Accessing Hard Disks

Oracle Database Tips by Donald Burleson

An important first step when learning a new operating system is determining how to view the contents of the disk drives and other devices such as CD ROMs and floppies.

To begin this process, launch a gnome-terminaland type the ls command. This command lists the contents of the current working directory, most likely the account's home directory, /root if logged in as root.

The ls command can be combined with a number of switches, giving it more functionality. For example, the ls ?l commandreturns a list of files along with all of the attributes of the files. This command will not return hidden files. Use the ls ?a commandto return a list of all files or the ls ?la commandto return all files including their attributes.

The ls command can be combined with a path such as ls /etc or a filename such as ls myfile.txt or both a path and a filename. It can also be used with wildcards such as * for multi-characters, or - for a single character.

The cd command is used to change to a different working directory. For example, cd /etc will place you into the /etc directory. The cd command alone will place you back into the home directory.

Using the ls command, you can now examine how Linux controls devices such as hard drives and CDROMs. In the directory /dev there are listings for hundreds of files that allow Linux to control devices, most of which are not in use at this time. These device files are used by Linux to pass data to and receive data from a device driver and communicate with devices. Linux uses the file /dev/hda to read from and write to the first hard disk.

If this hard disk is partitioned, it uses the /dev/hda1 file to access the first partition. Similarly, the /dev/hda2 file is used to access the second partition on the first drive.

The fdisk ‑l command is used to list the hard disks and their subsequent partitions on the system. This command will list all partitions, even those that are not mounted or formatted.

In the example shown in Figure 3.1, the first line reports that hard disk /dev/hda is 6,448 megabytes. The second line reports that it has 784 cylinders. The third line reports each cylinder is 8,225,280 bytes. Below the third line, the partitions are listed, which in this case is just one, /dev/hda1.

This partition starts on cylinder 1 and ends on cylinder 784, the final cylinder. A second drive, /dev/hdb is also listed. It has 13.5 gigabytes and three partitions: /dev/hdb1, /dev/hdb2, and /dev/hdb3. The start cylinder, end cylinder, and file system for each partition are listed.

Accessing a Hard Disk by Mounting its File System

In Linux, just because the system detects a hard disk partition does not necessarily mean that the files on that partition are accessible. In order to access the files on a disk, the disk has to be mounted. This brings up the obvious question, 'since the ls command works, does that mean files can be accessed on disk?? The answer to this question is yes, because the system has already mounted some or all of the partitions on the disks.

To determine what partitions or devices are mounted at any time, use either the mount ?l or the df ?h command as shown in Figure 3.2. In this example, the partition /dev/hdb2 is mounted on the mount-point /, and the partition /dev/hdb1 is mounted on the mount-point /boot. The swap partition /dev/hdb3 is not mounted because swap partitions are handled differently by the operating system. Note that the df ‑h command reports the used and available disk space on each mounted file system.

What is a Mount-Point?

To access a file system on a device such as a hard drive or floppy drive, the file system must be mounted. To mount a new file system, a directory must exist in the current file system. That directory does not have to be empty; there can be files within it even subdirectories. However, once a different file system is mounted to that directory, the files and subdirectories within that directory will become unavailable until the new file system is un‑mounted.

To keep things simple and to keep all files accessible, it is important to keep the directories intended for mounting other file systems empty. In fact, these directories should be created and used simply for mounting a specific file system that may or may not be mounted at boot time.

Fedora has a few mount-points already created for use. Type the command ls /mnt and you will see a list of the mount-points that Fedora has created to mount devices that are commonly mounted and that were detected when the operating system was installed.

The /etc/fstab File

The /etc/fstab file is a text file that tells Fedora what file systems to mount on boot. It also associates certain devices to certain mount-points, effectively abbreviating the command for mounting those devices. To view this file, type the command more /etc/fstab as shown in Figure 3.3.

The contents of the /etc/fstab file are listed in six columns, delimited by white-space, namely device, mount-point, file-system type, options, dump, and file-system check order. The details for each of these columns can be found in the manual page for the /etc/fstab file. Type the man fstab command to enter a manual page, and type q to exit a manual page.

In the example shown in Figure 3.3, the device with label ?/? is mounted at the mount-point /, and the device with label ?/boot? is mounted at /boot. There are two CDROMs and one floppy that have each been assigned its own mount-point.

Mounting a File System with the Mount Command

Figure 3.4 shows an example of mounting a file system using the full syntax and another example using the abbreviated syntax. The line beginning with ### is commented out. It is used to illustrate the various parts of the mount command.

Only a device or partition listed in the configuration file /etc/fstab can be mounted with the abbreviated syntax such as shown for mounting the floppy drive.

The umount command is used to un-mount a file system (note the missing ?n?). Always un-mount a file system before removing a floppy or CD; Linux does not gracefully handle unexpected file system removal.

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