The File System

The Linux file system can be thought of as a single abstract "tree." When you add files on a removable device—such as a floppy drive or CD-ROM—or a remote system, these subsidiary file systems are "grafted" onto the original file system at a mount point.

Mount points are usually empty directories.

A mount point—and, indeed, the entire abstract tree—has no relationship with physical reality. This behavior is different than in Microsoft Windows, where the files and directories on drive E are all on the logical or physical drive E.

Actually, the term file system has two meanings:

• The entire Linux abstract file system

• A subsection, or "branch," of the file system tree: in other words, a directory, possibly with subdirectories

Generally, it is apparent from the context which usage is meant.

There's no requirement that a Linux file system be organized in a particular way, but over time a kind of organizational convention has grown up. The root file system includes the root directory, the Linux kernel, /dev for device files, /etc for critical system files, and /bin for important utilities. Other standard directories and their contents are listed in Table 11.3.

It's also important to understand that every Linux file has a mode. The mode of a file controls who can read, write, and execute the contents of a file; it is also called the file's permissions. Generally only the owner of a file or the root superuser can change a file's permissions.

Mounting devices

File systems are extended by mounting devices, such as a CD-ROM, floppy drive, or removable hard drive.

To mount and dismount a file system:

• The general syntax for mounting is mount special-device directory
  
  The special-device argument indicates the device driver file: for example, /dev/fd0 for the first floppy disk drive, or /dev/cdrom for a CD-ROM drive. The directory argument can refer to any directory, which you can create if you want, but by convention, it is often a subdirectory of /mnt.
  
  The general syntax for unmounting is umount name where name is either the special-device file or its mount point.
  
  

  Standard Linux Directories and Their Contents
  Path Name	Contents
  /	The root directory
  /bin, /sbin	Important utilities necessary for minimum system operability
  /dev	Device files for terminals, disks, modems, and so on
  /etc	Critical system files
  /tmp	Temporary files that disappear between reboots
  /usr/bin	Executable files
  /usr/lib	Support files for standard Linux programs
  /usr/man	Man pages
  /usr/src	Source code
  /var/log	Log files
  /var/spool	Spooling directories for printers, mail, and so on
  /var/tmp	Temporary space (files that don't disappear between reboots)

  
  

To mount a CD-ROM:

1.	At the prompt, type mount /dev/cdrom /mnt/cdrom
2.	Press Enter.

To unmount a CD-ROM:

1.	At the prompt, type umount /dev/cdrom or umount /mnt/cdrom
2.	Press Enter.

To mount a floppy drive:

1.	At the prompt, type mount /dev/fd0 /mnt/floppy
2.	Press Enter.

To unmount a floppy drive:

1.	At the prompt, type umount /dev/fd0 or umount /mnt/floppy
2.	Press Enter.

To mount a device using Gnome:

1.	Log on as root.
2.	On the Gnome desktop, open the Linux Configuration applet on the control panel.
3.	Scroll down until you see Mount/Unmount File Systems in the list on the left (Figure 11.6).
4.	Select a device to mount it: for example, /dev/cdrom.
5.	Respond as appropriate when asked to confirm the mount (Figure 11.7).

Creating directories

Directories are the containers for other Linux directories and files. Most likely, every time you start a new project or want to group related files together, you will create a directory.

To create a new directory:

1.	Move to the location in the directory tree where you want to create the new directory.
2.	At the command prompt, type mkdir mynewdir
3.	Press Enter.
4.	Type ls -l to list the current directories. You will see mynewdir listed with a "d" in the leftmost column, indicating that it is a directory (Figure 11.8).

To delete a directory:

1.	Using the cd command, navigate to the parent of the directory you want to delete.
2.	At the prompt, use the rmdir command to remove the directory: for example, type rmdir mynewdir.
3.	Press Enter.

Creating files

If directories are the filing system, then files are the contents.

To list all files in a directory:

1.	At the prompt, type ls -al.
2.	Press Enter. The files in the current directory will be displayed (Figure 11.9).

To create a file:

1.	At the prompt, type touch followed by the new file name. For example, type touch happy.file
2.	Press Enter.

To copy a file (or directory):

• To copy a file (or directory), use the cp command. For example, enter
  
  

  									cp myfileordir yourfileordir
  								

  
  

To move a file (or directory):

• To move a file or directory, use the mv command. For example, enter
  
  

  									mv thefileordir /newloc/thefileordir
  								

  
  

To delete a file:

1.	At the prompt, type rm followed by the name of the file. For example, type rm this.file.is.history
2.	Press Enter.

To find a file:

• The find command is an extremely flexible means of finding files. (For more information on the find command, see Appendix A.)
  
  For example, to find all the files in the /home/hdavis/ch11 directory that contain the word fig in the name, use the following syntax:
  
  

  									find /home/hdavis/ch11 -name _   '*fig*' -print
  								

  
  This displays all the files that match the condition (Figure 11.10).
  
  

To find text within a file (or files):

• Use the grep command to search one or more files for specific text, or for text that matches a regular expression.
  
  

  Figure 11.11. The grep command is used to find text that matches expressions within a file.

  

  
  For example, the following grep command (See Figure 11.11) would search all the files in the current directory for the string elephant, printing three lines on either side of the occurrence of elephant and providing line numbers (because of the -n flag):
  
  

  									grep -n -3 elephant *
  								

  
  

To view the beginning of a file:

1.	Type head myfile.
2.	Press Enter. The first 10 lines of the file will be displayed.

To view the end of a file:

1.	Type tail myfile.
2.	Press Enter. The last 10 lines of the file will be displayed.