Beginning Ubuntu Linux - From Novice To Professional (2006)

Because nothing is specified before the +x, the shell assumes that the changes to be applied to the file are for the current user only.

To change the owner of a file, use the chown command. For security reasons this must be prefaced with the sudo command, which is to say that chown (and chgrp, to change the group), require superuser powers.

For example, to set the owner of myfile as frank, type this command:

sudo chown frank myfile

You can also change the owner and the group of a file using chown. Simply type each separated by a period:

sudo chown frank.mygroup myfile

This will change myfile so that its owner is frank and its group is mygroup.

To change the group of a file, you can use the chgrp command in exactly the same way as chown:

sudo chgrp mygroup myfile

The File System Explained

Now that you understand the principles of files and users, we can take a bird’s-eye view of the Linux file system and start to make sense of it.

You might already have ventured beyond the /home directory and wandered through the file system. You no doubt found it thoroughly confusing, largely because it’s not like anything you’re used to. The good news is that it’s not actually very hard to understand. If nothing else, you should be aware that nearly everything can be ignored during everyday use.

■Note The Ubuntu file system is referred to as a hierarchical file system. This means that it consists of a lot of directories that contain files. Windows also uses a hierarchical file system. Ubuntu refers to the very bottom level of the file system as the root. This has no connection with the root user.

You can switch to the root of the file system by typing the following shell command:

cd /

When used on its own, the forward slash is interpreted as a shortcut for root.

If I do this on my PC, and then ask for a long file listing (ls -l), I see the following:

The first thing you’ll notice from this is that the root of the file system contains largely directories and that all files and directories are owned by root.

Only users with administrative powers can write files to the root of the file system. That means if you wanted to write to the root of the file system or otherwise access those files, you would need to use the sudo command. This is to prevent damage, since most of the directories in the root of the file system are vital to the correct running of Linux and contain essential programs or data.

■Caution It’s incredibly easy to slip up when using the command-line shell and thereby cause a lot of damage. For example, simply mistyping a forward slash in a command can mean the difference between deleting the files in a directory and deleting the directory itself. This is just another reason why you should always be careful when working at the command line, especially if you use the sudo command.

As you can see from the file permissions of each directory in the root of the file system, most directories allow all users to browse them and access the files within (the last three characters of the permissions read r-x). You just won’t be able to create new files there or delete the

directories themselves. You might be able to modify or execute programs contained within the directory, but this will depend on the permissions of each individual file.

Table 14-3 provides a brief description of what each directory and file in the Ubuntu root file system contains. This is for reference only; there’s no need for you to learn this information. The Ubuntu file system broadly follows the principles in the Filesystem Hierarchy Standard, as do most versions of Linux, but it does have its own subtleties.

■Note The -> symbol in Table 14-3 indicates a symbolic link, which is similar to what Windows refers to as a shortcut.

Table 14-3. Directories and Files in the Ubuntu Root File System

Mounting

Described in technical terms, mounting is the practice of making a file system available under Linux. This can take the form of a partition on your hard disk, a CD-ROM, a network server, or many other things.

Mounting drives might seem a strange concept, but it actually makes everything much simpler than it might be otherwise. For example, once a drive is mounted, you don’t need to use any special commands or software to access its contents. You can use the same programs and tools that you use to access all of your other files. Mounting creates a level playing field on which everything is equal and can therefore be accessed quickly and efficiently.

Using the mount Command

Mounting is usually done via the mount command. Under Ubuntu, you must have administrator powers to use the mount command, which means prefacing it with sudo and providing your password.

With most modern versions of Linux, mount can be used in two ways: by specifying all the settings immediately after the command, or by making reference to an entry within the fstab file. fstab is a configuration file stored in the /etc directory that contains details of all file systems on the PC that can be mounted.

■Note The root file system is itself mounted automatically during bootup, shortly after the kernel has started and has all your hardware up and running. Every file system that Linux uses must be mounted at some point.

Let’s say that you insert a CD or DVD into your computer’s DVD-ROM drive. To mount the CD or DVD and make it available to Linux (something that is actually done automatically as soon as you put a disk in the drive, so this example is for demonstration purposes only), you would type the following:

sudo mount /cdrom

The mount command first looks in your fstab file in the etc directory to find what you’re referring to. Figure 14-4 shows an example of the contents of that file. (The example in the figure uses the cat command, which is discussed in Chapter 15). Using this information, the mount command attempts to make the contents of the CD available in the /cdrom directory. Note that this is done in a virtual way; the files are not literally copied into the directory. The directory is merely a magical conduit that allows you to read the CD’s contents.

There aren’t any special commands used to work with mounted drives. The shell commands discussed in Chapter 13 should do everything you need.

The mount command doesn’t see widespread usage by most users nowadays, because most removable storage devices like CDs, and even memory card readers, are mounted automatically under Ubuntu and an icon for them appears on the desktop. However, there may be occasions when you need to mount a drive manually.

Figure 14-4. Details of all frequently mounted file systems are held in the /etc/fstab file.

Mounting a Drive Manually

Let’s look at an example of when you might need to mount a drive manually. Suppose that you’ve just added a second hard disk to your PC that has previously been used on a Windows system. This has been added as the primary slave.

The first thing to do is create a mount point, which is an empty directory that will act as a location where you can tell mount to make the disk accessible. You can create this directory anywhere, but under Linux, the convention is to create it in the /mnt directory. Therefore, the following command should do the trick (note that you need to use the sudo command because writing to any directory other than your /home directory requires administrator privileges):

sudo mkdir /mnt/windows

You now need to know what kind of partition type is used on the disk, because you need to specify this when mounting. To find this out, use the fdisk command. Type the following exactly as it appears:

sudo fdisk -l /dev/hdb

This will list the partitions on the second disk drive (assuming an average PC system). With most hard disks used under Windows, you should find a single partition that will be either NTFS or FAT32. The examples here assume that this is hdb1.

■Caution Be aware that fdisk is a dangerous system command that can damage your system. The program is designed to partition disks and can wipe out your data if you’re not careful!

With this information in hand, you’re now ready to mount the disk. For a FAT32 disk, type the following:

sudo mount -t vfat –o umask=000 /dev/hdb1 /mnt/windows

For an NTFS disk, type the following:

sudo mount -t ntfs -o umask=0222 /dev/hdb1 /mnt/windows

The -t command option is used to specify the file system type. The -o flag indicates that you’re going to specify some more command options and you do so in the form of umask, which tells mount to ensure that the directory is readable (and writable in the case of the FAT32 drive). After this, you specify the relevant file in the /dev directory (this file is only virtual, of course, and merely represents the hardware), and then specify the directory that is acting as your mount point.

■Note You can only read from NTFS drives under Linux because writing to them is considered too risky and might result in data loss. However, you can both read and write to FAT32 partitions because of the much simpler technology used.

Now when you browse to the /mnt/windows directory, by typing cd /mnt/windows, you should find the contents of the hard disk accessible.

For more information about the mount command, read its man page (type man mount).

Removing a Mounted System

To unmount a system, you use the special command umount (notice there’s no n after the first u). Here’s an example of using the command to unmount the CD-ROM drive:

sudo umount /media/cdrom

You need to tell the umount command where to find the mounted contents. You can specify either the mount directory, as in the example, or the file in the /dev directory that refers to mounted resource (this is a little complicated, so in most cases, it’s better to simply specify the file system location of the mount).

■Note Sometimes you can automatically unmount a CD-ROM by hitting the eject button. This is because Ubuntu is clever enough to automatically unmount the CD if the button is pressed. However, if a program is accessing the CD, or if the CD is being browsed, the button won’t work until all access to the CD has ceased.

If you’re currently browsing the mounted directory, you’ll need to leave it before you can unmount it, no matter whether you choose to do this manually or by hitting an eject button. The same is true of all kinds of access to the mounted directory. If you’re browsing the mounted drive with Nautilus or if a piece of software is accessing it, you won’t be able to unmount it until you’ve quit the program and closed the Nautilus window (or browsed to a different part of the file system).

File Searches

Files frequently get lost. Well, technically speaking, they don’t actually get lost. We just forget where we’ve put them. But because of this, the shell includes some handy commands to search for files.

Using the find Command

Like the Search option on the Windows XP Start menu, the find command manually searches through all the files on the hard disk. It’s not a particularly fast way of finding a file, but it is reliable.

Here’s an example:

find /home/keir -name "myfile"

This will search for myfile using /home/keir as a starting point (which is to say that it will search all directories within /home/keir, and any directories within those directories, and so on, because it’s recursive).

You can search the entire file system by leaving out the initial path. In this case, find will assume you want the search to start from /, the root of the file system.

If the file is found, you’ll see it appear in the output of the command. The path will be shown next to the filename.

Period punctuation symbols have interesting meanings within file listings and therefore within the output of the find command. As you learned in Chapter 13, .. refers to the parent directory of the one you’re currently browsing. In a similar way, a single . refers to the directory you’re in at the moment; it’s shorthand for “right here.” So, if find returns a result like ./myfile, it means that myfile is right here in the current directory.

However, when a single period is used at the beginning of a filename, such as in .bashrc, it has the effect of hiding the file. In other words, that file won’t appear when you type ls (although you can type ls -a to see all files, even those that are hidden).

If you give find a try, you’ll see that it’s not a particularly good way of searching. Apart from being slow, it will also return a lot of error messages about directories it cannot search. This is because, when you run the find command, it takes on your user permissions. Whenever find comes across a directory it cannot access, it will report it to you, as shown in the example in Figure 14-5. There are frequently so many of these warnings that the output can hide the instances where find actually locates the file in question!

Figure 14-5. The find command is useful for finding files but isn’t problem-free.

You can avoid these error messages in various ways, but perhaps the quickest solution is to preface the find command with sudo to invoke superuser powers. In this way, you’ll have access to every file on the hard disk, so the find command will be unrestricted in where it can search and won’t run into any directories it doesn’t have permission to enter.

■Caution Using the sudo command with find may represent an invasion of privacy if you have more than one user on your system. The find command will search other users’ /home directories and report any instances of files found there, too.

However, an even better solution for finding files is to use the locate command.

Using the locate Command

The alternative to using find is to use the locate command. This is far quicker than find because it relies on a central database of files, which is periodically updated. In other words, it doesn’t literally search the file system each time.

The problem is that if you’ve saved a file recently and are hoping to find it, there’s a chance that it won’t yet appear in locate’s database, so it won’t turn up in the list of results.

Using locate is easy. You can use the following command to search for a file (you don’t need to precede the command with sudo):

locate myfile

It’s possible to update the locate database manually, although this might take a few minutes to work through. Simply issue the command:

sudo updatedb

After this, all files in the system should be indexed, making your search results more accurate.

Using the whereis Command

One other command worth mentioning in the context of searching is whereis. This locates where programs are stored and is an excellent way of exploring your system. Using it is simply a matter of typing something like this:

whereis cp

This will tell you where the cp program is located on your hard disk. It will also tell you were its source code and man page are located (if applicable). However, the first path returned by the search will be the location of the program itself.

File Size and Free Space

Often, it’s necessary to know how large files are and to know how much space they’re taking up on the hard disk. In addition, it’s often handy to know how much free space is left on a disk.

Viewing File Sizes

Using the ls -l command option will tell you how large each file is in terms of bytes. Adding the -h option converts these file sizes to kilobytes, megabytes, and even gigabytes, depending on how large they are.

In order to get an idea of which are the largest files and which are the smallest, you can add the -S command option. This will order the files in the list in terms of the largest and smallest files.

The following will return a list of all the files in the current directory, in order of size (largest first), detailing the sizes in kilobytes, megabytes, or gigabytes:

ls -Slh

There’s another more powerful way of presenting this information: using the du command, which stands for disk usage. When used on its own without command switches, du simply presents the size of directories alongside their names (starting in the current directory). It will show any hidden directories (directories whose names start with a period) and will also present a total at the end of the list. This will probably be quite a long list. Once again, you can add the -h command option to force the du command to produce human-readable measurements of kilobytes and megabytes.

If you specify a file or directory when using the du command, along with the -s command option, you can find out its total file size:

du -sh mydirectory

This will show the size taken up on the disk by mydirectory, adding to the total any files or subdirectories it contains.