📄 ch9.htm
字号:
<HTML>
<HEAD>
<TITLE>Chapter 9 -- Using Files</TITLE>
<META>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910">
<H1><FONT SIZE=6 COLOR=#FF0000>Chapter 9</FONT></H1>
<H1><FONT SIZE=6 COLOR=#FF0000>Using Files</FONT></H1>
<HR>
<P>
<CENTER><B><FONT SIZE=5>CONTENTS</FONT></B></CENTER>
<UL>
<LI><A HREF="#SomeFilesAreStandard">
Some Files Are Standard</A>
<UL>
<LI><A HREF="#ExampleUsingSTDIN">
Example: Using STDIN</A>
<LI><A HREF="#ExampleUsingRedirectiontoChangeSTDINandSTDOUT">
Example: Using Redirection to Change STDIN and STDOUT</A>
<LI><A HREF="#ExampleUsingtheDiamondOperatorltgt">
Example: Using the Diamond Operator (<>)</A>
</UL>
<LI><A HREF="#FileTestOperators">
File Test Operators</A>
<UL>
<LI><A HREF="#ExampleUsingFileTests">
Example: Using File Tests</A>
</UL>
<LI><A HREF="#FileFuNCtionsBR">
File FuNCtions<BR>
</A>
<UL>
<LI><A HREF="#ExampleOpeningFiles">
Example: Opening Files</A>
<LI><A HREF="#ExampleBinaryFiles">
Example: Binary Files</A>
<LI><A HREF="#ExampleGettingFileStatistics">
Example: Getting File Statistics</A>
<LI><A HREF="#ExampleUsingtheDirectoryFuNCtions">
Example: Using the Directory FuNCtions</A>
<LI><A HREF="#ExamplePrintingRevisited">
Example: Printing Revisited</A>
</UL>
<LI><A HREF="#Globbing">
Globbing</A>
<UL>
<LI><A HREF="#ExampleAssigningaGlobtoanArray">
Example: Assigning a Glob to an Array</A>
</UL>
<LI><A HREF="#UsingDataStructureswithFiles">
Using Data Structures with Files</A>
<UL>
<LI><A HREF="#ExampleSplittingaRecordintoFields">
Example: Splitting a Record into Fields</A>
</UL>
<LI><A HREF="#Summary">
Summary</A>
<LI><A HREF="#ReviewQuestions">
Review Questions</A>
<LI><A HREF="#ReviewExercises">
Review Exercises</A>
</UL>
<HR>
<P>
If you've read the previous chapters and have executed some of
the programs, then you already know that a file is a series of
bytes stored on a disk instead of inside the computer's memory.
A <I>file</I> is good for long-term storage of information. Information
in the computer's memory is lost when the computer is turned off.
Information on a disk, however, is persistent. It will be there
when the computer is turned back on.
<P>
Back in <A HREF="ch1.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch1.htm" >Chapter 1</A> "Getting Your Feet Wet," you saw
how to create a file using the edit program that comes with Windows
95 and Windows NT. In this chapter, you'll see how to manipulate
files with Perl.
<P>
There are four basic operations that you can do with files. You
can open them, read from them, write to them, and close them.
Opening a file creates a connection between your program and the
location on the disk where the file is stored. Closing a file
shuts down that connection.
<P>
Every file has a unique <I>fully qualified </I>name so that it
can't be confused with other files. The fully qualified name iNCludes
the name of the disk, the directory, and the file name. Files
in different directories can have the same name because the operating
system considers the directory name to be a part of the file name.
Here are some fully qualified file names:
<BLOCKQUOTE>
<PRE>
c:/windows/win95.txt
c:/windows/command/scandisk.ini
c:/a_long_directory_name/a_long_subdirectory_name/a_long_file_name.doc
<BR>
</PRE>
</BLOCKQUOTE>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD><B>Caution</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
You may be curious to know if spaces can be used inside file names. Yes, they can. But, if you use spaces, you need to surround the file name with quotes when referring to it from a DOS or UNIX command line.</BLOCKQUOTE>
</TD></TR>
</TABLE>
</CENTER>
<P>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD><B>Note</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
It is very important that you check for errors when dealing with files. To simplify the examples in this chapter, little error checking will be used in the example. Instead, error checking information will be discussed in <A HREF="ch13.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch13.htm" >Chapter
13</A>, "Handling Errors and Signals."</BLOCKQUOTE>
</TD></TR>
</TABLE>
</CENTER>
<P>
<H2><A NAME="SomeFilesAreStandard"><FONT SIZE=5 COLOR=#FF0000>
Some Files Are Standard</FONT></A></H2>
<P>
In an effort to make programs more uniform, there are three connections
that always exist when your program starts. These are <TT>STDIN</TT>,
<TT>STDOUT</TT>, and <TT>STDERR</TT>.
Actually, these names are <I>file handles</I>. File handles are
variables used to manipulate files. Just like you need to grab
the handle of a hot pot before you can pick it up, you need a
file handle before you can use a file. Table 9.1 describes the
three file handles.<BR>
<P>
<CENTER><B>Table 9.1 The Standard File Handles</B></CENTER>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD WIDTH=73><I>Name</I></TD><TD WIDTH=517><I>Description</I>
</TD></TR>
<TR><TD WIDTH=73><TT>STDIN</TT></TD>
<TD WIDTH=517>Reads program input. Typically this is the computer's keyboard.
</TD></TR>
<TR><TD WIDTH=73><TT>STDOUT</TT></TD>
<TD WIDTH=517>Displays program output. This is usually the computer's monitor.
</TD></TR>
<TR><TD WIDTH=73><TT>STDERR</TT></TD>
<TD WIDTH=517>Displays program errors. Most of the time, it is equivalent to <TT>STDOUT</TT>, which means the error messages will be displayed on the computer's monitor.
</TD></TR>
</TABLE>
</CENTER>
<P>
<P>
You've been using the <TT>STDOUT</TT>
file handle without knowing it for every <TT>print()</TT>
statement in this book. The <TT>print()</TT>
fuNCtion uses <TT>STDOUT</TT> as the
default if no other file handle is specified. Later in this chapter,
in the "Examples: Printing Revisited" section, you will
see how to send output to a file instead of to the monitor.
<H3><A NAME="ExampleUsingSTDIN">
Example: Using STDIN</A></H3>
<P>
Reading a line of input from the standard input, <TT>STDIN</TT>,
is one of the easiest things that you can do in Perl. This following
three-line program will read a line from the keyboard and then
display it. This will continue until you press <TT>Ctrl+Z</TT>
on DOS systems or <TT>Ctrl-D</TT>
on UNIX systems.
<HR>
<BLOCKQUOTE>
<B>Listing 9.1 09LST01.PL-Read from Standard Input
Until an End-of-File Character Is Found<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<PRE>
while (<STDIN>) {
print();
}
</PRE>
</BLOCKQUOTE>
<HR>
<P>
The <TT><></TT> characters,
when used together, are called the <I>diamond</I> operator. It
tells Perl to read a line of input from the file handle inside
the operator. In this case, <TT>STDIN</TT>.
Later, you'll use the diamond operator to read from other file
handles.
<P>
In this example, the diamond operator assigned the value of the
input string to <TT>$_ </TT>. Then,
the <TT>print()</TT> fuNCtion was
called with no parameters, which tells <TT>print()</TT>
to use <TT>$_</TT> as the default
parameter. Using the <TT>$_ </TT>
variable can save a lot of typing, but I'll let you decide which
is more readable. Here is the same program without using <TT>$_</TT>.
<BLOCKQUOTE>
<PRE>
while ($inputLine = <STDIN>) {
print($inputLine);
}
</PRE>
</BLOCKQUOTE>
<P>
When you pressed <TT>Ctrl+Z</TT> or
<TT>Ctrl+D</TT>, you told Perl that
the input file was finished. This caused the diamond operator
to return the undefined value which Perl equates to false and
caused the <TT>while</TT> loop to
end. In DOS (and therefore in all of the flavors of Windows),
26-the value of <TT>Ctrl+Z</TT>-is
considered to be the end-of-file indicator. As DOS reads or writes
a file, it monitors the data stream and when a value of 26 is
eNCountered the file is closed. UNIX does the same thing when
a value of 4-the value of <TT>Ctrl+D</TT>-is
read.<BR>
<p>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR><TD><B>Tip</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
When a file is read using the diamond operator, the newline character that ends the line is kept as part of the input string. Frequently, you'll see the <TT>chop()</TT> fuNCtion used to remove the newline. For instaNCe, <TT>chop($inputLine =
<INPUT_FILE>);</TT>. This statement reads a line from the input file, assigns its value to <TT>$inputLine</TT> and then removes that last character from <TT>$inputLine</TT>-which is almost guaranteed to be a newline character. If you fear that the
last character is not a newline, use the <TT>chomp()</TT> fuNCtion instead.
</BLOCKQUOTE>
</TD></TR>
</TABLE>
</CENTER>
<P>
<H3><A NAME="ExampleUsingRedirectiontoChangeSTDINandSTDOUT">
Example: Using Redirection to Change STDIN and STDOUT</A></H3>
<P>
DOS and UNIX let you change the standard input from being the
keyboard to being a file by changing the command line that you
use to execute Perl programs. Until now, you probably used a command
line similar to:
<BLOCKQUOTE>
<PRE>
perl -w 09lst01.pl
</PRE>
</BLOCKQUOTE>
<P>
In the previous example, Perl read the keyboard to get the standard
input. But, if there was a way to tell Perl to use the file <TT>09LST01.PL</TT>
as the standard input, you could have the program print itself.
Pretty neat, huh? Well, it turns out that you can change the standard
input. It's done this way:
<BLOCKQUOTE>
<PRE>
perl -w 09lst01.pl < 09lst01.pl
</PRE>
</BLOCKQUOTE>
<P>
The < character is used to <I>redirect</I> the standard input
to the 09LST01.PL file. You now have a program that duplicates
the fuNCtionality of the DOS type command. And it only took three
lines of Perl code!
<P>
You can redirect standard output to a file using the <TT>></TT>
character. So, if you wanted a copy of <TT>09LST01.PL</TT>
to be sent to <TT>OUTPUT.LOG,</TT>
you could use this command line:
<BLOCKQUOTE>
<PRE>
perl -w 09lst01.pl <09lst01.pl >output.log
</PRE>
</BLOCKQUOTE>
<P>
Keep this use of the <TT><</TT>
and <TT>></TT> characters in mind.
You'll be using them again shortly when we talk about the <TT>open()</TT>
fuNCtion. The <TT><</TT> character
will signify that files should be opened for input and the <TT>></TT>
will be used to signify an output file. But first, let's continue
talking about accessing files listed on the command line.
<H3><A NAME="ExampleUsingtheDiamondOperatorltgt">
Example: Using the Diamond Operator (<>)</A></H3>
<P>
If no file handle is used with the diamond operator, Perl will
examine the <TT>@ARGV</TT> special
variable. If <TT>@ARGV</TT> has no
elements, then the diamond operator will read from <TT>STDIN</TT>-either
from the keyboard or from a redirected file. So, if you wanted
to display the contents of more than one file, you could use the
program shown in Listing 9.2.
<HR>
<BLOCKQUOTE>
<B>Listing 9.2 09LST02.PL-Read from Multiple Files
or from </B><TT><I><B><FONT FACE="Courier">STDIN<BR>
</FONT></B></I></TT>
</BLOCKQUOTE>
<BLOCKQUOTE>
<PRE>
while (<>) {
print();
}
</PRE>
</BLOCKQUOTE>
<HR>
<P>
The command line to run the program might look like this:
<BLOCKQUOTE>
<PRE>
perl -w 09lst02.pl 09lst01.pl 09lst02.pl
</PRE>
</BLOCKQUOTE>
<P>
And the output would be:
<BLOCKQUOTE>
<PRE>
while (<STDIN>) {
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -