Lowe D. Networking For Dummies

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Book IX

Chapter 8

Running FTP

757

Configuring FTP

If you disable anonymous access, your users will have to have a Linux

account on the server to access the FTP site. By default, each user will

be directed to his or her home directory.

✦ Allowing local users: By default, local users are allowed to access the

FTP site. To prevent local users from accessing the FTP site, change the

local_enable line to:

local_enable=NO

Note that you may want to disable anonymous users or local users, but

you shouldn’t disable both!

✦ Creating a read-only FTP site: If you want to make your FTP site read-

only, so that users can download files but not upload, change the

write_enable line to:

write_enable=NO

✦ Customizing the welcome banner: You can customize the welcome

banner by uncommenting the ftpd_banner line and adding your

banner. For example:

Ftpd_banner=Welcome to my FTP site!

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758

Book IX: Managing Linux Systems

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Chapter 9: Linux Commands

In This Chapter

✓ Discovering the basics of command shells

✓ Identifying file and directory commands

✓ Discovering commands that help with packages and services

✓ Figuring out commands for managing users and groups

✓ Becoming familiar with networking commands

inux has several nice graphical user interfaces to choose from, and

many of the more common Linux networking functions have graphical

configuration utilities. Still, many Linux configuration tasks can be done only

from a command shell. In many cases, the configuration utility provides

access only to the most basic configuration parameters. So if you want to

configure advanced features, you have to use commands. In fact, some net-

work features don’t have a graphical configuration utility, so you have no

choice but to use commands.

Even when GNOME-based alternatives are available, you’ll often resort to

using commands because, frankly, that’s what Linux was built to do. Unlike

Windows, Linux relies on commands as its primary means of getting things

done. So if you’re going to work with Linux, knowing the basic commands

presented in this chapter is a must.

Command Shell Basics

A shell is a program that accepts commands from a command prompt and

executes them. The shell displays a prompt to let you know it’s waiting for a

command. When you type the command and press the Enter key, the system

reads your command, interprets it, executes it, displays the results, and then

displays the prompt again so that you can enter another command.

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Command Shell Basics

Getting to a shell

You can work with Linux commands directly from one of the six virtual con-

soles. If you like the responsiveness of text mode, virtual consoles are for

you. To switch to a virtual console, press Ctrl+Alt+Fx. For example, press

Ctrl+Alt+F1 to switch to virtual console 1. When you’re in a virtual console,

you have to answer the logon prompt with a valid username and password.

To return to GNOME, press Ctrl+Alt+F7.

The alternative is to work in a terminal window within the GNOME environ-

ment. If you have an older computer, you may find that the terminal window

is a little unresponsive. If your computer is relatively new, however, the

terminal window will be just as responsive as the text-mode virtual console.

Plus, you’ll have the benefit of a scroll bar that lets you scroll to see text that

otherwise would have flown off the screen.

To open a terminal window, choose Applications➪System Tools➪Terminal.

This opens a command shell in a window right on the GNOME desktop, as

shown in Figure 9-1. Because this shell runs within the user account GNOME

is logged on as, you don’t have to log on. You can just start typing com-

mands. When you’re done, type Exit to close the window.

Figure 9-1:

A terminal

window.

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Command Shell Basics

For normal Linux users, the command shell prompt character is a dollar sign

($). If you see a hash mark (#) as the prompt character, it means you’re

logged on as root. Whenever you see a hash prompt, you should be extra

careful about what you do because you can easily get yourself into trouble

by deleting important files or otherwise corrupting the system.

Editing commands

Most of the time, you just type commands using the keyboard. If you make

a mistake, you just type the command again, being careful not to repeat the

mistake. However, Linux shells have several built-in editing features that can

simplify the task of correcting a mistaken command or entering a sequence

of similar commands:

✦ If you want to repeat a command that you’ve used recently, press the

up-arrow key. This action recalls your most recently executed com-

mands. You can press Enter to execute a command as is, or you can edit

the command before you execute it.

✦ The shell has a handy auto-complete feature that can finish partially

spelled directory, file, or command names. Just type part of the name

and then press the Tab key. If you’ve typed enough for the shell to figure

out what you mean, the shell finishes the name for you. Otherwise, it

just beeps, in which case you can type a few more letters and try again.

In some cases, the shell displays a list of items that match what you’ve

typed so far to help you know what to type next.

Wildcards

Wildcards are one of the most powerful features of command shells. With

wildcards, you can process all the files that match a particular naming pat-

tern with a single command. For example, suppose that you have a folder

with 500 files in it, and you want to delete all the files that contain the letters

Y2K and end with .doc, which happens to be 50 files. If you try to do this in

GNOME, you’ll spend ten minutes picking these files out from the list. From a

shell, you can delete them all with the single command rm *Y2K*.doc.

You can use two basic wildcard characters. An asterisk (*) stands for any

number of characters, including zero, while an exclamation mark (!) stands

for just one character. Thus, !Text.doc matches files with names like

aText.doc, xText.doc, and 4Text.doc, but not abcText.doc or just

Text.doc. However, *Text.doc would match any of the names I mentioned.

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Command Shell Basics

You can also use brackets to indicate a range of characters to choose

from. For example, report[123] matches the files report1, report2, or

report3. You can also specify report[1-5] to match report1, report2,

report3, report4, or report5. The wildcard r[aeiou]port matches

files named raport, report, riport, roport, or ruport. As you can see,

the possibilities are almost endless.

Redirection and piping

Redirection and piping are related techniques. Redirection lets you specify

an alternative destination for output that will be displayed by a command or

specify an alternative source for input that should be fed into a command.

For example, you can save the results of an ifconfig command to /home/

doug/myconfig like this:

$ ifconfig > /home/doug/myconfig

Here, the greater-than sign (>) is used to redirect the command’s console

output.

If a command accepts input from the keyboard, you can use input redirec-

tion to specify a file that contains the input that you want to feed to the

command. For example, you can create a text file named lookup.commands

with subcommands for a command such as nslookup. Then, you can feed

those scripted subcommands to the nslookup command, like this:

$ nslookup < /home/doug/lookup.commands

Piping is a similar technique. It takes the console output from one command

and feeds it into the next command as input. One of the most common uses

of piping is to send the output of a command that displays a lot of informa-

tion to the more program, which displays the output one page at a time. For

example:

$ ifconfig | more

The vertical bar ( | ) is often called the pipe character because it’s the

symbol used to indicate piping.

Environment variables

The shell makes several environment variables available to commands. You

can use an environment variable anywhere in a command by typing $ (dollar

sign) followed by the environment variable name. For example, this command

$ echo This is $HOSTNAME running on an $HOSTTYPE

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Command Shell Basics

displays a line such as

This is LSERVER running on an i386

Table 9-1 lists some of the more useful environment variables that are avail-

able to you and your commands.

Table 9-1 Environment Variables

Variable Description

HOME

The current user’s home directory.

HOSTNAME

The computer’s host name.

HOSTTYPE

The host computer type.

OSTYPE

The operating system.

PATH

The search order for executable programs.

PROMPT_COMMAND

The command used to generate the prompt.

PWD

The present working directory.

SHELL

The shell being used.

USERNAME

The current username.

Shell scripts

A shell script is simply a text file that contains one or more commands. Shell

scripts are similar to MS-DOS/Windows batch files, but shell scripts can be

much more complex. The simplest shell scripts are just lists of commands,

but advanced shell scripts can include complicated scripting statements

that border on a full-featured programming language.

You can create shell scripts by using any text editor. The easiest text editor

to use is gedit, which you can access from the GNOME desktop by choos-

ing Main Menu➪Accessories➪Text Editor. If you want your friends to think

you’re a Linux guru, however, take a few moments to learn how to use vi, a

powerful text mode editor. To create or edit a file in vi, type the command

vi followed by a filename. Then, type away. To use a vi command, press the

Escape key and then type one of the commands listed in Table 9-2.

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Directory and File Handling Commands

Table 9-2 Common vi Commands

Command Explanation

Enters insert mode so that you can enter text at the cursor location.

Move the cursor to the point where you want to enter the text first.

When you’re finished inserting text, press Esc to return to com-

mand mode.

Saves the file (w stands for write).

Quit.

:wq

Write and then quit.

:q!

Quit without saving.

/string

Search forward for string.

?string

Search backward for string.

Repeat the last search.

Undo the previous command.

After you’ve created a shell script, you have to grant yourself execute per-

mission to run the script. For example, to grant yourself permission to run a

script named myscript, use this command:

$ chmod 755 myscript

To run a shell script, you use the sh command and provide the name of the

script file. For example:

$ sh myscript

Directory and File Handling Commands

Because much of Linux administration involves working with configuration

files, you frequently need to use the basic directory and file handling com-

mands presented in this section.

The pwd command

This command displays the current directory, which is called the present

working directory — hence the command name pwd. Here’s the syntax:

pwd

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Directory and File Handling Commands

Enter this command, and you get output similar to the following:

$ pwd

/home/doug

The cd command

The cd command changes the current working directory. The syntax is as

follows:

cd directory

You may want to follow the cd command up with a pwd command to make

sure that you changed to the right directory. For example:

$ cd /etc/mail

$ pwd

/etc/mail

To change to a subdirectory of the current directory, omit the leading slash

from the directory name. For example:

$ pwd

/home

$ cd doug

$ pwd

/home/doug

You can also use the double-dot (..) to represent the parent of the current

directory. Thus, to move up one level, use the command cd .. as follows:

$ pwd

/home/doug

$ cd ..

$ pwd

/home

The mkdir command

To create a new directory, you use the mkdir command. It has the following

syntax:

mkdir directory

Here’s an example that creates a subdirectory named images in the current

directory:

$ mkdir images

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Directory and File Handling Commands

This example creates a directory named /home/doug/images:

$ mkdir /home/doug/images

The rmdir command

The rmdir command removes a directory. It has the following syntax:

rmdir directory

Here’s an example:

$ rmdir /home/doug/images

Here, the /home/doug/images directory is deleted. Note that the directory

must be empty to be removed, so you have to first delete any files in the

directory.

The ls command

The ls command lists the contents of the current directory. Here’s the

syntax:

ls [options] directory

The following paragraphs describe the more important options for the ls

command:

✦ -a: Lists all the files in the directory, including files that start with a

period.

✦ -c: Sorts entries by the time the files were last modified.

✦ -d: Lists only directory names.

✦ -l: Displays in long format.

✦ -r: Displays files in reverse order.

✦ -R: Lists the contents of all subdirectories, and subdirectories of subdi-

rectories, and subdirectories of subdirectories of subdirectories . . . in

other words, lists subdirectories recursively.

✦ -s: Displays file sizes.

✦ -S: Sorts files by size.

✦ -t: Sorts files by timestamp.

✦ -u: Sorts files by the time the files were last accessed.

✦ -X: Sorts files by their extensions.

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