Formatting & Partioning Hard disks

Before you use fdisk to create or revise partitions, you should check your free space and the partitions that are currently mounted. You can do this with df and mount commands. The following example illustrates how the df command displays the total, used, and available free space on all currently mounted filesystems.

Note the numbers under the 1k-blocks column. In this case (except for the mounted DVD), they add up to about 35GB of allocated space. If your hard drive is larger, you may have unallocated space that you can use for another partition. Just remember to leave room for expansion in appropriate directories, such as /home, /tmp, and /var.

[root@Enterprise root]# df
Filesystem 1k-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
            9903432   2333948   7058304  25% /
/dev/hda3     101105     19821     76063  21% /boot
tmpfs         451200         0   451200    0% /dev/shm
/dev/mapper/VolGroup00-LogVol02
            4951688    149164   4546936   4% /home
/dev/md0      302684     10326    276730   4% /tmp
/dev/mapper/VolGroup00-LogVol03
            4951688    194308   4501792   5% /var
/dev/hdc        7384      7384         0 100%
                      /media/Red Hat Enterprise Linux Server
/dev/hda2   11715984   9509432   2206552  82% /DosD
[root@Enterprise root]#

The second command, mount, includes the filesystem type. In this case, examine the partition represented by device /dev/hda2 mounted with the VFAT file type on the /DosD directory. It provides direct access to the D: drive of the Windows operating system. For the following example, I've set up the data shown from the mount command in columns for clarity.

[root@Enterprise root]# mount
/dev/mapper/VolGroup00-LogVol00 on / type ext3         (rw)
proc      on /proc type proc                           (rw)
sysfs     on /sys type sysfs                           (rw)
devpts    on /dev/pts type devpts                      (rw,gid=5,mode=620)
/dev/hda3 on /boot type ext3                           (rw)
tmpfs     on /dev/shm type tmpfs                       (rw)
/dev/mapper/VolGroup00-LogVol02 on /home type ext3     (rw)
/dev/md0  on /tmp type ext3                            (rw)
/dev/mapper/VolGroup00-LogVol01 on /var type ext3      (rw)
none      on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
sunrpc    on /var/lib/nfs/rpc_pipefs type rpc_pipefs   (rw)
/dev/hdc  on /media/RHEL-5 i386 Disc 1type iso9660     (ro,noexec,nosuid,nodev,uid=500)
/dev/hda2 on DosD type vfat                            (rw)
[root@Enterprise root]#

One of the benefits is that you can move and copy files between the Linux and the DOS partitions using standard Linux commands. You cannot, however, run any Windows applications within Linux unless you run a DOS or Windows emulation package such as Wine (www.winehq.org) or CrossOver Office (www.codeweavers.com).

The fdisk Utility

The fdisk tility is a universally available tool that you should know well. There are many commands within fdisk, more in expert mode, but you need to know only the few discussed here.

Though you can modify the physical disk partition layout using many programs, this section explores the Linux implementation of fdisk. FDISK.EXE from DOS has the same name and is also used for creating partitions, but it doesn't incorporate any Linux-compatible features. It also uses a different interface.

Using fdisk: Starting, Getting Help, and Quitting

The following screen output lists commands that show how to start the fdisk program, how to get help, and how to quit the program. The /dev/hda drive is associated with the first PATA/IDE drive on a regular PC. Your computer may have a different hard drive; you can check the output from the df and mount commands for clues.

As you can see, once you start fdisk, it opens its own command line prompt:

# fdisk /dev/hda
Command (m for help): m
Command action
  a   toggle a bootable flag
  b   edit bsd disklabel
  c   toggle the dos compatibility flag
  d   delete a partition
  l   list known partition types
  m   print this menu
  n   add a new partition
  o   create a new empty DOS partition table
  p   print the partition table
  q   quit without saving changes
  s   create a new empty Sun disklabel
  t   change a partition's system id
  u   change display/entry units
  v   verify the partition table
  w   write table to disk and exit
  x   extra functionality (experts only)

Command (m for help): q

There are a wide variety of commands associated with fdisk and more when you run the x command to access fdisk's extra functionality.

Using fdisk: In a Nutshell

At the fdisk command line prompt, start with the print command (p) to print the partition table. This allows you to review the current entries in the partition table. Assuming you have free space, you then create a new (n) partition, either primary (p) or logical (l). If it doesn't already exist, you can also create an extended partition (e) to contain your logical partitions. Remember that you can have up to four primary partitions, which would correspond to numbers 1 through 4. One of the primary partitions can be redesignated as an extended partition. The remaining partitions are logical partitions, numbered 5 and above. The Linux fdisk utility won't allow you to create more than 16 partitions on the drive.

When you assign space to a partition, you're assigning a block of cylinders on that hard disk. If you have free space, the fdisk default starts the new partition at the first available cylinder. The actual size of the partition depends on disk geometry; do not worry about exact size here.

Using fdisk: Deleting Partitions

The following example removes the only configured partition. The sample output screen first starts fdisk. Then you print (p) the current partition table, delete (d) the partition by number (1 in this case), write (w) the changes to the disk, and quit (q) from the program. Needless to say, do not perform this action on any partition where you need the data.

# fdisk /dev/hdb
Command (m for help): p
Disk /dev/hdb: 255 heads, 63 sectors, 525 cylinders
Units = cylinders of 16065 * 512 bytes

Device    Boot    Start       End    Blocks   Id  System
/dev/hdb1   *         1       525   4217031    6  FAT16
Command (m for help): d
Partition number (1-1): 1

This is the last chance to change your mind before deleting the current partition. If you want to change your mind, exit from fdisk with the q command. If you're pleased with the changes that you've made and want to make them permanent, proceed with the w command:

Command (m for help): w

You did it! Now you have an empty hard disk or hard disk area where you can create the partitions you need.

You no longer have to reboot to get Linux to read the new partition table. Now, the partprobe command rereads the partition table without a reboot.

Using fdisk: Creating Partitions

The following screen output sample shows the steps used to create (n) the first (/boot) partition, make it bootable (a), and then finally write (w) the partition information to the disk. (Note that although you may ask for a 100MB partition, the geometry of the disk may not allow that precise size, as shown in the example.)

# fdisk /dev/hdb

Command (m for help): n
Command action
  e   extended
  p   primary partition (1-4)
p
Partition number (1-4):

First cylinder (1-256, default 1): 1
Last cylinder or +size or +sizeM or +sizeK (2-256,def 256): +100M

Command (m for help): a
Partition number (1-4): 1

Command (m for help): p
Disk /dev/hdb: 255 heads, 63 sectors, 256 cylinders
Units = cylinders of 16065 * 512 bytes
  Device Boot Start         End     Blocks   Id  System
/dev/hdb1   *      1          12      98163   83  Linux

Command (m for help):

Repeat the commands to create any other partitions that you might need. One possible group of partitions is illustrated here:

Command (m for help): p

Disk /dev/hdb: 255 heads, 63 sectors, 256 cylinders
Units = cylinders of 16065 * 512 bytes
  Device Boot Start   End   Blocks  Id  System
/dev/hdb1   *      1     2    16044  83  Linux
/dev/hdb2          3    18    64176  82  Linux swap / Solaris
/dev/hdb3         19   169 1203300   83  Linux
/dev/hdb4        170   250  649782    5  Extended
/dev/hdb5        170   201  248682   83  Linux
/dev/hdb6        202   257  449232   83  Linux

Command (m for help): w

On the Job

The number of blocks that you see may vary slightly depending on the size of your hard disk; the number of heads, sectors, and cylinders on that disk; as well as the version of fdisk that you're using.

Using fdisk: A New PC with No Partitions

After installing Linux on a new PC, you'll want to use fdisk to configure additional physical disks attached to the system. For example, if the additional disk is the first disk attached to the secondary IDE controller, run the fdisk /dev/hdc command. Remember the limitations on partitions. If you need more than four partitions on the new physical disk, configure type Primary for the first three partitions, and then Extended for the rest of the disk as partition 4. You can then creating logical partitions 5–16 within the extended partition.

Using fdisk: Creating a Swap Partition

You need to create a partition before you can reassign it as a swap partition. At the fdisk prompt, run the l command. You'll see a large number of file types, listed as hex codes. When you create a partition, fdisk creates a Linux Native type partition by default. As you can see from the output of the l command, the associated hex code is (83).

It's easy to reassign a partition as a swap partition. Run the p command. Remember the number of the partition you want to change. Make sure that partition doesn't contain data that you want to save.

Now run the t command. Type in the number associated with the partition that you want to change. Type in the hex code for the type you want-in this case, 82 for a Linux swap partition. For example, I could run the following sequence of commands to set up a new swap partition on the second IDE hard drive. The commands that I type are in boldface. The details of what you see depend on the partitions that you may have created. It'll be a 1GB swap space on the first primary partition (/dev/hdb1).

# fdisk /dev/hdb
Command (m for help): n
Command action
    e  extended
    p  primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-10402, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-10402, default 10402): +1000M

Command (m for help): p

Disk /dev/hdb: 5368 MB, 5368709120 bytes
16 heads, 63 sectors/track, 10402 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

  Device Boot    Start       End    Blocks    Id  System
/dev/hdb1             1      1939    977224+   83  Linux

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 82
Changed system type of partition 1 to 82 (Linux swap / Solaris)

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
#

The fdisk utility doesn't actually write the changes to your hard disk until you run the write (w) command. You have a chance to cancel your changes with the quit (q) command. To make sure Linux rereads the partition table after fdisk writes it, run the partprobe command

Crontab details

Here is the format of a line in crontab.

#minute, hour, day of month, month, day of week, command
*        *     *             *      *            command

Entries in a crontab Command Line

Field	Value
minute	0–59
hour	Based on a 24-hour clock; for example, 23 = 11 P.M.
day of month	1–31
month	1–12, or jan, feb, mar, etc.
day of week	0–7; where 0 and 7 are both Sunday; or sun, mon, tue, etc.
command	The command you want to run

If you see an asterisk in any column, cron runs that command for all possible values of that column. For example, an * in the minute field means that the command is run every minute during the specified hour(s). Consider another example, as shown here:

1  5  3  4  *  ls

This line runs the ls command every April 3 at 5:01 A.M. The asterisk in the day of week column simply means that it does not matter what day of the week it is; crontab still runs the ls command at the specified time.

The crontab file is flexible. For example, a 7–10 entry in the hour field would run the specified command at 7:00 A.M., 8:00 A.M., 9:00 A.M., and 10:00 A.M. A list of entries in the minute field such as: 0,5,10,15,20,25,30,35,40,45,50,55 would run the specified command every five minutes. The cron daemon also recognizes abbreviations for months and the day of the week.

The actual command is the sixth field. You can set up new lines with a percent (%) symbol. This is useful for formatting standard input. The example of a cron file follows formats input for an e-mail message:

# crontab -l
# Sample crontab file
#
# Force /bin/sh to be my shell for all of my scripts.
SHELL=/bin/sh
# Run 15 minutes past Midnight every Saturday
15 0 * * sat   $HOME/scripts/scary.script
# Do routine cleanup on the first of every Month at 4:30 AM
30 4 1 * *     /usr/scripts/removecores >> /tmp/core.tmp 2>>&1
# Mail a message at 10:45 AM every Friday
45 10 * * fri  mail -s "Project Update employees%Can I have a status
update on your project?%%Your Boss.%
# Every other hour check for alert messages
0 */2 * * * /usr/scripts/check.alerts

For more examples, review some of the scripts in the /etc/cron.daily directory.