Perl file manipulation

Perl uses a variable called a file handle type to manipulate files.

Reading or writing data from a file requires a file handle.

A file handle is the name of an I/O connection.

Perl provides three file handles: STDIN, STDOUT, and STDERR, which represent standard input, standard output, and standard error output respectively.

You can open files in Perl using the following methods:

 open FILEHANDLE, EXPRopen FILEHANDLE, FILENAME, MODE, PERMSsysopen FILEHANDLE, FILENAME, MODE

Parameter description:

FILEHANDLE: File handle, used to store a unique file identifier.

EXPR: An expression composed of file name and file access type.

MODE: file access type.

PERMS: access permission bits (permission bits).

Open function

In the following code, we use the open function to open the file file.txt in read-only mode (<):

 open(DATA, "<file.txt");

< indicates read-only mode.

DATA in the code is the file handle used to read the file. The following example will open the file and output the file content:

Example

The following code opens the file file.txt for writing ( > ):

 open(DATA, ">file.txt") or die "file.txt file cannot be opened, $!";

> Indicates the writing method.

If you need to open the file for reading and writing, you can add a + sign before the > or < character:

 open(DATA, "+<file.txt"); or die "file.txt file cannot be opened, $!";

This method will not delete the original content of the file. If you want to delete it, the format is as follows:

 open DATA, "+>file.txt" or die "file.txt file cannot be opened, $!";

If you want to append data to a file, you only need to open the file in append mode before appending the data:

 open(DATA,">>file.txt") || die "file.txt file cannot be opened, $!";

>> means appending data to the end of the existing file. If you need to read the content of the file to be appended, you can add the + sign:

 open(DATA,"+>>file.txt") || die "file.txt file cannot be opened, $!";

The following table lists the different access modes:

Sysopen function

The sysopen function is similar to the open function, except that their parameter forms are different.

The following example opens a file for reading and writing (+<filename):

 sysopen(DATA, "file.txt", O_RDWR);

If you need to clear the file before updating it, the writing method is as follows:

 sysopen(DATA, "file.txt", O_RDWR|O_TRUNC );

You can use O_CREAT to create a new file, O_WRONLY is write-only mode, O_RDONLY is read-only mode.

The PERMS parameter is an octal attribute value, indicating the permissions after file creation. The default is 0x666 .

The following table lists possible mode values:

Close function

After the file is used, the file must be closed to refresh the input and output buffers associated with the file handle. The syntax for closing the file is as follows:

 closeFILEHANDLEclose

FILEHANDLE is the specified file handle, and returns true if it is closed successfully.

 close(DATA) || die "Cannot close file";

Read and write files

There are several different ways to read and write information to files:

<FILEHANDL> operator

The primary method of reading information from an open file handle is the <FILEHANDLE> operator. In a scalar context, it returns a single row from the file handle. For example:

Example

When we use the <FILEHANDLE> operator, it will return a list of each line in the file handle, for example we can import all the lines into an array.

Implement the creation of the import.txt file with the following content:

 $ cat import.txt 123

Read import.txt and put each line into the @lines array:

Example

Executing the above program, the output result is:

 123

getc function

The xgetc function returns a single character from the specified FILEHANDLE, or STDIN if not specified:

 getcFILEHANDLEgetc

If an error occurs, or the file handle is at the end of the file, undef is returned.

read function

The read function is used to read information from the buffer's file handle.

This function is used to read binary data from a file.

 read FILEHANDLE, SCALAR, LENGTH, OFFSETread FILEHANDLE, SCALAR, LENGTH

Parameter description:

FILEHANDLE: File handle, used to store a unique file identifier.

SCALAR: Stores the result. If OFFSET is not specified, the data will be placed at the beginning of SCALAR. Otherwise the data is placed after the OFFSET byte in SCALAR.

LENGTH: The length of the read content.

OFFSET: offset.

Returns the number of bytes read if the read is successful, 0 if the end of file is reached, and undef if an error occurs.

print function

For all functions that read information from file handles, the main writing function in the backend is print:

 print FILEHANDLE LISTprint LISTprint

The results of program execution can be sent to the output device (STDOUT: standard output) using the file handle and the print function, for example:

 print "Hello World!n";

file copy

In the following example, we will open an existing file file1.txt, read each line of it and write it to the file file2.txt:

Example

File rename

In the following example, we rename the existing file file1.txt to file2.txt, and the specified directory is under /usr/codercto/test/:

 #!/usr/bin/perlrename ("/usr/codercto/test/file1.txt", "/usr/codercto/test/file2.txt" );

The function renames only accepts two parameters and only renames existing files.

Delete files

The following example demonstrates how to use the unlink function to delete files:

Example

Specify file location

You can get the location of a file using the tell function, and specify the location within the file by using the seek function:

tell function

The tell function is used to get the file location:

 tell FILEHANDLEtell

If FILEHANDLE is specified, this function returns the position of the file pointer, in bytes. If not specified, returns the default selected file handle.

seek function

The seek() function reads or writes files by moving the file read-write pointer through the file handle, and reads and writes in bytes:

 seek FILEHANDLE, POSITION, WHENCE

Parameter description:

FILEHANDLE: File handle, used to store a unique file identifier.

POSITION: Indicates the number of bytes to be moved by the file handle (read and write position pointer).

WHENCE: Indicates the starting position when the file handle (read-write position pointer) starts to move. The possible values ​​are 0, 1, and 2; indicating the beginning of the file, the current position, and the end of the file respectively.

The following example reads 256 bytes from the beginning of the file:

 seek DATA, 256, 0;

File information

Perl's file operations can also first test whether the file exists, whether it can be read and written, etc.

We can first create the file1.txt file, as follows:

 $ cat file1.txt www.codercto.com

Example

Executing the above program, the output result is:

 file1.txt information: is a text file, 15 bytes

The file test operators are shown in the following table: