ch07.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

<A NAME="Heading7<FONT COLOR="#000077"><B>NOTE:</B> </FONT>As with basic Linux

	commands, case is important! For instance, <TT>ls</TT> has an <TT>R</TT> option (recursive:

	show files in subdirectories, too) that gives much different results from the <TT>r</TT>

	option.

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<A NAME="Heading8<FONT COLOR="#000077"><B>TIP: </B></FONT>You can think of

	<TT>a</TT> as the &quot;all files&quot; option, <TT>l</TT> as the &quot;long list&quot;

	option, <TT>t</TT> as the &quot;sort by time&quot; option, <TT>r</TT> as the &quot;reverse

	sort&quot; option, and so on. In fact, most options in Linux are mnemonic--the option

	letter stands for a word or phrase. Some option letters mean the same thing in many

	different Linux commands. For instance, <TT>v</TT> often means verbose--in other

	words, &quot;Give me lots of detail.&quot; However, do not assume that, on any unfamiliar

	command, certain options will work in the &quot;usual&quot; way! For instance, <TT>r</TT>

	is the recursive option for many Linux commands; however, in the case of <TT>ls</TT>,

	reverse sort is more commonly used, and therefore it gets the easier-to-type lowercase

	r, while recursive is left with the capital R. It might seem like not much extra

	effort to press the Shift key to get the capital letter, but try typing a string

	of four or five options, one of which is capitalized!

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<A NAME="Heading9<FONT COLOR="#000077"><B>NOTE:</B> </FONT>You can easily find

	out which options are available for any Linux command by using the <TT>man</TT> command.

	See the section &quot;The Linux Man Pages&quot; later in this chapter.

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<H4 ALIGN="CENTER"><A NAME="Heading10<FONT COLOR="#000077">Other Parameters</FONT></H4>

<P>Linux commands often use parameters that are not actual command options. These

parameters, such as filenames or directories, are not preceded by a dash.</P>

<P>For instance, by default <TT>ls</TT> lists the files in your current directory.

You can, however, tell <TT>ls</TT> to list the files in any other directory simply

by adding the directory to the command line. For instance, <TT>ls /bin</TT> will

list everything in the <TT>/bin</TT> directory. This can be combined with command

options, so that <TT>ls -l /bin</TT> gives you detailed listings of the files in

<TT>/bin</TT>. Try this. You will be impressed by the number of files in the <TT>/bin</TT>

directory!</P>

<P>You can also specify <TT>ls</TT> to list information about any particular file

by entering its filename. For instance, <TT>ls -la .lessrc</TT> gives detailed information

only about the <TT>.lessrc</TT> file.

<H4 ALIGN="CENTER"><A NAME="Heading11<FONT COLOR="#000077">Input and Output

Redirection</FONT></H4>

<P>Many Linux commands let you specify which file or directory they are to act upon,

as we saw with the example <TT>ls -l /bin </TT>earlier.</P>

<P>You can also &quot;pipe&quot; the output from a command so that it becomes another

command's input. This is done by typing two or more commands separated by the <TT>|</TT>

character. (This character normally is found on the same key as the <TT>\</TT> character.

You must hold down the Shift key or you will get <TT>\</TT> instead of <TT>|</TT>.)

The <TT>|</TT> character means &quot;Use the output from the previous command as

the input for the next command.&quot; Therefore, typing <TT>command_1|command_2</TT>

does both commands, one after the other, before giving you the results.</P>

<P>Using our assembly-line metaphor, we are processing items through two black boxes

instead of just one. When we use piping, it's like hooking up the first command's

output conveyor belt to become the input conveyor belt for the second command.



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<A NAME="Heading12<FONT COLOR="#000077"><B>TIP: </B></FONT>Although Linux doesn't

	care whether <TT>|</TT> is set off by spaces, if <TT>command_1 | command_2</TT> is

	easier for you to read and understand than <TT>command_1|command_2</TT>, by all means

	use spaces around <TT>|</TT>.

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<P>You will have noticed that the output of <TT>ls -l /bin</TT> is many lines long,

so that much of the information scrolls off the screen before you can read it. You

can pipe this output to a formatting program called <TT>more</TT>, which displays

information in screen-sized chunks. When you enter <TT>ls -l /bin | more</TT>, you

will see the following:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">darkstar:~$ ls -l /bin | more

total 1611

-rwxr-xr-x   1 root     bin          1248 Sep 17 04:25 arch*

-rwxr-xr-x   1 root     bin        295940 Sep  5 01:45 bash*

-rwxr-xr-x   1 root     bin          4840 Nov 24  1993 cat*

-rwxr-xr-x   1 root     bin          9220 Jul 20 12:06 chgrp*

-rwxr-xr-x   1 root     bin         13316 Jul 20 12:06 chmod*

-rwxr-xr-x   1 root     bin         13316 Jul 20 12:06 chown*

lrwxrwxrwx   1 root     root           17 Dec  7 13:37 compress -&gt; /usr/bin/comp

ress*

-rwxr-xr-x   1 root     bin         21508 Jul 20 12:06 cp*

-rwxr-xr-x   1 root     bin         41988 May  1  1994 cpio*

lrwxrwxrwx   1 root     root            4 Dec  7 13:40 csh -&gt; tcsh*

-rwxr-xr-x   1 root     bin          5192 Nov 24  1993 cut*

-rwxr-xr-x   1 root     bin         19872 Mar 23  1994 date*

-rwxr-xr-x   1 root     bin         17412 Jul 20 12:06 dd*

-rwxr-xr-x   1 root     bin         13316 Jul 20 12:06 df*

-rwxr-xr-x   1 root     bin         66564 Jun  9  1994 dialog*

-rwxr-xr-x   1 root     bin          1752 Sep 17 04:25 dmesg*

lrwxrwxrwx   1 root     root            8 Dec  7 13:37 dnsdomainname -&gt; hostname*

-rwxr-xr-x   1 root     bin         13316 Jul 20 12:06 du*

-rwxr-xr-x   1 root     bin          3312 Mar 23  1994 echo*

-rwxr-xr-x   1 root     bin         36684 May  4  1994 ed*

-rwxr-xr-x   1 root     bin           326 Mar 23  1994 false*

--More--

</FONT></PRE>

<P>The <TT>--More--</TT> at the bottom of the screen tells you that there's more

text to come. To go to the next screen of text, press the spacebar. Every time you

press the spacebar, <TT>more</TT> displays another screenful of text. When the last

screenful of text has been displayed, <TT>more</TT> returns you to the Linux prompt.



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<A NAME="Heading13<FONT COLOR="#000077"><B>TIP:</B> </FONT>The <TT>more</TT>

	command can do many other things. For instance, to move back one screen at a time,

	type <TT>b</TT> for &quot;back.&quot; Another useful command is <TT>q</TT> for &quot;quit.&quot;

	This lets you leave immediately, without having to go through all the remaining screens

	of text. While in <TT>more</TT>, type <TT>h</TT> for &quot;help.&quot; This will

	list the commands available within <TT>more</TT>.

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<A NAME="Heading14<FONT COLOR="#000077"><B>NOTE: </B></FONT>The Linux system

	sometimes uses the command <TT>less</TT> instead of <TT>more</TT>. One difference

	you will notice is that, unlike <TT>more</TT>, <TT>less</TT> requires you to type

	<TT>q</TT> to return to the command line, even if you're at the end of the text to

	be displayed. This might seem cumbersome, but it prevents you from accidentally exiting

	the program by pressing the spacebar once too often. The name <TT>less</TT> is a

	play on <TT>more</TT>. Originally, <TT>less</TT> was designed to have many features

	that <TT>more</TT> lacked. The version of <TT>more</TT> included in your Linux system

	has most of these features, however. The Linux <TT>man</TT> program, discussed later,

	uses <TT>less</TT> to display text. Most other UNIX systems use <TT>more</TT> by

	default. Don't get confused. Remember to type <TT>q</TT> to exit from <TT>less</TT>!

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<P>Another thing you can do in Linux is to send output to a file instead of the screen.

There are many different reasons why you might want to do this. You might want to

save a &quot;snapshot&quot; of a command's output as it was at a certain time, or

you might want to save a command's output for further examination. You might also

want to save the output from a command that takes a very long time to run, and so

on.</P>



<P>To send output to a file, use the <TT>&gt;</TT> symbol (found above the period

on your keyboard). For instance, you can place the output of the <TT>ls -l /bin</TT>

command into a file called <TT>test</TT> by typing <TT>ls -l /bin &gt; test</TT>.

Again, spaces around <TT>&gt;</TT> are optional and not strictly necessary, but they

do make the command much more readable.</P>

<P>If you now do an <TT>ls</TT> or <TT>ls -l</TT>, you will see that you've created

a new file called <TT>test</TT> in your own directory.</P>

<P>To see the contents of a file, you can again use the <TT>more</TT> command. Just

specify the name of the file you want to look at. In this case, you would type <TT>more

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<A NAME="Heading15<FONT COLOR="#000077"><B>WARNING:</B> </FONT>Be careful!

	When you use <TT>&gt;</TT>, you completely overwrite the previous contents of the

	file you specify to take the output (if that file existed). For example, if we already

	had a file called <TT>test</TT> in our directory, its old contents would be completely

	replaced by the output from <TT>ls -l /bin</TT>. Linux will not warn you that you

	are about to do this! Be particularly careful if you're not in your usual directory,

	or if you're logged in as <TT>root</TT>. You could, for instance, accidentally clobber

	the Linux program <TT>test</TT>, which exists as a file named <TT>test</TT>--fortunately,

	not in the directory where we created our <TT>test</TT> file! It's a good idea to

	check if the output file already exists before using <TT>&gt;</TT>. In our example,

	we could have typed <TT>ls -l test</TT> beforehand. If no information is displayed,

	the file does not exist.

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<P>You can specify that you want to add your output to the end of the file, rather

than replace the file's contents, by using <TT>&gt;&gt;</TT>. Type <TT>who &gt;&gt;

test</TT> to add the output of the <TT>who</TT> command to the end of the text already

in the file <TT>test</TT>.</P>



<P>You can examine the results by using either <TT>more</TT> or <TT>less</TT> and

paging through to the end of the file, or by using the Linux command <TT>tail</TT>,

which displays the last few lines of the specified file. In this case, you would

type <TT>tail test</TT> to see the last few lines of the file <TT>test</TT>. Try

using <TT>tail</TT>!



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<A NAME="Heading16<FONT COLOR="#000077"><B>NOTE:</B> </FONT>For a more detailed

	discussion of redirection and piping, see Chapter 10, &quot;<TT>bash</TT>.&quot;

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<H3 ALIGN="CENTER"><A NAME="Heading17<FONT COLOR="#000077">Notational Conventions

Used to Describe Linux Commands</FONT></H3>



<H3 ALIGN="CENTER"><FONT COLOR="#000077"></FONT></H3>

<P>There is a set of accepted notational conventions used to describe, in a concise

and consistent way, the correct syntax for any given Linux command. This specifies

what options or parameters you must use, what options or parameters you can use or

not use, and so on. Sometimes this set of conventions is used to give a complete

and exhaustive listing of a command's syntax, showing every possible command and

parameter. Sometimes it is used to make a particular example more general and the

command's basic usage clearer.</P>



<P>If you remember the following five basic rules, you will be able, in principle,

to understand the syntax of any Linux or UNIX command.

<H4 ALIGN="CENTER"><A NAME="Heading19<FONT COLOR="#000077">Six Basic Rules

of Linux Notation</FONT></H4>



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	<DD><B>1.</B> Any text standing by itself, and not within <TT>[]</TT>, <TT>&lt;&gt;</TT>,

	or <TT>{}</TT>, must be typed exactly as shown.<BR>

	<B><BR>

	2.</B> Any text within square brackets (<TT>[]</TT>) is optional. You can type it

	or not type it. For instance, the syntax <TT>ls [-l]</TT> means you must type <TT>ls</TT>

	(per the first rule), while adding <TT>-l</TT> is optional, but not necessary. Do

	not type the square brackets themselves! In our example, type <TT>ls</TT> or <TT>ls

	-l</TT>. Don't type <TT>ls [-l]</TT>.<BR>

	<B><BR>

	3. </B>Angle brackets (<TT>&lt;&gt;</TT>) and the text within them must be replaced

	by appropriate text (usually a name or value). The text within the brackets usually

	indicates the nature of the replacement. For instance, the syntax <TT>more &lt;</TT>filename<TT>&gt;</TT>

	means that you should replace &lt;filename&gt; with the name of the file you wish

	to examine using <TT>more</TT>. If you want to look at the file <TT>test</TT>, you

	would type <TT>more test</TT>. Remember, do not use the angle brackets when you actually

	type the command!<BR>

	<B><BR>

	4.</B> Curly braces (<TT>{}</TT>) indicate that you must choose one of the values

	given within the braces. The values are separated by <TT>|</TT> (which in this case

	means or, not pipe!). For example, the syntax <TT>command -{a|b}</TT> means you must

	enter either <TT>command -a</TT> or <TT>command -b</TT>.<BR>

	<B><BR>

	5.</B> An ellipsis (<TT>...</TT>) means &quot;and so on.&quot; They are normally

	used with parameters such as filenames, as described later.<BR>

	<B><BR>

	6. </B>The sixth basic rule states that the brackets can be combined as necessary.

	For instance, you don't have to type a filename with the <TT>more</TT> command. This

	would be indicated as <TT>more [&lt;</TT>filename<TT>&gt;]</TT>. The outer set of

	square brackets makes the entire parameter optional. If you do decide to use the

	parameter, replace the inner set of angle brackets with the appropriate value. Because

	the <TT>more</TT> command enables one or more filenames to be specified, the syntax

	becomes <TT>more [&lt;</TT>filename<TT>&gt;...]</TT>. The ellipsis means you can

	have as many <TT>&lt;</TT>filenames<TT>&gt;</TT> as you want.