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Input/Output, File I/O, Command-Line Arguments

Now that we have the basics of Perl down, we need to tackle some key issues: Input/Output and processing command-line arguments.

Input/Output :: Handling STDIN and STDOUT

Standard in, or STDIN, is the default file handle for reading in text or data streams.
Input can be handled like this:
```
  $input = <STDIN>
  @linput = <STDIN>
```
where $input would store the input line. You can even store STDIN into a list as @linput line shows. You can continuously read in STDIN by:
```
  while (<>) {
    print $_;
  }
```
The above example shows the diamond operator in action. The diamond operator is the shorthand for <STDIN>, and we are printing from the scratch list @_.
All prints are directed by default to Standard Out.
Perl also provides a STDERR stream.
You can use the C functions, sprintf and printf in Perl to format your text! Very powerful!

File Input/Output :: Filehandles

You can redirect STDIN and STDOUT by using filehandles. Perl provides a very easy mechanism for this:
```
  open(infile, "/some/directory/file.in");
  open(outfile, ">/some/directory/file.out");
  while () {
    print outfile "$_\n";
  }
  close(outfile);
```
This block of code basically copies the contents of file.in into file.out line by line. Notice the > symbol to signify a write-clobber. You can use >> to append to a file. Don't forget to close your output file handles!

File Input/Output :: Other Functions

flock - Provides a file lock on the filehandle.
seek - Provides the ability to move the file pointer within the file.
Example of the above functions:
```
  open(outf, ">>log.out");
  flock(outf, 2);
  seek(outf, 0, 2);
  # ... some code
  close(outf);
```
We are opening a log.out file for appending, and immediately lock the file with 2, which is LOCK_EX for an exclusive lock. This is typically used for writing. You can flock files for reading using 0 for LOCK_SH or a shared lock. The seek seeks to the end of the file if there was writing done to the log.out before the flock was called. seek takes in the filehandle, offset, and whence arguments. 0 is the beginning of the file, 1 is the current position in the file, 2 is for the end of the file. Note that closing any filehandles automatically frees all file locks.

File Input/Output :: Error Handling

What if we can get a filehandle? We should handle this case graciously by using die. For example:
```
  open (inf, "/some/directory/file.in") || die "cannot open: $!";
```
So what does this do? Perl tries to open file.in OR it calls die with the string. The $! contains the most recent system error, so it will append a useful tag to the output of die. You could even make a dienice subroutine that could be more helpful. You can exit a Perl program immediately by calling exit;.
Notice how we used the logical operator, || as a control structure! It's basically saying open this file or die! You can even use &&.

File Input/Output :: File Tests

For further fool-proof file Error Handling and the Misc. you can pass file tests to filehandles. To test the existence of a file you can do this for example:
```
  $filename = "hooray.index";
  if (-e $filename) {
    open(outf, ">>hooray.index");
  } else {
    open(outf, ">>hooray-hooray.index");
  }
```
There are many other file tests: -T tests for ASCII file, -B tests for a Binary file, -d tests for a directory, -l tests for a symlink. There are tons of other tests listed on Schwartz pgs. 112-113. You can run these tests of filehandles also.
Perl provides a way to query information about a particular file also:
```
  ($dev, $ino, $mode, $nlink, $uid, $gid, $rdev, $size, $atime, $mtime,
   $ctime, $blksize, $blocks) = stat ($filename);
```
The stat function provides all these fields about the particular file $filename. Refer to Wall Pgs. 800-801 for more information about stat. You can also get information about a file using the File::stat package:
```
  use File::stat;
  $sb = stat($filename);
  print "$sb->size\n";
```
This would print the size of the file. More on File::stat in the Larry Wall book.

Command-Line Arguments :: @ARGV

So how do we get command line arguments? Perl automatically stores any command line arguments into a global list called @ARGV.
Each element in @ARGV sequentially contains the command-line arguments called with the program. You can parse these manually by hand, or use the more "sophisticated" getopt method below.

Command-Line Arguments :: Getopt::Std and Getopt::Long

Perl provides getopt-like functionality using the packages Getopt::Std and Getopt::Long. For example:
```
  use Getopt::Std;

  getopt('oDI');    # -o, -D & -I take arg.  Sets opt_* as a side effect.
  getopt('oDI', \%opts);    # -o, -D & -I take arg.  Values in %opts
  getopts('oif:');  # -o & -i are boolean flags, -f takes an argument
                    # Sets opt_* as a side effect.
  getopts('oif:', \%opts);  # options as above. Values in %opts
```
This example is taken from the man pages. getopt works much in the same way as getopt in C, but simpler. In the last example, getopts will take in -o, -i, -f with -f having arguments. The hash %opts contains the arguments, with the switches as the keys for the hash.
Getopt::Long can take in switches longer than one character.

Tangent :: Using Packages/Modules

You've probably noticed by now that we can call libraries, packages, or modules (whatever you want to call them), using the use keyword followed by the module.
In the above example, we used the Getopt::Std package. There are tons of standard modules and even more non-standard modules. Think of these as ANSI C Standard libraries and non-standard libraries for Perl. Other packages include File::Copy, Text::ParseWords, Math::Complex, and CGI. There are many, many more. See Schwartz Pgs. 238-242, and Wall Ch. 30-32 (pgs. 831-915).

I/O, File I/O, Command-Line Args Review

Perl provides three standard streams (STDIN, STDOUT, STDERR) as well as the ability to specify your own filehandles. You can write, read, and append to these files using Perl's builtin functions.
You can parse command-line arguments using @ARGV list or using the Getopt packages.