ch9.htm

this is a book on pearl , simple example with explanation is given here. it could be beneficial for

    print();

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

This program displays:<BR>

<BLOCKQUOTE>

<PRE>

while (&lt;STDIN&gt;) {

    print();

}

while (&lt;&gt;) {

    print();

}

</PRE>

</BLOCKQUOTE>

<P>

Next, we will take a look at the ways that Perl lets you test

files, and following that, the fuNCtions that can be used with

files.

<H2><A NAME="FileTestOperators"><FONT SIZE=5 COLOR=#FF0000>

File Test Operators</FONT></A></H2>

<P>

Perl has many operators that you can use to test different aspects

of a file. For example, you can use the <TT>-e</TT>

operator to ensure that a file exists before deleting it. Or,

you can check that a file can be written to before appending to

it. By checking the feasibility of the impending file operation,

you can reduce the number of errors that your program will eNCounter.

Table 9.2 shows a complete list of the operators used to test

files.<BR>

<P>

<CENTER><B>Table 9.2&nbsp;&nbsp;Perl's File Test Operators</B></CENTER>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD WIDTH=121><I>Operator</I></TD><TD WIDTH=469><I>Description</I>

</TD></TR>

<TR><TD WIDTH=121><TT>-A</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Returns the access age of <TT>OPERAND</TT> when the program started.

</TD></TR>

<TR><TD WIDTH=121><TT>-b</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a block device.

</TD></TR>

<TR><TD WIDTH=121><TT>-B</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a binary file. If <TT>OPERAND</TT> is a file handle, then the current buffer is examined, instead of the file itself.

</TD></TR>

<TR><TD WIDTH=121><TT>-c OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a character device.

</TD></TR>

<TR><TD WIDTH=121><TT>-C</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Returns the inode change age of <TT>OPERAND</TT> when the program started.

</TD></TR>

<TR><TD WIDTH=121><TT>-d</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a directory.

</TD></TR>

<TR><TD WIDTH=121><TT>-e</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> exists.

</TD></TR>

<TR><TD WIDTH=121><TT>-f</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a regular  file as opposed to a directory, symbolic link or other type of file.

</TD></TR>

<TR><TD WIDTH=121><TT>-g</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> has the setgid bit set.

</TD></TR>

<TR><TD WIDTH=121><TT>-k</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> has the sticky bit set.

</TD></TR>

<TR><TD WIDTH=121><TT>-l</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a symbolic link. Under DOS, this operator always will return false.

</TD></TR>

<TR><TD WIDTH=121><TT>-M</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Returns the age of <TT>OPERAND</TT> in days when the program started.

</TD></TR>

<TR><TD WIDTH=121><TT>-o</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is owned by the effective uid. Under DOS, it always returns true.

</TD></TR>

<TR><TD WIDTH=121><TT>-O</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is owned by the read uid/gid. Under DOS, it always returns true.

</TD></TR>

<TR><TD WIDTH=121><TT>-p</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a named pipe.

</TD></TR>

<TR><TD WIDTH=121><TT>-r</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> can be read from.

</TD></TR>

<TR><TD WIDTH=121><TT>-R</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> can be read from by the real uid/gid. Under DOS, it is identical to -r.

</TD></TR>

<TR><TD WIDTH=121><TT>-s</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Returns the size of <TT>OPERAND</TT> in bytes. Therefore, it returns true if <TT>OPERAND</TT> is non-zero.

</TD></TR>

<TR><TD WIDTH=121><TT>-S</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a socket.

</TD></TR>

<TR><TD WIDTH=121><TT>-t</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is opened to a tty.

</TD></TR>

<TR><TD WIDTH=121><TT>-T</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> is a text file. If <TT>OPERAND</TT> is a file handle, then the current buffer is examined, instead of the file itself.

</TD></TR>

<TR><TD WIDTH=121><TT>-u</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> has the setuid bit set.

</TD></TR>

<TR><TD WIDTH=121><TT>-w</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> can be written to.

</TD></TR>

<TR><TD WIDTH=121><TT>-W</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> can be written to by the real uid/gid. Under DOS, it is identical to -w.

</TD></TR>

<TR><TD WIDTH=121><TT>-x</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> can be executed.

</TD></TR>

<TR><TD WIDTH=121><TT>-X</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> can be executed by the real uid/gid. Under DOS, it is identical to -x.

</TD></TR>

<TR><TD WIDTH=121><TT>-z</TT> <TT>OPERAND</TT>

</TD><TD WIDTH=469>Tests if <TT>OPERAND</TT> size is zero.

</TD></TR>

</TABLE>

</CENTER>

<P>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

If the <TT>OPERAND</TT> is not specified in the file test, the <TT>$ </TT>variable will be used instead.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

The operand used by the file tests can be either a file handle

or a file name. The file tests work by internally calling the

operating system to determine information about the file in question.

The operators will evaluate to true if the test succeeds and false

if it does not.

<P>

If you need to perform two or more tests on the same file, you

use the special underscore (<TT>_</TT>)

file handle. This tells Perl to use the file information for the

last system query and saves time. However, the underscore file

handle does have some caveats. It does not work with the <TT>-t</TT>

operator. In addition, the <TT>lstat()</TT>

fuNCtion and <TT>-l</TT> test will

leave the system buffer filled with information about a symbolic

link, not a real file.

<P>

The <TT>-T</TT> and <TT>-B</TT>

file tests will examine the first block or so of the file. If

more than 10 percent of the bytes are non-characters or if a null

byte is eNCountered, then the file is considered a binary file.

<I>Binary</I> files are normally data files, as opposed to text

or human-readable files. If you need to work with binary files,

be sure to use the <TT>binmode()</TT>

file fuNCtion, which is described in the section, &quot;Example:

Binary Files,&quot; later in this chapter.

<H3><A NAME="ExampleUsingFileTests">

Example: Using File Tests</A></H3>

<P>

For our first example with file tests, let's examine a list of

files from the command line and determine if each is a regular

file or a special file.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start a </I><TT><I>foreach</I></TT><I>

loop that looks at the command line array. Each element in the

array is assigned to the default loop variable </I><TT><I>$_</I></TT><I>.

<BR>

Print the file name contained in </I><TT><I>$_</I></TT><I>.

<BR>

Print a message indicating the type of file by checking the evaluation

of the </I><TT><I>-f</I></TT><I> operator.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.4&nbsp;&nbsp;09LST04.PL-Using the </B><TT><I><B><FONT FACE="Courier">-f</FONT></B></I></TT><B>

Operator to Find Regular Files Inside a </B><TT><I><B><FONT FACE="Courier">foreach</FONT></B></I></TT><B>

Loop<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

foreach (@ARGV) {

    print;

    print((-f) ? &quot; -REGULAR\n&quot; : &quot; -SPECIAL\n&quot;)

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When this program is run using the following command line:

<BLOCKQUOTE>

<PRE>

perl -w 09lst01.pl \perl5 perl.exe \windows

</PRE>

</BLOCKQUOTE>

<P>

the following is displayed:

<BLOCKQUOTE>

<PRE>

09lst01.pl -REGULAR

\perl5 -SPECIAL

perl.exe -REGULAR

\windows -SPECIAL

</PRE>

</BLOCKQUOTE>

<P>

Each of the directories listed on the command line were recognized

as special files. If you want to ignore all special files in the

command line, you do so like this:

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start a </I><TT><I>foreach</I></TT><I>

loop that looks at the command line array.<BR>

If the current file is special, then skip it and go on to the

next iteration of the </I><TT><I>foreach</I></TT><I>

loop.<BR>

Print the current file name that is contained in </I><TT><I>$_</I></TT><I>.

<BR>

Print a message indicating the type of file.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.5&nbsp;&nbsp;09LST05.PL-Using the </B><TT><I><B><FONT FACE="Courier">-f</FONT></B></I></TT><B>

Operator to Find Regular Files Inside a </B><TT><I><B><FONT FACE="Courier">foreach</FONT></B></I></TT><B>

Loop<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

foreach (@ARGV) {

    next unless -f;    # ignore all non-normal files.

    print;

    print((-f) ? &quot; -REGULAR\n&quot; : &quot; -SPECIAL\n&quot;)

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When this program is run using the following command line:

<BLOCKQUOTE>

<PRE>

perl -w 09lst01.pl \perl perl.exe \windows

</PRE>

</BLOCKQUOTE>

<P>

the following is displayed:

<BLOCKQUOTE>

<PRE>

09lst01.pl -REGULAR

perl.exe -REGULAR

</PRE>

</BLOCKQUOTE>

<P>

Notice that only the regular file names are displayed. The two

directories on the command line were ignored.

<P>

As mentioned above, you can use the underscore file handle to

make two tests in a row on the same file so that your program

can execute faster and use less system resources. This could be

important if your application is time critical or makes many repeated

tests on a large number of files.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start a </I><TT><I>foreach</I></TT><I>

loop that looks at the command line array.<BR>

If the current file is special, then skip it and go on to the

next iteration of the </I><TT><I>foreach</I></TT><I>

loop.<BR>

Determine the number of bytes in the file with the </I><TT><I>-s</I></TT><I>

operator using the underscore file handle so that a second operating

system call is not needed.<BR>

Print a message indicating the name and size of the file.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.6&nbsp;&nbsp;09LST06.PL-Finding the Size in Bytes

of Regular Files Listed on the Command Line<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

foreach (@ARGV) {

    next unless -f;

    $fileSize = -s _;

    print(&quot;$_ is $fileSize bytes long.\n&quot;);

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When this program is run using the following command line:

<BLOCKQUOTE>

<PRE>

perl -w 09lst06.pl \perl5 09lst01.pl \windows perl.exe

</PRE>

</BLOCKQUOTE>

<P>

the following is displayed:

<BLOCKQUOTE>

<PRE>

09lst01.pl is 36 bytes long.

perl.exe is 61952 bytes long.<BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD><B>Tip</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

Don't get the underscore file handle confused with the <TT>$_</TT> special variable. The underscore file handle tells Perl to use the file information from the last system call and the <TT>$ </TT>variable is used as the default parameter for a variety of 
fuNCtions.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H2><A NAME="FileFuNCtionsBR"><FONT SIZE=5 COLOR=#FF0000>

File FuNCtions<BR>

</FONT></A></H2>

<P>

<CENTER><B>Table 9.3&nbsp;&nbsp;Perl's File FuNCtions</B></CENTER>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD WIDTH=241><I>FuNCtion</I></TD><TD WIDTH=349><I>Description</I>

</TD></TR>

<TR><TD WIDTH=241><TT>binmode</TT>(<TT>FILE_HANDLE</TT>)

</TD><TD WIDTH=349>This fuNCtion puts FILE_HANDLE into a binary mode. For more information, see the section, &quot;Example: Binary Files,&quot; later in this chapter.

</TD></TR>

<TR><TD WIDTH=241><TT>chdir</TT>(<TT>DIR_NAME</TT>)

</TD><TD WIDTH=349>Causes your program to use DIR_NAME as the current directory. It will return true if the change was successful, false if not.

</TD></TR>

<TR><TD WIDTH=241><TT>chmod</TT>(<TT>MODE</TT>, <TT>FILE_LIST</TT>)

</TD><TD WIDTH=349>This UNIX-based fuNCtion changes the permissions for a list of files. A count of the number of files whose permissions was changed is returned. There is no DOS equivalent for this fuNCtion.

</TD></TR>

<TR><TD WIDTH=241><TT>chown</TT>(<TT>UID</TT>, <TT>GID</TT>, <TT>FILE_LIST</TT>)

</TD><TD WIDTH=349>This UNIX-based fuNCtion changes the owner and group for a list of files. A count of the number of files whose ownership was changed is returned. There is no DOS equivalent for this fuNCtion.