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The basics of handling files are simple: you associate a filehandle with an external entity (usually a file) and then use a variety of operators and functions within Perl to read and update the data stored within the data stream associated with the filehandle.
A filehandle is a named internal Perl structure that associates a physical file with a name. All filehandles are capable of read/write access, so you can read from and update any file or device associated with a filehandle. However, when you associate a filehandle, you can specify the mode in which the filehandle is opened.
Three basic file handles are - STDIN, STDOUT, and STDERR.
Opening and Closing Files
There are following two functions with multiple forms which can be used to open any new or existing file in Perl.
open FILEHANDLE, EXPR open FILEHANDLE sysopen FILEHANDLE, FILENAME, MODE, PERMS sysopen FILEHANDLE, FILENAME, MODE
Here FILEHANDLE is the file handle returned by open function and EXPR is the expression having file name and mode of opening the file.
Following is the syntax to open file.txt in read-only mode. Here less than < signe indicates that file has to be opend in read-only mode
Here DATA is the file handle which will be used to read the file. Here is the example which will open a file and will print its content over the screen.
Following is the syntax to open file.txt in writing mode. Here less than > signe indicates that file has to be opend in writing mode
This example actually truncates (empties) the file before opening it for writing, which may not be the desired effect. If you want to open a file for reading and writing, you can put a plus sign before the > or < characters.
For example, to open a file for updating without truncating it:
To truncate the file first:
You can open a file in append mode. In this mode writing point will be set to the end of the file
A double >> opens the file for appending, placing the file pointer at the end, so that you can immediately start appending information. However, you can.t read from it unless you also place a plus sign in front of it:
Following is the table which gives possible values of different modes
The sysopen function is similar to the main open function, except that it uses the system open() function, using the parameters supplied to it as the parameters for the system function:
For example, to open a file for updating, emulating the +<filename format from open:
or to truncate the file before updating:
You can use O_CREAT to create a new file and O_WRONLY- to open file in write only mode and O_RDONLY - to open file in read only mode.
The PERMS argument specifies the file permissions for the file specified if it has to be created. By default it takes 0x666
Following is the table which gives possible values of MODE
To close a filehandle, and therefore disassociate the filehandle from the corresponding file, you use the close function. This flushes the filehandle's buffers and closes the system's file descriptor.
close FILEHANDLE close
If no FILEHANDLE is specified, then it closes the currently selected filehandle. It returns true only if it could successfully flush the buffers and close the file.
Reading and Writing Filehandles
Once you have an open filehandle, you need to be able to read and write information. There are a number of different ways of reading and writing data into the file.
The <FILEHANDL> Operator
The main method of reading the information from an open filehandle is the <FILEHANDLE> operator. In a scalar context it returns a single line from the filehandle. For example:
When you use the <FILEHANDLE> operator in a list context, it returns a list of lines from the specified filehandle. For example, to import all the lines from a file into an array:
The getc function returns a single character from the specified FILEHANDLE, or STDIN if none is specified:
getc FILEHANDLE getc
If there was an error, or the filehandle is at end of file, then undef is returned instead.
The read function reads a block of information from the buffered filehandle: This function is used to read binary data from the file.
read FILEHANDLE, SCALAR, LENGTH, OFFSET read FILEHANDLE, SCALAR, LENGTH
The length of the data read is defined by LENGTH, and the data is placed at the start of SCALAR if no OFFSET is specified. Otherwise data is placed after OFFSET bytes in SCALAR. The function returns the number of bytes read on success, zero at end of file, or undef if there was an error.
For all the different methods used for reading information from filehandles, the main function for writing information back is the print function.
print FILEHANDLE LIST print LIST print
The print function prints the evaluated value of LIST to FILEHANDLE, or to the current output filehandle (STDOUT by default). For example:
Here is the example which opens an existing file file1.txt and read it line by line and generate another copy file2.txt
Renaming a file
Here is an example which shows how we can rename a file file1.txt to file2.txt. Assuming file is available in /usr/test directory.
This function rename takes two arguments and it just rename existing file
Deleting an exiting file
Here is an example which shows how to delete a file file1.txt using unlink function.
Locating Your Position Within a File
You can use to tell function to know the current position of a file and seek function to point a particular position inside the file.
The first requirement is to find your position within a file, which you do using the tell function:
tell FILEHANDLE tell
This returns the position of the file pointer, in bytes, within FILEHANDLE if specified, or the current default selected filehandle if none is specified.
The seek function positions the file pointer to the specified number of bytes within a file:
seek FILEHANDLE, POSITION, WHENCE
The function uses the fseek system function, and you have the same ability to position relative to three different points: the start, the end, and the current position. You do this by specifying a value for WHENCE.
Zero sets the positioning relative to the start of the file. For example, the line sets the file pointer to the 256th byte in the file.
Getting File Information
You can test certain features very quickly within Perl using a series of test operators known collectively as -X tests.
For example, to perform a quick test of the various permissions on a file, you might use a script like this:
Here is the list of features which you can check for a file
Working with Directories
Following are the standard functions used to play with directories.
Here is an example which opens a directory and list out all the files available inside this directory.
Another example to print the list of C source code files, you might use
You can make a new directory using the mkdir function:
To remove a directory, use the rmdir function:
To change the directory you can use chdir function.
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