NOTES_LICENCE/Basic Root FS.txt

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+CREDIT:  http://www.tldp.org/HOWTO/Bootdisk-HOWTO/buildroot.html
+
+
+    /dev -- Device files, required to perform I/O
+
+    /proc -- Directory stub required by the proc filesystem
+
+    /etc -- System configuration files
+
+    /sbin -- Critical system binaries
+
+    /bin -- Essential binaries considered part of the system
+
+    /lib -- Shared libraries to provide run-time support
+
+    /mnt -- A mount point for maintenance on other disks
+
+    /usr -- Additional utilities and applications
+
+
+
+Three of these directories will be empty on the root filesystem, so they only need to be created with mkdir.
+The /proc directory is basically a stub under which the proc filesystem is placed. The directories /mnt and /usr
+are only mount points for use after the boot/root system is running. Hence again, these directories only need to be created.
+
+
+4.3.1. /dev
+A /dev directory containing a special file for all devices to be used by the system is mandatory for any Linux system.
+The directory itself is a normal directory, and can be created with mkdir in the normal way. The device special files,
+however, must be created in a special way, using the mknod command.
+
+There is a shortcut, though — copy devices files from your existing hard disk /dev directory. The only requirement is
+that you copy the device special files using -R option. This will copy the directory without attempting to copy the contents
+of the files. Be sure to use an upper case R. For example: 
+
+    cp -dpR /dev/fd[01]* /mnt/dev
+    cp -dpR /dev/tty[0-6] /mnt/dev
+
+
+assuming that the diskette is mounted at /mnt. The dp switches ensure that symbolic links are copied as links, rather
+than using the target file, and that the original file attributes are preserved, thus preserving ownership information.
+
+If you want to do it the hard way, use ls -l to display the major and minor device numbers for the devices you want,
+and create them on the diskette using mknod.
+
+However the devices files are created, check that any special devices you need have been placed on the rescue diskette.
+For example, ftape uses tape devices, so you will need to copy all of these if you intend to access your floppy tape drive
+from the bootdisk.
+
+Note that one inode is required for each device special file, and inodes can at times be a scarce resource, especially on
+diskette filesystems. You'll need to be selective about the device files you include. For example, if you do not have SCSI
+disks you can safely ignore /dev/sd*; if you don't intend to use serial ports you can ignore /dev/ttyS*.
+
+If, in building your root filesystem, you get the error No space left on device but a df command shows space still available,
+you have probably run out of inodes. A df -i will display inode usage.
+
+Important:   Be sure to include the following files from this directory: console, kmem, mem, null, ram0 and tty1.
+
+
+4.3.2. /etc
+
+The /etc directory contains configuration files. What it should contain depends on what programs you intend to run. On most
+systems, these can be divided into three groups:
+
+    Required at all times, e.g. rc, fstab, passwd.
+
+    May be required, but no one is too sure.
+
+    Junk that crept in. 
+
+Files which are not essential can usually be identified with the command: 
+
+    ls -ltru
+
+This lists files in reverse order of date last accessed, so if any files are not being accessed, they can be omitted from
+a root diskette. On my root diskettes, I have the number of config files down to 15. This reduces my work to dealing with
+three sets of files:
+
+    The ones I must configure for a boot/root system:
+
+        rc.d/* -- system startup and run level change scripts
+
+        fstab -- list of file systems to be mounted
+
+        inittab -- parameters for the init process, the first process started at boot time.
+
+        gettydefs -- parameters for the init process, the first process started at boot time.
+
+    The ones I should tidy up for a boot/root system:
+
+        passwd -- Critical list of users, home directories, etc.
+
+        group -- user groups.
+
+        shadow -- passwords of users. You may not have this.
+
+        termcap -- the terminal capability database.
+
+If security is important, passwd and shadow should be pruned to avoid copying user passwords off the system, and so that
+unwanted logins are rejected when you boot from diskette. Be sure that passwd contains at least root. If you intend other
+ users to login, be sure their home directories and shells exist. termcap, the terminal database, is typically several
+hundred kilobytes. The version on your boot/root diskette should be pruned down to contain only the terminal(s) you use,
+which is usually just the linux or linux-console entry. The rest. They work at the moment, so I leave them alone.
+
+Out of this, I only really have to configure two files, and what they should contain is surprisingly small. 
+
+rc should contain:
+
+        #!/bin/sh       
+        /bin/mount -av
+        /bin/hostname Kangaroo
+
+Be sure it is executable, be sure it has a "#!" line at the top, and be sure any absolute filenames are correct.
+You don't really need to run hostname — it just looks nicer if you do.
+
+fstab should contain at least:
+
+        /dev/ram0       /               ext2    defaults
+        /dev/fd0        /               ext2    defaults
+        /proc           /proc           proc    defaults
+
+
+You can copy entries from your existing fstab, but you should not automatically mount any of your hard disk partitions;
+use the noauto keyword with them. Your hard disk may be damaged or dead when the bootdisk is used.
+
+Your inittab should be changed so that its sysinit line runs rc or whatever basic boot script will be used.
+Also, if you want to ensure that users on serial ports cannot login, comment out all the entries for getty which include
+a ttys or ttyS device at the end of the line. Leave in the tty ports so that you can login at the console.
+
+A minimal inittab file looks like this:
+
+        id:2:initdefault:
+        si::sysinit:/etc/rc
+        1:2345:respawn:/sbin/getty 9600 tty1
+        2:23:respawn:/sbin/getty 9600 tty2
+
+The inittab file defines what the system will run in various states including startup, move to multi-user mode, etc.
+Check carefully the filenames mentioned in inittab; if init cannot find the program mentioned the bootdisk will hang,
+and you may not even get an error message.
+
+Note that some programs cannot be moved elsewhere because other programs have hardcoded their locations. For example,
+on my system, /etc/shutdown has hardcoded in it /etc/reboot. If I move reboot to /bin/reboot, and then issue a shutdown
+command, it will fail because it cannot find the reboot file.
+
+For the rest, just copy all the text files in your /etc directory, plus all the executables in your /etc directory that
+you cannot be sure you do not need. As a guide, consult the sample listing in Appendix C. Probably it will suffice to copy
+only those files, but systems differ a great deal, so you cannot be sure that the same set of files on your system is equivalent
+to the files in the list. The only sure method is to start with inittab and work out what is required.
+
+Most systems now use an /etc/rc.d/ directory containing shell scripts for different run levels. The minimum is a single rc script,
+but it may be simpler just to copy inittab and the /etc/rc.d directory from your existing system, and prune the shell scripts in
+the rc.d directory to remove processing not relevent to a diskette system environment.
+
+
+4.3.3. /bin and /sbin
+
+The /bin directory is a convenient place for extra utilities you need to perform basic operations, utilities such as ls, mv,
+cat and dd. See Appendix C for an example list of files that go in a /bin and /sbin directories. It does not include any of
+the utilities required to restore from backup, such as cpio, tar and gzip. That is because I place these on a separate utility
+diskette, to save space on the boot/root diskette. Once the boot/root diskette is booted, it is copied to the ramdisk leaving
+the diskette drive free to mount another diskette, the utility diskette. I usually mount this as /usr.
+
+Creation of a utility diskette is described below in Section 9.2. It is probably desirable to maintain a copy of the same version
+of backup utilities used to write the backups so you don't waste time trying to install versions that cannot read your backup tapes.
+
+Important:   Be sure to include the following programs: init, getty or equivalent, login, mount, some shell capable of running
+your rc scripts, a link from sh to the shell.
+
+
+4.3.4. /lib
+In /lib you place necessary shared libraries and loaders. If the necessary libraries are not found in your /lib directory then the
+system will be unable to boot. If you're lucky you may see an error message telling you why.
+
+Nearly every program requires at least the libc library, libc.so.N, where N is the current version number. Check your /lib directory.
+The file libc.so.N is usually a symlink to a filename with a complete version number:
+
+% ls -l /lib/libc*
+-rwxr-xr-x   1 root     root      4016683 Apr 16 18:48 libc-2.1.1.so*
+lrwxrwxrwx   1 root     root           13 Apr 10 12:25 libc.so.6 -> libc-2.1.1.so*
+
+In this case, you want libc-2.1.1.so. To find other libraries you should go through all the binaries you plan to include and check
+their dependencies with ldd. For example:
+
+        % ldd /sbin/mke2fs
+        libext2fs.so.2 => /lib/libext2fs.so.2 (0x40014000)
+        libcom_err.so.2 => /lib/libcom_err.so.2 (0x40026000)
+        libuuid.so.1 => /lib/libuuid.so.1 (0x40028000)
+        libc.so.6 => /lib/libc.so.6 (0x4002c000)
+        /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
+
+Each file on the right-hand side is required. The file may be a symbolic link.
+
+Note that some libraries are quite large and will not fit easily on your root filesystem. For example, the libc.so listed above is
+about 4 meg. You will probably need to strip libraries when copying them to your root filesystem. See Section 8 for instructions.
+
+In /lib you must also include a loader for the libraries. The loader will be either ld.so (for A.OUT libraries, which are no longer
+common) or ld-linux.so (for ELF libraries). Newer versions of ldd tell you exactly which loader is needed, as in the example above,
+but older versions may not. If you're unsure which you need, run the file command on the library. For example:
+
+% file /lib/libc.so.4.7.2 /lib/libc.so.5.4.33 /lib/libc-2.1.1.so
+/lib/libc.so.4.7.2: Linux/i386 demand-paged executable (QMAGIC), stripped
+/lib/libc.so.5.4.33: ELF 32-bit LSB shared object, Intel 80386, version 1, stripped
+/lib/libc-2.1.1.so: ELF 32-bit LSB shared object, Intel 80386, version 1, not stripped
+
+The QMAGIC indicates that 4.7.2 is for A.OUT libraries, and ELF indicates that 5.4.33 and 2.1.1 are for ELF.
+
+Copy the specific loader(s) you need to the root filesystem you're building. Libraries and loaders should be checked carefully against
+the included binaries. If the kernel cannot load a necessary library, the kernel may hang with no error message.
+
+
+4.6. Some final details
+Some system programs, such as login, complain if the file /var/run/utmp and the directory /var/log do not exist. So:
+
+        mkdir -p /mnt/var/{log,run}
+        touch /mnt/var/run/utmp
+
+Finally, after you have set up all the libraries you need, run ldconfig to remake /etc/ld.so.cache on the root filesystem.
+The cache tells the loader where to find the libraries. You can do this with:
+
+        ldconfig -r /mnt
+
+4.7. Wrapping it up
+
+When you have finished constructing the root filesystem, unmount it, copy it to a file and compress it:
+
+        umount /mnt
+        dd if=DEVICE bs=1k | gzip -v9 > rootfs.gz
+
+When this finishes you will have a file rootfs.gz. This is your compressed root filesystem. You should check its size to
+make sure it will fit on a diskette; if it doesn't you'll have to go back and remove some files. Some suggestions for
+reducing root filesystem size appear in Section 8.
+
+Notes [1]  The directory structure presented here is for root diskette use only. Real Linux systems have a more complex
+and disciplined set of policies, called the Filesystem Hierarchy Standard, for determining where files should go.)
+
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