Working with the Linux File System

Test Objective Covered:

3. Manage the Linux file system.

To effectively manage a Linux server, it's imperative that you have a sound understanding of the Linux file system. We've already introduced this topic earlier in this chapter, but now we're going to dive in a little deeper in the sections that follow.

Default Linux File System Structure

If you're familiar with Windows or DOS, you will probably be completely lost when you first encounter the default Linux directory structure. There's little in the way of similarity between them.

That's because they are based on completely different standards. The Linux directory structure is based on the Linux Standards Base (LSB). The goal behind the LSB project is to encourage all the various Linux distributions to conform to a set of operating system standards, including how the file system is organized. The purpose is to ensure compatibility between distributions.

Within the LSB is the File Systems Hierarchy (FSH) standard. This defines directory structures, naming standards, and usage. For more details on the LSB, visit http://www.linuxbase.org.

A full review of all the default Linux directories is beyond the scope of this study guide. The folks at Novell, however, felt it important enough that they included this objective in the CLE. Here are the common Linux directories you need to be familiar with to be successful in the CLE Practicum:

• /bin ” This directory contains most of the useful command-line programs and shells you learned about earlier in this chapter. These include echo, bash, zsh, su, and netstat, among others.

• /dev ” This directory has a special purpose in the Linux file system ”it contains files that represent special links to system hardware. For example, in /dev is a file named sda. This file points to the first SCSI hard disk drive, if it exists, in the system. If you have two SCSI hard disk drives , the sdb file would reference the second drive. The hda file points to the first IDE hard disk drive in the system. The fd0 file is a link to the floppy drive A.

• /etc ” This directory contains configuration files for the various applications and utilities on the system. For example, the fstab file, mentioned earlier in this chapter, is located in this directory. Your hosts file is also located in this directory. If you needed to configure your web server, you would use the httpd.conf file located in the httpd subdirectory of /etc . Because the files in this directory are used to configure the local system, the files in the /etc directory on one Linux system will be different from those on other Linux systems.

• /home ” This directory contains home directories for every user in the system except for root.

• /mnt ” This directory is used as a mount point for remote NFS file systems. It is also used to mount a Samba share on a remote server. On some distributions, such as Red Hat, it is used to mount removable media such as floppy disks and CDs.

• /media ” This directory is unique to SUSE Linux. It is used as a mount point for removable media.

• /root ” This directory is the root user's home directory. By default, only root can see this directory.

• /tmp ” This directory is used for storage of temporary files created by applications.

• /usr ” This directory is used to store binary program files. For example, gcalc, an X Window System calculator program, is stored in the /usr/bin directory.

  Real World

  Many people think that usr stands for "user." That isn't true. The acronym usr stands for Unix System Resources .

  
  

• /var ” This directory has a variety of uses. Manual pages are stored here, as are log files. Data files for services such as named and dhcpd are stored here. You will also see that NNLS stores many of its data files here, as well.

Warning

Because the data stored in /var can grow very large ( especially with NNLS installed), you should make sure the partition where /var is located has lots of free space. Some NNLS administrators will actually add an additional disk to the system, partition it, and mount it at /var to ensure they don't run out of space .

As you will be working a great deal with these directories in this course, it would be a good idea to commit their names and uses to memory.

Keep in mind that these are the default system-created directory structures. There's nothing to stop you from modifying the structure or creating your own additional structures. If you do, however, be sure to document your changes. An administrator down the road might inherit your system from you and will need to know what you have put where.

Command-Line File System Utilities

Just as with other operating systems, Linux provides a suite of utilities you can use to manage the file system. These include tools to make directories, copy files, move files, delete files, remove directories, view the contents of files, and so on.

It's been my experience that system administrators who learned their trade in a graphical environment tend to become a little nervous when presented with command-line file system utilities. I've heard arguments from them to the effect that, if graphical utilities are available, in this case X Window System, why bother learning how to use the command-line version of these utilities?

In a Windows world, there is some validity to their argument. Windows command-line utilities generally aren't as powerful as their graphical counterparts.

The same can't be said for Linux. There are two good reasons for leaning how to use command-line Linux tools. First and foremost, many Linux system administrators and implementers actually install their systems without X Window System. They want their server systems to run as fast as possible. They don't want to "waste" CPU cycles redrawing graphical screens. Second, the Linux command-line file system utilities tend to be much more powerful than their graphical counterparts. Most experienced administrators share this opinion, preferring command-line utilities over their graphical equivalent.

In short, to be a good Linux system administrator, you simply must know how to effectively use the command line. With that said, let's review some of the most commonly used command-line file system management utilities. Note that this study guide doesn't go into great depth on the syntax required to use these commands. You can use the man command to learn the specifics of how to use each of those utilities.

Real World

Remember that these commands are case sensitive!

• cat ” This command is used to display the contents of a text file on the screen. The DOS equivalent would be the TYPE command. The syntax is cat file_name . For example, to print the contents of the system log to screen, you would enter cat /var/log/ messages.

• cp ” This command is used to copy files. The syntax used is very similar to that used with the COPY command used by MS-DOS. You simply type cp source_file destination_file .

• cd ” This command is very similar to its DOS counterpart command: CD . It is used to change directories within a file system. The syntax is cd directory_name .

• dir ” This command is very similar to the DOS command DIR . It is used to display a listing of the contents of a directory. The syntax is dir directory_name . If no directory name is supplied, it displays the contents of the current directory. If you add an “o switch, the file owners and permission assignments, discussed in the next chapter, are displayed as well.

• find ” This command is used to search for a directory or file. For example, you could enter find /tmp to find all the files in the /tmp directory and its subdirectories.

• grep ” This command is very powerful. It is used to search for an expression within a file. The syntax is grep expression file_name . If you use the “ i switch, grep will ignore case as it searches the file. This utility is very useful for searching for specific text within large log files.

• less ” This command is related to the more command. It is usually used in conjunction with another command, such as dir , ls , or cat , to pause long displays one screen at a time. For example, if you were to execute dir “o for a large directory, such as /usr/bin , the output quickly scrolls off the screen before you can read it. If, instead, you were to enter dir /usr/bin “o less , the output will pause at each screen, allowing you to read the text. This command also allows you to page forward or backward through the display (which the more command can't do).

• locate ” This command is similar to the find command. It is used to search for files in the file system. It uses a special search database that speeds up the search process. The updatedb command is used to initially build the database. To locate a file, enter locate file_name . For example, you could enter locate gnome- term -linux.png to locate the graphic file you used for your terminal icon in GNOME.

• ls ” This command is similar to the dir command. It is used to display the contents of a directory. The syntax is ls directory_name . If no directory name is provided, it will display the contents of the current directory. If you append the “l switch, the owners and permissions assigned to the files in the directory will also be displayed.

• mkdir ” This command is used to create a directory in the file system. It is similar to the MD command in DOS. The syntax is mkdir path /directory_name . If you omit the path, the directory will be created in the current directory.

• more ” This command is similar to the less command. It is used in conjunction with another command, such as dir , ls , or cat , to pause long displays one screen at a time. For example, if you were to enter dir /usr/bin “o more , the output will pause at each screen, allowing you to read the text.

• mv ” The mv command is similar the MOVE command under DOS. It is used to move files within the file system. It can also be used to rename a file in place. The syntax for this command is mv source_path/file destination_path .

• pwd ” This command displays the current directory.

• rm ” This command is similar to the DEL command under DOS. It is used to delete a file in the file system. The syntax is rm file_name .

• rmdir ” This command is similar to the RD command under DOS. It's used to delete a directory from the file system. The syntax is rmdir directory_name .

• tail ” This is a very useful command for viewing entries in text files, such as a log file. Log files can become quite large over time. There will be many times when you will actually only need to view the last few entries in the file. The tail command is used in these circumstances ”it displays the last 10 lines of a text file (although you can configure it to display more). The syntax is tail file_name . When troubleshooting a Linux system, one of the most useful tools I've found is to use tail on the /var/log/messages file. By using the “f parameter, you can continuously monitor the file.

The preceding commands are very useful when you're maintaining a Linux system, especially one with NNSL installed. At this point, we need to discuss in depth several utilities that you must know how to use in order to install and configure NNLS on your Linux system. These include mount, tar, gunzip , and vi .

Using the mount Command

In order to install NNLS on your Linux system, you must know how to use the mount command. This command wasn't included in the preceding list because you need to know how to use it in depth. You will probably need to use the mount command whenever you insert a floppy diskette or a CD/DVD disc in your Linux server's drives.

Tip

Some Linux distributions use an automount daemon that detects a new diskette or disc in the system and will automatically mount it .

The syntax for the mount command is as follows :

 mount -t  filesystem_type device mount_point  

The mount command supports many different file system types. There are really only two that you need to be concerned with for the CLE certification. The first is the ISO9660 file system. This is used by most data CDs. The other is the VFAT file system. This used by most floppy diskettes.

Tip

Technically, a floppy diskette can be formatted using any file system that supports the media type. However, in this book, all the floppy diskettes we will work with will be formatted using the FAT file system. This allows them to be readable in both Windows and Linux systems .

Check the man page for mount to see a complete list of supported file systems.

To mount a floppy diskette on a SUSE Linux system, you would insert the disk and then enter mount -t vfat /dev/fd0/media/floppy .

Note

Remember that you don't have to use the /media/floppy direcrory. You can mount the disk anywhere in the file system. However, many applications are hard-coded to look in /media/floppy when they are looking for a floppy diskette .

On a Red Hat system, you would use the same command, but you would mount the diskette in the /mnt/floppy directory.

If you have the correct entry in the fstab file, you don't have to specify the file system type or the mount point. Consider the fstab file shown in Figure 2.14.

Figure 2.14. The fstab file.

Notice that /dev/fd0 is configured to automatically mount to /media/floppy . Because of this, you can enter the following command to mount a floppy diskette:

 mount /dev/fd0 

You can customize your fstab file to make the mount process even easier. To do this, use the following command:

 ln -s /dev/fd0 /dev/floppy 

Note

To learn more about how ln is used to create links, check out its man page by entering man ln .

When executed on your system, this command creates a file in /dev called floppy and creates a symbolic link between it and the /dev/fd0 file. After it's executed, you can mount a floppy diskette on your system by simply entering the following:

 mount /dev/floppy 

CDs are mounted in a similar fashion as floppies. On a SUSE Linux system, insert the CD in the CD drive and then enter the following command:

 mount -t iso9660 /dev/cdrom /media/cdrom 

A Red Hat system is mounted in the same way except that the default mount point is /mnt/cdrom .

Notice in Figure 2.14 that there is an entry in the fstab file for /dev/cdrom that configures it to automatically mount to the /media/cdrom directory. Because of this, you can mount a CD using a simplified mount command as shown here:

 mount /dev/cdrom 

If your system doesn't have an entry for /dev/cdrom in the fstab file, you can create your own symbolic link, as we did for the floppy diskette drive. Check the fstab file and entry for the CD drive. If your CD drive is the master drive on the secondary IDE channel (which is how most PC manufacturers configure them), its corresponding link in /dev will be hdc . You can create a symbolic link for /dev/cdrom by entering the following command:

 ln -s /dev/hdc /dev/cdrom 

Once this link has been created, the preceding mount command will work properly. To access a mounted floppy diskette or CD, simply change directories to the mount point. The files on the media will be accessible in that directory.

It is critical that you remember that mounted media must not be removed unless you first dismount it. You'll notice that, once mounted, you can't open the CD drive door until it is dismounted. There is nothing to stop you, however, from removing a mounted floppy diskette from its drive.

This can be a serious issue. Changes made to the files on the diskette may not be immediately written to the disk. Dismounting the media immediately writes all changes to disk. If you remove it without dismounting , files may become corrupted.

To dismount a mounted floppy diskette, enter the following command:

 umount /dev/floppy 

or

 umount /dev/fd0 

To dismount a mounted CD, enter the following command:

 umount /dev/cdrom 

Tip

You can also use the mount point directory with the umount command. For example, you could dismount a floppy diskette by entering umount /media/floppy .

Managing Archive Files

With the concept of mounting removable media behind us, we still need to cover a few more topics in this chapter. Novell wants you to understand how to work with Linux archive files in preparation for your CLE practical exam.

Most operating systems have programs written for them that can compress multiple files into a single zip file. You've probably downloaded a zip file before from the Internet, or had one emailed to you.

Linux also has utilities for working with compressed files. It's a little bit different, however, than those you've probably worked with on the Windows or Macintosh platforms. Linux uses two separate utilities: tar and gzip . The tar utility, which stands for tape archive , is used to take multiple files from the Linux file system and combine them into a single file. Tape archive files usually end with a .tar extension. This archive file is not compressed.

The gzip utility is used to compress files. It can be used to compress any file in the file system, but it is most frequently used to compress archive files. The gunzip file is used to decompress a compressed file. Files compressed with gzip usually have a .gz extension.

If you see a Linux file that has a .tar.gz extension, you know that this is an archive file that has been compressed with gzip.

Note

Compressed archive files are sometimes called tarballs .

There are two ways to decompress and extract a tarball file. The first way is to use the gunzip command to decompress the file, and the syntax is gunzip file_name . After the file has been decompressed, you'll notice that it automatically loses its .gz extension. For example, if you were decompressing mystuff.tar.gz , the resulting file from the gunzip process would be mystuff.tar .

After decompression , the archive is extracted using the tar command, and the syntax is tar -xvf file_name . The x switch tells the tar utility to extract an existing archive file. The v switch tells the tar utility to display the progress of the extraction process on the screen. The f switch tells the tar utility that the string of text that follows is the name of the file to be processed .

Warning

The v switch is optional, but the x and f switches are not .

The tar utility can also be used to both decompress and extract, and this is the second method for managing tarball files. To do this, simply add a “ z switch to the tar command and run it against the . tar.gz file. This will automatically decompress and then extract the archive. For example, if you have a tarball named mystuff.tar.gz , you could decompress and unarchive it by entering tar -zxvf mystuff.tar.gz .

Editing Text Files

The last topic we will address in this chapter is that of editing text files. Another way that Linux is unique from Windows is the fact that most Linux configuration changes are made by editing text files.

If you're familiar with Windows, you know that most of its configuration parameters are stored in the Registry. Data in the Registry is stored in a series of binary files that can only be edited with a binary file editor such as REGEDIT or REGEDT32.

Linux configuration parameters, on the other hand, are stored in a series of text files that are located, for the most part, in the /etc directory.

Which is the better method? I suspect this question is very subjective . Personally, I much prefer editing a text file. For me, they are much more intuitive. Too many times have I opened up Registry keys in Windows and had to do a lot of head scratching to understand them. With a text file, programmers can (and usually do) include documentation for the various parameters as comments within the file.

Most Linux distributions include a number of text editors, both graphical and command-line, that you can use to edit these configuration files.

Using vi from the Command Line

One of the most commonly used editors is vi ”a command-line text editor. Depending upon your distribution, you might have vi or vim (which stands for vi improved ) installed. To open a file for editing in vi, enter the following command:

 vi /  path_to_file/file_name  

When you do, vi is executed, and the specified file is loaded. For example, if you need to edit the smb.conf file (which is used to configure the Samba service), you could enter vi /etc/samba/smb.conf . When you do, the following appears, as shown in Figure 2.15.

Figure 2.15. The vi command mode.

If you're used to text editors such as Notepad under Windows, you might find vi a little confusing at first. You need to understand that vi operates in two different modes: Insert mode and Command mode.

Insert mode is used for actually editing the file. When the file is first opened in vi, however, it is opened in Command mode. This mode is what is shown in Figure 2.15. To edit the file, you must enter Insert mode by pressing Ins or by pressing i , as shown in Figure 2.16.

Figure 2.16. Editing in vi Insert mode.

Notice in Figure 2.16 that the text “ INSERT - appears at the bottom of the screen when you are in Insert mode. When you are in Command mode, this text is absent, as shown in Figure 2.15 In Insert mode, you can edit the file just as you would with any other text editor. To return to command mode, press the Esc key.

In Command mode, you can enter commands to work with the file. To enter a command in Command mode, you must first type:. This brings up a command prompt of sorts where you enter vi commands (see Figure 2.17).

Figure 2.17. Entering vi commands.

Notice that you now see a colon (:) and a cursor at the bottom of the screen, as shown in Figure 2.17. This cursor is used to issue commands to vi. For example, to exit vi and save all changes made to the currently open file, you would enter exit or q at the command prompt. To exit vi without saving changes to the file, you would enter quit! or q! .

A full review of all vi commands is beyond the scope of this book. Fortunately, there is an extensive help system available for vi. At the shell prompt, enter vimtutor to learn more.

Editing Text Files Using Graphical Utilities

If text-based editors make you uncomfortable, you can also use one of several graphical text editors from the X Window System environment to edit Linux configuration files.

On a Red Hat system, you can use the gedit utility. Open a terminal session and enter gedit from the shell prompt. You can also select Main Menu, Programs, Applications, gedit . When you do, the screen looks like Figure 2.18.

Figure 2.18. Editing text files with gedit.

You'll find that gedit works in a manner very similar to the Notepad utility on a Windows system.

On a SUSE Linux system, you can use the KWrite utility to edit text files in a graphical environment. You can run KWrite by selecting Applications, KDE Menu, Editors, KWrite . When you do, the screen shown in Figure 2.19 appears.

Figure 2.19. Editing text files with KWrite.

When you understand how to use mount, tar, gunzip, and vi, you've got the skills you need to complete an initial NNLS installation. Before continuing, make sure you can use these utilities in Lab Exercise 2-3.