounted where when your system starts up, this is the file you will need to edit. There is a man page for fstab. 6.2. Exercises Find the rcN.d directory for the default run-level of your system and do a ls -l to see what the files are links to. Change the number of gettys that run on your system. Remove any subsystems that you don't need from your default run-level. See how little you can get away with starting. Set up a floppy disk with lilo, a kernel and a statically linked "hello world" program called /sbin/init and watch it boot up and say hello. Watch carefully as your system starts up, and take notes about what it tells you is happening. Or print a section of your system log /var/log/messages from start up time. Then starting at inittab, walk through all the scripts and see what code does what. You can also put extra start up messages in, such as echo "Hello, I am rc.sysinit" This is a good exercise in learning Bash shell scripting too, some of the scripts are quite complicated. Have a good Bash reference handy. 6.3. More Information · see ``downloads'' for source code download url's · There are man pages for the inittab and fstab files. Type (eg) man inittab into a shell to see it. · The Linux System Administrators Guide has a good section on init. 7. The Filesystem In this section, I will be using the word ``filesystem'' in two different ways. There are filesystems on disk partitions and other devices, and there is the filesystem as it is presented to you by a running Linux system. In Linux, you ``mount'' a disk filesystem onto the system's filesystem. In the previous section I mentioned that init scripts check and mount the filesystems. The commands that do this are fsck and mount respectively. A hard disk is just a big space that you can write ones and zeros on. A filesystem imposes some structure on this, and makes it look like files within directories within directories... Each file is represented by an inode, which says who's file it is, when it was created and where to find its contents. Directories are also represented by inodes, but these say where to find the inodes of the files that are in the directory. If the system wants to read /home/greg/bigboobs.jpeg, it first finds the inode for the root directory / in the ``superblock'', then finds the inode for the directory home in the contents of /, then finds the inode for the directory greg in the contents of /home, then the inode for bigboobs.jpeg which will tell it which disk blocks to read. If we add some data to the end of a file, it could happen that the data is written before the inode is updated to say that the new blocks belong to the file, or vice versa. If the power cuts out at this point, the filesystem will be broken. It is this kind of thing that fsck attempts to detect and repair. The mount command takes a filesystem on a device, and adds it to the heirarchy that you see when you use your system. Usually, the kernel mounts its root file system read-only. The mount command is used to remount it read-write after fsck has checked that it is ok. Linux supports other kinds of filesystem too: msdos, vfat, minix and so on. The details of the specific kind of filesystem are abstracted away by the virtual file system (VFS). I won't go into any detail on this though. There is a discussion of it in ``The Linux Kernel'' (see section ``The Linux Kernel'' for a url) 7.1. Configuration There are parameters to the command mke2fs which creates ext2 filesystems. These control the size of blocks, the number of inodes and so on. Check the mke2fs man page for details. What gets mounted where on your filesystem is controlled by the /etc/fstab file. It also has a man page. 7.2. Exercises Make a very small filesystem, and view it with a hex viewer. Identify inodes, superblocks and file contents. I believe there are tools that give you a graphical view of a filesystem. Find one, try it out, and email me the url and a review! Check out the ext2 filesystem code in the Kernel. 7.3. More Information · Chapter 9 of the LDP book ``The Linux Kernel'' is an excellent description of filesystems. You can find it at the Australian LDP mirror · The mount command is part of the util-linux package, there is a link to it in ``downloads''. · man pages for mount, fstab, fsck and mke2fs · EXT2 File System Utilities ext2fsprogs home page ext2fsprogs Australian mirror. There is also a Ext2fs-overview document here, although it is out of date, and not as readable as chapter 9 of ``The Linux Kernel'' · Unix File System Standard Another link to the Unix File System Standard. This describes what should go where in a Unix file system, and why. It also has minimum requirements for the contents of /bin, /sbin and so on. This is a good reference if your goal is to make a minimal yet complete system. 8. Kernel Daemons Unfortunately, this section contains more conjectures and questions than facts. Perhaps you can help? If you issue the ps aux command, you will see something like the following: USER PID %CPU %MEM SIZE RSS TTY STAT START TIME COMMAND root 1 0.1 8.0 1284 536 ? S 07:37 0:04 init [2] root 2 0.0 0.0 0 0 ? SW 07:37 0:00 (kflushd) root 3 0.0 0.0 0 0 ? SW 07:37 0:00 (kupdate) root 4 0.0 0.0 0 0 ? SW 07:37 0:00 (kpiod) root 5 0.0 0.0 0 0 ? SW 07:37 0:00 (kswapd) root 52 0.0 10.7 1552 716 ? S 07:38 0:01 syslogd -m 0 root 54 0.0 7.1 1276 480 ? S 07:38 0:00 klogd root 56 0.3 17.3 2232 1156 1 S 07:38 0:13 -bash root 57 0.0 7.1 1272 480 2 S 07:38 0:01 /sbin/agetty 38400 tt root 64 0.1 7.2 1272 484 S1 S 08:16 0:01 /sbin/agetty -L ttyS1 root 70 0.0 10.6 1472 708 1 R Sep 11 0:01 ps aux This is a list of the processes running on the system. Note that init is process number one. Processes 2, 3, 4 and 5 are kflushd, kupdate, kpiod and kswapd. There is something strange here though: notice that in both the virtual storage size (SIZE) and the Real Storage Size (RSS) columns, these processes have zeroes. How can a process use no memory? These processes are really part of the kernel. The kernel does not show up on process lists at all, and you can only work out what memory it is using by subtracting the memory available from the amount on your system. The brackets around the command name could signify that these are kernel processes(?) kswapd moves parts of programs that are not currently being used from real storage (ie RAM) to the swap space (ie hard disk). kflushd writes data from buffers to disk. This allows things to run faster. What programs write can be kept in memory, in a buffer, then written to disk in larger more efficient chunks. I don't know what kupdate and kpiod are for. This is where my knowledge ends. What do these last two daemons do? Why do kernel daemons get explicit process numbers rather than just being anonymous bits of kernel code? Does init actually start them, or are they already running when init arrives on the scene? I put a script to mount /proc and do a ps aux in /sbin/init. Process 1 was the script itself, and processess 2, 3, 4 and 5 were the kernel daemons just as under the real init. The kernel must put these processes there, because my script certainly didn't! The following ramblings were contributed by David Leadbeater: These processes seem to take care of disk reads and writes, they seem to be started by the kernel but after it runs the init process, it seems that being run as kernel processes rather than seperate processess they are protected from being killed (kill -9 dosen't stop them), I am not sure why they are run as seperate threads (it seems to be something with disk access) kflushd and kupdate These two processes are started to flush dirty (changed) buffers back to disk. kflushd is run when the buffers are full and kupdate runs periodically (5 seconds?) to sync the disk and the buffers in memory. kpiod and kswapd These deal with paging out pages (sections) of memory into the swap file so main memory never gets exhausted, these are similar to kflushd and kupdate in that one is run when needed kpiod and the other kswapd is run peridically (1 second intervals) Other Kernel Daemons On a default install of RH6 kupdate is missing but update is running as a user space daemon so it seems it needs to be run! Also another daemon mdrecoveryd is there, this seems to be dealing with software RAID, looking at the kernel source it seems that some SCSI drivers also start seperate processes. I am still unsure of the meaning of the brackets but it seems that they appear when the RSS of a process is 0 meaning it isn't using any memory? (end of ramble, thanks David) 8.1. Configuration I don't know of any configuration for these kernel daemons. 8.2. Exercises Find out what these processes are for, how they work, and write a new ``Kernel Daemons'' section for this document and send it to me! 8.3. More Information The Linux Documentation Project's ``The Linux Kernel'' (see section ``The Linux Kernel'' for a url), and the kernel source code are all I can think of. 9. System Logger Init starts the syslogd and klogd daemons. They write messages to logs. The kernel's messages are handled by klogd, while syslogd handles log messages from other processes. The main log is /var/log/messages. This is a good place to look if something is going wrong with your system. Often there will be a valuable clue in there. 9.1. Configuration The file /etc/syslog.conf tells the loggers what messages to put where. Messages are identified by which service they come from, and what priority level they are. This configuration file consists of lines that say messages from service x with priority y go to z, where z is a file, tty, printer, remote host or whatever. NOTE: Syslog requires the /etc/services file to be present. The services file allocates ports. I am not sure whether syslog needs a port allocated so that it can do remote logging, or whether even local logging is done through a port, or whether it just uses /etc/services to convert the service names you type /etc/syslog.conf into port numbers. 9.2. Exercises Have a look at your system log. Find a message you don't understand, and find out what it means. Send all your log messages to a tty. (set it back to normal once done) 9.3. More Information Australian sysklogd Mirror 10. Getty and Login Getty is the program that enables you to log in through a serial device such as a virtual terminal, a text terminal, or a modem. It displays the login prompt. Once you enter your username, getty hands this over to login which asks for a password, checks it out and gives you a shell. There are many getty's available. Some distributions, including Red Hat use a very small one called mingetty that only works with virtual terminals. The login program is part of the util-linux package, which also contains a getty called agetty, which works fine. This package also contains mkswap, fdisk, passwd, kill, setterm, mount, swapon, rdev, renice, more (the program) and more (ie more programs). 10.1. Configuration The message that comes on the top of your screen with your login prompt comes from /etc/issue. Gettys are usually started in /etc/inittab. Login checks user details in /etc/passwd, and if you have password shadowing, /etc/shadow. 10.2. Exercises Create a /etc/passwd by hand. Passwords can be set to null, and changed with the program passwd once you log on. See the man page for this file Use man 5 passwd to get the man page for the file rather than the man page for the program. 11. Bash If you give login a valid username and password combination, it will check in /etc/passwd to see which shell to give you. In most cases on a Linux system this will be bash. It is bash's job to read your commands and see that they are acted on. It is simultaneously a user interface, and a programming language interpreter. As a user interface it reads your commands, and executes them itself if they are ``internal'' commands like cd, or finds and executes a program if they are ``external'' commands like cp or startx. It also does groovy stuff like keeping a command history, and completing filenames. We have already seen bash in action as a programming language interpreter. The scripts that init runs to start the system up are usually shell scripts, and are executed by bash. Having a proper programming language, along with the usual system utilities available at the command line makes a very powerful combination, if you know what you are doing. For example (smug mode on) I needed to apply a whole stack of ``patches'' to a directory of source code the other day. I was able to do this with the following single command: for f in /home/greg/sh-utils-1.16*.patch; do patch -p0 < $f; done; This looks at all the files in my home directory whose names start with sh-utils-1.16 and end with .patch. It then takes each of these in turn, and sets the variable f to it and executes the commands between do and done. In this case there were 11 patch files, but there could just as easily have been 3000. 11.1. Configuration The file /etc/profile controls the system-wide behaviour of bash. What you put in here will affect everybody who uses bash on your system. It will do things like add directories to the PATH, set your MAIL directory variable. The default behaviour of the keyboard often leaves a lot to be desired. It is actually readline that handles this. Readline is a separate package that handles command line interfaces, providing the command history and filename completion, as well as some advanced line editing features. It is compiled into bash. By default, readline is configured using the file .inputrc in your home directory. The bash variable INPUTRC can be used to override this for bash. For example in Red Hat 6, INPUTRC is set to /etc/inputrc in /etc/profile. This means that backspace, delete, home and end keys work nicely for everyone. Once bash has read the system-wide configuration file, it looks for your personal configuration file. It checks in your home directory for .bash_profile, .bash_login and .profile. It runs the first one of these it finds. If you want to change the way bash behaves for you, without changing the way it works for others, do it here. For example, many applications use environment variables to control how they work. I have the variable EDITOR set to vi so that I can use vi in Midnight Commander (an excellent console based file manager) instead of its editor. 11.2. Exercises The basics of bash are easy to learn. But don't stop there: there is an incredible depth to it. Get into the habit of looking for better ways to do things. Read shell scripts, look up stuff you don't understand. 11.3. More Information · source code download see ``downloads'' · There is a ``Bash Reference Manual'' with this, which is comprehensive, but heavy going. · There is an O'Rielly book on Bash, not sure if it's good. · I don't know of any good free up to date bash tutorials. If you do, please email me a url. 12. Commands You do most things in bash by issuing commands like cp. Most of these commands are small programs, though some, like cd are built into the shell. The commands come in packages, most of them from the Free Software Foundation (or GNU). Rather than list the packages here, I'll direct you to the Linux From Scratch HOWTO . It has a full and up to date list of the packages that go into a Linux system as well as instructions on how to build them. 13. Building A Minimal Linux System From Source So far I have focussed on what the packages do. Here I will offer what clues I can about making a minimal Linux system from source. This is a toy system we are making here. If you want to build a real system to be used for real work, see the Linux From Scratch HOWTO . It is possible to get a bash prompt without installing everything I mention here. What I describe is a base system, without nasty kludges, that can be built on easily. 13.1. What You Will Need We will install a Linux distribution like Red Hat in one partition, and use that to build a new Linux system in another partition. I will call the system we are building the ``target'' and the system we are using to build it with, the ``source'' (not to be confused with source code which we will also be using.) So you are going to need a machine with two spare partitions on it. If you can, use a machine with nothing important on it. You could use an existing Linux installation as the source system, but I wouldn't recommend that. If you leave a parameter out of one of the commands we will issue, you could accidentally install stuff to this system. This could lead to incompatibilites and strife. Older PC hardware, mostly 486's and earlier, have an annoying limitation in their bios. They can not read from a hard disk past the first 512M. This is not too much of a problem for Linux, because once it is up, it does its own disk io, bypassing the bios. But for Linux to get loaded by these old machines, the kernel has to reside somewhere below 512M. If you have one of these machines you will need to have a separate partition completely below the 512M mark, to mount as /boot for any partitions that are over that 512M mark. Last time I did this, I used Red Hat 6.1 as a source system. I installed the base system plus · cpp · egcs · egcs-c++ · patch · make · dev86 · ncurses-devel · glibc-devel · kernel-headers I also had X-window and Mozilla so I could read documentation easily, but that's not really necessary. By the time I had finished working, it had used about 350M of disk space. (Seems a bit high, I wonder why?) The finished target system took 650M, but that includes all the source code and intermediate build files. If space is tight, you should do a make clean after each package is built. Still, this mind boggling bloat is a bit of a worry. Finally, you are going to need the source code for the system we are going to build. These are the ``packages'' that I have discussed in this document. These can be obtained from a source cd, or from the internet. I'll give URL's for the USA sites and for Australian mirrors. · MAKEDEV USA Another USA site · Lilo USA , Australia . · Linux Kernel Use one of the mirrors listed at home page rather than USA because they are always overloaded. Australia · GNU libc itself, and the linuxthreads addon are at USA Australia · GNU libc addons You will also need the linuxthreads and libcrypt addons. If libcrypt is not there it is because of some US export laws. You can get it at libcrypt The linuxthreads addon is in the same places as libc itself · GNU ncurses USA Australia · SysVinit USA Australia · GNU Bash USA Australia · GNU sh-utils USA Australia · util-linux Somewhere else Australia This package contains agetty and login. To sum up then, you will need: · A machine with two spare partitions of about 400M and 700M respectively though you could probably get away with less · A Linux distribution (eg. a Red Hat cd) and a way of installing it (eg. a cdrom drive) · The source code tarballs listed above I'm assuming that you can install the source system yourself, without any help from me. From here on, I'll assume that its done. The first milestone in this little project is getting the kernel to boot up and panic because it can't find an init. This means we are going to have to install a kernel, and install lilo. To install lilo nicely though, we will need the device files in the target /dev directory. Lilo needs them to do the low level disk access necessary to write the boot sector. MAKEDEV is the script that creates these device files. (You can just copy them from the source system of course, but that's cheating!) But first of all, we need a filesystem to put all of this into. 13.2. The Filesystem Our new system is going to live in a file system. So first, we have to make that file system using mke2fs. Then mount it somewhere. I'd suggest /mnt/target. In what follows, I'll assume that this is where it is. You could save yourself a bit of time by putting an entry in /etc/fstab so that it mounts there automatically when the source system comes up. When we boot up the target system, the stuff that's now in /mnt/target will be in /. We need a directory structure on target. Have a look at the File Heirarchy Standard (see section ``Filesystem'') to work out what this should be, or just cd to where the target is mounted and blindly do mkdir bin boot dev etc home lib mnt root sbin tmp usr var cd var; mkdir lock log run spool cd ../usr; mkdir bin include lib local sbin share src cd share/; mkdir man; cd man mkdir man1 man2 man3 ... man9 Since the FHS and most packages disagree about where man pages should go, we need a symlink cd ..; ln -s share/man man 13.3. MAKEDEV We will put the source code in the target /usr/src directory. So for example, if your target file system is mounted on /mnt/target and your tarballs are in /root, you would do cd /mnt/target/usr/src tar -xzvf /root/MAKEDEV-2.5.tar.gz Don't be completely lame and copy the tarball to the place where you are going to extract it ;-> Normally when you install software, you are installing it onto the system that is running. We don't want to do that though, we want to install it as though /mnt/target is the root filesystem. Different packages have different ways of letting you do this. For MAKEDEV you do ROOT=/mnt/target make install You need to look out for these options in the README and INSTALL files or by doing a ./configure --help. Have a look in MAKEDEV's Makefile to see what it does with the ROOT varible that we set in that command. Then have a look in the man page by doing man ./MAKEDEV.man to see how it works. You'll find that the way to make our device files is to cd /mnt/target/dev and do ./MAKEDEV generic. Do an ls to see all the wonderful device files it has made for you. 13.4. Kernel Next we make a kernel. I presume you've done this before, so I'll be brief. It is easier to install lilo if the kernel it is meant to boot is already there. Go back to the target usr/src directory, and unpack the linux kernel source there. Enter the linux source tree (cd linux) and configure the kernel using your favourite method, for example make menuconfig. You can make life slightly easier for yourself by configuring a kernel without modules. If you configure any modules, then you will have to edit the Makefile, find INSTALL_MOD_PATH and set it to /mnt/target. Now you can make dep, make bzImage, and if you configured modules: make modules, make modules_install. Copy the kernel arch/i386/boot/bzImage and the system map System.map to the target boot directory /mnt/target/boot, and we are ready to install lilo. 13.5. Lilo Lilo comes with a neat script called QuickInst. Unpack the lilo source into the target source directory, run this script with the command ROOT=/mnt/target ./QuickInst. It will ask you questions about how you want lilo installed. Remember, since we have set ROOT, to the target partition, you tell it file names relative to that. So when it asks what kernel you want to boot by default, answer /boot/bzImage not /mnt/target/boot/bzImage. I found a little bug in the script, so it said ./QuickInst: /boot/bzImage: no such file But if you just ignore it, it's ok. Where should we get QuickInst to put the boot sector? When we reboot we want to have the choice of booting into the source system or the target system, or any other systems that are on this box. And we want the instance of lilo that we are building now to load the kernel of our new system. How are we going achieve both of these things? Let's digress a little and look at how lilo boots DOS on a dual boot Linux system. The lilo.conf file on such a system probably looks something like this: prompt timeout = 50 default = linux image = /boot/bzImage label = linux root = /dev/hda1 read-only other = /dev/hda2 label = dos If the machine is set up this way, then the master boot record gets read and loaded by the bios, and it loads the lilo bootloader, which gives a prompt. If you type in dos at the prompt, lilo loads the boot sector from hda2, and it loads DOS. What we are going to do is just the same, except that the boot sector in hda2 is going to be another lilo boot sector - the one that QuickInst is going to install. So the lilo from the Linux distribution will load the lilo that we have built, and that will load the kernel that we have built. You will see two lilo prompts when you reboot. To cut a long story short, when QuickInst asks you where to put the boot sector, tell it the device where your target filesystem is, eg. /dev/hda2. Now modify the lilo.conf on your source system, so it has a line like other = /dev/hda2 label = target run lilo, and we should be able to do our first boot into the target system. 13.6. Glibc Next we want to install init, but like almost every program that runs under Linux, init uses library functions provided by the GNU C library, glibc. So we will install that first. Glibc is a very large and complicated package. It took 90 hours to build on my old 386sx/16 with 8M RAM. But it only took 33 minutes on my Celeron 433 with 64M. I think memory is the main issue here. If you only have 8M of RAM (or, shudder, less!) be prepared for a long build. The glibc install documentation recommends building in a separate directory. This enables you to start again easily, by just blowing that directory away. You might also want to do that to save yourself about 265M of disk space! Unpack the glibc-2.1.3.tar.gz (or whatever version) tarball into /mnt/target/usr/src as usual. Now, we need to unpack the ``add-ons'' into glibc's directory. So cd glibc-2.1.3, and then unpack the glibc- crypt-2.1.3.tar.gz and glibc-linuxthreads-2.1.3.tar.gz tarballs there. Now we can create the build directory, configure, make and install glibc. These are the commands I used, but read the documentation yourself and make sure you do what is best for your circumstances. Before you do though, you might want to do a df command to see how much free space you have. You can do another after you've built and installed glibc, to see what a space-hog it is. cd .. mkdir glibc-build ../glibc-2.1.3/configure --enable-add-ons --prefix=/usr make make install_root=/mnt/target install Notice that we have yet another way of telling a package where to install. 13.7. SysVinit Making and installing the SysVinit binaries is pretty straight forward. I'll just be lazy and give you the commands, assuming that you have unpacked and entered the SysVinit source code directory: cd src make ROOT=/mnt/target make install There are also a lot of scripts associated with init. There are example scripts with the SysVinit package, which work fine. But you have to install them manually. They are set up in a heirarchy under debian/etc in the SysVinit source code tree. You can just copy them straight across into the target etc directory, with something like cd ../debian/etc; cp -r * /mnt/target/etc. Obviously you will want to have a look before you copy them across! Everything is in place now for the target kernel to load up init when we reboot. The problem this time should be that the scripts won't run, becasue bash isn't there to interpret them. Also, init will try to run getty's, but there is no getty for it to run. Reboot now and make sure there is nothing else wrong. 13.8. Ncurses The next thing we need is Bash, but bash needs ncurses, so we'll install it first. Ncurses replaces termcap as the way of handling text screens, but it can also provide backwards compatibility by supporting the termcap calls. In the interests of having a clean simple modern system, I think its best to disable the old termcap method. You might strike trouble later on if you are compiling an older application that uses termcap. But at least you will know what is using what. If you need to you can recompile ncurses with termcap support. The commands I used are ./configure --prefix=/usr --with-install-prefix=/mnt/target --with-shared --disable-termcap make make install 13.9. Bash It me took quite a lot of reading and thinking and trial and error to get Bash to install itself where I thought it should go. The configuration options I used are ./configure --prefix=/mnt/target/usr/local --exec-prefix=/mnt/target --with-curses Once you have made and installed Bash, you need to make a symlink like this cd /mnt/target/bin; ln -s bash sh. This is because scripts usually have a first line like this #!/bin/sh If you don't have the symlink, your scripts won't be able to run, because they will be looking for /bin/sh not /bin/bash. You could reboot again at this point if you like. You should notice that the scripts actually run this time, though you still can't login, because there are no getty or login programs. 13.10. Util-linux (getty and login) The util-linux package contains agetty and login. We need both of these to be able to log in and get a bash prompt. After it is instlalled, make a symlink from agetty to getty in the target /sbin directory. getty is one of the programs that is supposed to be there on all Unix-like systems, so the link is a better idea than hacking inittab to run agetty. I have one remaining problem with the compilation of util-linux. The package also contains the program more, and I have not been able to persuade the make process to have more link against the ncurses 5 library on the target system rather than the ncurses 4 on the source system. I'll be having a closer look at that. You will also need a /etc/passwd file on the target system. This is where the login program will check to find out if you are allowed in. Since this is only a toy system at this stage, we can do outrageous things like setting up only the root user, and not requiring any password!! Just put this in the target /etc/passwd root::0:0:root:/root:/bin/bash The fields are separated by colons, and from left to right they are user id, password (encrypted), user number, group number, user's name, home directory and default shell. 13.11. Sh-utils The last package we need is GNU sh-utils. The only program we need from here at this stage is stty, which is used in /etc/init.d/rc which is used to change runlevels, and to enter the initial runlevel. I actually have, and used a package that contains only stty, but I can't remember where it came from. Its a better idea to use the GNU package, because there is other stuff in there that you will need if you add to the system to make it useable. Well that's it. You should now have a system that will boot up and prompt you for a login. Type in ``root'', and you should get a shell. You won't be able to do much with it. There isn't even an ls command here for you to see your handiwork. Press tab twice so you can see the available commands. This was about the most satisfying thing I found to do with it. 13.12. Towards Useability It might look like we have made a pretty useless system here. But really, there isn't that far to go before it can do some work. One of the first things you would have to do is have the root filesystem mount read-write. There is a script from the SysVinit package, in /etc/init.d/mountall.sh which does this, and issues a mount -a so that everything gets mounted the way you specify in /etc/fstab. Put a symlink called something like S05mountall to it in the target's etc/rc2.d. You may find that this script will use commands that you haven't installed yet. If so, find the package that contains the commands and install it. See section ``Random Tips'' for clues on how to find packages. Look at the other scripts in /etc/init.d. Most of them will need to be included in any serious system. Add them in one at a time, make sure everthing is running smoothly before adding more. Check the File Heirarchy Standard (see section ``Filesystem''). It has lists of the commands that should be in /bin and /sbin. Make sure that you have all these commands installed. Even better, find the Posix documentation that specifies this stuff. >From there, it's really just a matter of throwing in more and more packages until everything you want it there. The sooner you can put the build tools such as gcc and make in the better. Once that is done, you can use the target system to build itself, which is much less complicated. 13.13. Random Tips If you have a command called thingy on a Linux system with RPM, and want a clue about where to get the source from, you can use the command: rpm -qif `which thingy` And if you have a Red Hat source CD, you can install the source code using rpm -i /mnt/cdrom/SRPMS/what.it.just.said-1.2.srpm This will put the tarball, and any Red Hat patches into /usr/src/redhat/SOURCES. 13.14. More Information · There is a mini-howto on building software from source, the Software Building mini-HOWTO . · There is also a HOWTO on building a Linux system from scratch. It focuses much more on getting the system built so it can be used, rather than just doing it as a learning exercise. The Linux From Scratch HOWTO 14. Conclusion One of the best things about Linux, in my humble opinion, is that you can get inside it and really find out how it all works. I hope that you enjoy this as much as I do. And I hope that this little note has helped you do it. 15. Administrivia 15.1. Copyright This document is copyright (c) 1999, 2000 Greg O'Keefe. You are welcome to use, copy, distribute or modify it, without charge, under the terms of the GNU General Public Licence . Please acknowledge me if you use all or part of this in another document. 15.2. Homepage The lastest version of this document lives at From Powerup To Bash Prompt 15.3. Feedback I would like to hear any comments, criticisms and suggestions for improvement that you have. Please send them to me Greg O'Keefe 15.4. Acknowledgements Product names are trademarks of the respective holders, and are hereby considered properly acknowledged. There are some people I want to say thanks to, for helping to make this happen. Everyone on the learning@TasLUG mailing list Thanks for reading all my mails and asking interesting questions. You can join this list by sending a message to majordomo with subscribe learning in the message body. Michael Emery For reminding me about Unios. Tim Little For some good clues about /etc/passwd sPaKr on #linux in efnet Who sussed out that syslogd needs /etc/services, and introduced me to the phrase ``rolling your own'' to describe building a system from source code. Alex Aitkin For bringing Vico and his ``verum ipsum factum'' (understanding arises through making) to my attention. Dennis Scott For correcting my hexidecimal arithmetic. jdd For pointing out some typos. David Leadbeater For contributing some ``ramblings'' about the kernel deamons. 15.5. Change History 15.5.1. 0.6 -> 0.7 · more emphasis on explanation, less on how to build a system, building info gathered together in a separate section and the system built is trimmed down, direct readers to Gerard Beekmans' ``Linux From Scratch'' doc for serious building · added some ramblings contributed by David Leadbeater · fixed a couple of url's, added link to unios download at learning.taslug.org.au/resources · tested and fixed url's · generally rewrite, tidy up 15.5.2. 0.5 -> 0.6 · added change history · added some todos 15.6. TODO · explain kernel modules, depmod, modprobe, insmod and all that (I'll have to find out first!) · mention the /proc filesystem, potential for exercises here · convert to docbook sgml · add more exercises, perhaps a whole section on larger exercises, like creating a minimal system file by file from a distro install.