Files

1
Files
2
File Overview

• A file is a sequence of bytes stored on disk.
• Provides a means of stable store.
• Perl has 3 predefined files
• One for keyboard input.
• One for screen output.
• One for error messages.
• Most languages and systems originating in the
  Unix environment have these.

3
Filehandles

• Files are referenced through special variables
  called filehandles.
• A filehandle is a variable that we associate with
  an actual file.
• By convention, filehandles are all uppercase
• Avoid name conflicts with reserved words.
• e.g. STDIN, STDOUT, STDERR.

4
Open/close File
5
Open file (example)
6
Open Failures

• open can fail for several reasons
• File for input does not exist.
• No more filehandles.
• open returns true if it succeeds.
• Can check why open fails with die.
• open (INDAT, score) or die Error !
• ! contains error message from the system.

7
Close

• Some programmers prefer to explicitly close a
  file.
• Closing a file can fail too
• Not enough disk space.
• The function call is
• close (filehandle)
• Can detect cause of failure with
• close (RESULTS) or die Error - !

8
Reading and Writing Files

• We have read and written to files before
• Read lines from the keyboard and written to
  screen.
• Reading a line from a file is similar
• ltfilehandlegt
• Writing a line to a file is easy too
• print filehandle OUTPUT

9
An Example

• !/usr/bin/perl -w
• Open the files.
• source shift _at_ARGV
• destination shift _at_ARGV
• open (IN, lt, source) or die Cant read source
  file source !\n
• open (OUT, gt, destination)
• or die Cant write to file destination
  !\n
• Read until EOF.
• print Copying source to destination\n
• while (ltINgt)
• print OUT
• Close the filehandles.
• close (IN)
• close (OUT)

10
File Tests

• Sometimes, a program needs to know information
  about a file before using it.
• (EX) Does a file with the same name exists?
• File tests are unary operators.
• Operand is an expression that evaluates to a
  string.
• String must be a file or filehandle.

11
File Tests (cont.)

• -e True if file exists.
• -r True if file is readable.
• -w True if file is writeable.
• -x True if file is executable.
• -d True if file is a directory.

• -f True if file is a regular file.
• -T True if file is a text file.
• -B True if file is a binary file.
• -s Length of file in bytes.
• -z True if its zero size