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Linux disk devices are named after the type of interface they sit on. Legacy hard drive interfaces, such as IDE and EIDE are often named /dev/hda*, while more modern SATA or SCSI are named /dev/sda*.
Rather than use the whole disk for one purpose almost all operating systems allow hard drives to be partitioned up into partitions. Each partition is effectively a chunk of the disk, and can have its own operating system, boot information, or simply be a different volume in a single operating system.
Partitions can be primary or secondary partitions. In effect the first 4 partitions can be primary, while the remainder are considered secondary. There are some technical advantages to primary partitions, but by and large it is normal to make your bootable partitions as primary partitions (although this is not a requirement), and then to split up the disk how you like.
> sfdisk -l /dev/sda Disk /dev/sda: 19449 cylinders, 255 heads, 63 sectors/track Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0 Device Boot Start End #cyls #blocks Id System /dev/sda1 * 0+ 1274- 1275- 10240000 83 Linux /dev/sda2 1274+ 3824- 2550- 20480000 82 Linux swap / Solaris /dev/sda3 3824+ 19449- 15625- 125506560 83 Linux /dev/sda4 0 - 0 0 0 Empty
The sfdisk command in linux is one of the many commands offered by linux to investigate and change partition table information. The above example is the drive structure of one of my servers. Here you see 3 partitions (all primary).
In this structure sda1 is 10GB and is /, and sda3 is /home (125GB or so). There are is one other partition, sda2, which is the swap partition for use with virtual memory. It is customary unless you have other information to make the swap space double your main memory size. The swap size in this case is 2GB.
The fstab file explains to Linux what partitions are to be mounted where. It may contain magical partitions which dont really exist, and these are to do with special filesystems for handling kernel virtual directories, such as the /proc directory.
UUID=d40d9bef-1306-491f-bcba-61990e1bf886 / ext4 defaults 1 1 UUID=f9d23007-9414-498f-9cc6-553eeb685213 /home ext4 defaults 1 2 UUID=5501a6af-ee7f-4c73-81a7-cf5c75cb8661 swap swap defaults 0 0 # /dev/sda2 swap swap defaults 0 0 tmpfs /dev/shm tmpfs defaults 0 0 devpts /dev/pts devpts gid=5,mode=620 0 0 sysfs /sys sysfs defaults 0 0 proc /proc proc defaults 0 0
The first column indicates what to mount. This could be the device name, such as /dev/sda2. In modern systems each device, when formatted, is given a block device identifier, or UUID. You can use that in place of the device name. This is useful as it allows a drive to be moved from one drive slot to another (which may change the device name) and still have Linux recognise it and mount it correctly. Next is the location that the partition should be mounted, its file system type (such as swap or ext4 or ntfs), then some flags to be used, then two numbers. The first number is 1 if this drive should be backed up (assuming you do backups) and the last number is the order that the drives are checked for damage (root first then the others).
% blkid /dev/sda1: UUID="d40d9bef-1306-491f-bcba-61990e1bf886" TYPE="ext4" /dev/sda2: UUID="5501a6af-ee7f-4c73-81a7-cf5c75cb8661" TYPE="swap" /dev/sda3: UUID="f9d23007-9414-498f-9cc6-553eeb685213" TYPE="ext4"
The blkid command allows you to find out the UUIDs for all attached block device partitions, along with their types if known.
Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda2 10080520 3142968 6425484 33% / /dev/sda1 101086 9665 86202 11% /boot none 1038660 0 1038660 0% /dev/shm /dev/sda6 56340828 3853984 49624868 8% /home
The df command lets you find out the current Disk Usage, which reports mounted partition utilisation of all mountpoints which are present as a directory. So it does not report swap space usage (but this can be found in /proc/swaps). It can be hard to read as it is in blocks, but "du -h" reports the information in a human readable form, e.g. MB or GB.
In a UML-power virtual machine, there are no IDE or SCSI drives. The disks are called /dev/ubd/n where n is a number. They are actually implemented by files in the host operating system, but this is hidden from you. If you have a UML virtual machine then the drive partitions are:
However if you are using the standard QEMU/KVM virtualisation then the drives are the normal /dev/sda1 style devices.
If you want to find out how much disk space a directory is using, the "du" command does this easily.
$ du -s /usr/lib 477464 /usr/lib $ du -sh /usr/lib 467M /usr/lib
"-s" is useful, otherwise it tells you about all subdirectories sizes too one subdirectory at a time. With this summary flag, it still check the subdirectories but only reports the total size. Again "-h" puts it into human readable form using MB or GB.
From switch-on:
The linux loader, e.g. grub, gives the user an interface for selecting different operating systems or different kernel configurations. For Redhat it looks like this:
The grub loaded is very configurable, and the example above is configured as follows:
default=1
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Fedora Core (2.6.6-1.435.2.3)
root (hd0,0)
kernel /vmlinuz-2.6.6-1.435.2.3 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.6-1.435.2.3.img
title Fedora Core (2.6.5-1.358)
root (hd0,0)
kernel /vmlinuz-2.6.5-1.358 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.5-1.358.img
Here it says that unless within 10 seconds you touch the keyboard, it will automatically boot the first entry by default. The background image is called the splashimage. There are two entries, each for a different version of the kernel. This is standard Linux practice. Each time you update your linux distribution kernel, the previous few versions are kept to allow you to recover from a problem caused by the new kernel. Problems with new kernels is uncommon but they can happen, and having a broken kernel may result in an unusable machine.
As linux boots, it runs various system scripts. These do general housekeeping functions, but eventually it enters one for your standard "runlevel". Runlevel scripts enable the services (like ssh and apache") which you may want to start. Runlevel startup scripts all live in /etc/init.d/.
For example, apache, the web server in Linux, is looked after by the following init script:
/etc/init.d/httpd
All the scripts in the init.d directory can be executed directly, and take a range of parameters. You could call them by hand, but it is best to use system commands to do this instead. The scripts in init.d can:
You should not call these scripts directly, as this can interfere with different security models which are in use (such as SELinux). Instead you need to use the "service" command. For instance, sending "start" to the httpd control script in init.d is performed as follows:
> service httpd start
The run level determines what init.d files run. As you enter a run level services not running which should run at that run level start. As you leave a run level services which should not be running at the new run level stop. There are 7 default runlevels, numbered 0 to 6.
What services start and stop are determined by the soft links found in the different /etc/rd?.d directories (where the ? can be the numbers 0 to 6). Usually all we need to know is the stardard runlevel is 5.
K01yum K35vncserver K74ypxfrd S13portmap S80sendmail K05saslauthd K35winbind K89netplugd S14nfslock S90crond K10dc_server K45named K95kudzu S18rpcgssd S90xfs K10psacct K50netdump K96init.cssd S19rpcsvcgssd S95anacron K12dc_client K50snmpd S00microcode_ctl S20random S95atd K12mysqld K50snmptrapd S04readahead_early S25netfs S96init.cssd K20nfs K50tux S06cpuspeed S26apmd S96readahead K24irda K54dovecot S08iptables S28autofs S97messagebus K25squid K70aep1000 S09isdn S44acpid S97rhnsd K34dhcrelay K70bcm5820 S10network S55sshd S99local K34yppasswdd K74ntpd S12syslog S56rawdevices S99mdmonitor K35dhcpd K74ypserv S13irqbalance S56xinetd
The filenames all take the following format:
S/K priority service-name
If the name starts with an S then it Starts at that runlevel, while a K indicates that is is stopped (Killed) at that runlevel. A priority of 00 is executed
before priority 01, which is before priority 50, all the way to 99.
So for instance, consider S99mdmonitor:
$ ls -l /etc/rc5.d/S99mdmonitor lrwxrwxrwx. 1 root root 19 Jul 27 13:00 S99mdmonitor -> ../init.d/mdmonitor
> chkconfig -list mdmonitor mdmonitor 0:off 1:off 2:on 3:on 4:on 5:on 6:off > chkconfig --levels 345 mdmonitor off > chkconfig -list mdmonitor mdmonitor 0:off 1:off 2:on 3:off 4:off 5:off 6:off
One problem with init.d is that it is linear. Each service is started in turn before the next service starts. There are newer service managers available which use tree dependency models and parallel execution. Fedora 15 currently uses a system based on systemd. However it still uses init.d for some services which have not been migrated to the new manager. "systemd" is much more complex to understand than the simple init.d approach, but it is much faster to boot as many services can start in parallel. Service managers is still very much an area under active development
Starting system message bus: [ OK ] Starting mdmonitor : [ OK ]
service telnet
{
flags = REUSE
socket_type = stream
wait = no
user = root
server = /usr/sbin/in.telnetd
log_on_failure += USERID
disable = no
}
> ps aux | grep sshd root 1796 ... /usr/sbin/sshd > kill -s SIGKILL 1796
> ls -a /etc/skel/ .bash_logout .bash_profile .bashrc .gtkrc .kde
colorls.csh gnome-ssh-askpass.csh krb5.csh less.csh vim.csh colorls.sh gnome-ssh-askpass.sh krb5.sh less.sh vim.sh glib2.csh kde.csh lang.csh qt.csh which-2.sh glib2.sh kde.sh lang.sh qt.sh
if [ -n "$BASH_VERSION" -o -n "$KSH_VERSION" -o -n "$ZSH_VERSION" ]; then # for bash, pdksh and zsh, only if no alias is already set alias vi >/dev/null 2>&1 || alias vi=vim fi
$ man adduser
$ adduser -h adduser: invalid option -- h Usage: useradd [options] LOGIN Options: -b, --base-dir BASE_DIR base directory for the new user account ... -g, --gid GROUP force use GROUP for the new user account ... $ adduser jim -g staff $ tail -1 /etc/passwd jim:x:502:100::/home/jim:/bin/bash $ grep 100 /etc/group staff:x:100:
$ chown jim.staff filename $ chown jim filename $ chgrp staff filename
$ grep "gordon" /etc/* /etc/group:gordon:x:500: /etc/group-:gordon:x:500: /etc/gshadow:gordon:!!:: /etc/gshadow-:gordon:!!:: /etc/passwd:gordon:x:500:100:Dr Gordon ...
$ find /etc -name '*host*' /etc/hosts.deny /etc/ghostscript /etc/ssh/ssh_host_dsa_key.pub
$ find . -name core -print -exec rm {} \;
gordon:x:44:In which file in /etc would you expect to see such a line, and what does it mean?
$ mkdir temp $ cd temp/ $ mkdir txt.txt/ $ cd txt.txt/ $ touch hello $ cd .. $ ls *.*What is printed on the screen in response to the last line of the commands?
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