files.md

Files

This chapter introduces the idea of “persistent” programs that keep data in permanent storage, and shows how to use different kinds of permanent storage, like files and databases.

Persistence

Most of the programs we have seen so far are transient in the sense that they run for a short time and produce some output, but when they end, their data disappears. If you run the program again, it starts with a clean slate.

Other programs are persistent: they run for a long time (or all the time); they keep at least some of their data in permanent storage (a hard drive, for example); and if they shut down and restart, they pick up where they left off.

Examples of persistent programs are operating systems, which run pretty much whenever a computer is on, and web servers, which run all the time, waiting for requests to come in on the network.

One of the simplest ways for programs to maintain their data is by reading and writing text files. We have already seen programs that read text files; in this chapter we will see programs that write them.

An alternative is to store the state of the program in a database. In this chapter I will present a simple database and a module, Marshal, that makes it easy to store program data.

Reading and writing

A text file is a sequence of characters stored on a permanent medium like a hard drive, flash memory, or CD-ROM. We saw how to open and read a file in Section Reading word lists.

To write a file, you have to open it with mode 'w' as a second parameter:

>> fout = File.open('output.txt', 'w')
=> #<File:output.txt>

If the file already exists, opening it in write mode clears out the old data and starts fresh, so be careful! If the file doesn’t exist, a new one is created.

File.open returns a File object that provides methods for working with the file. The write method puts data into the file.

>> line1 = "This here's the wattle,\n"
=> "This here's the wattle,\n"
>> fout.write(line1)
=> 24

The return value is the number of characters that were written. The file object keeps track of where it is, so if you call write again, it adds the new data to the end of the file.

>> line2 = "the emblem of our land.\n"
=> "the emblem of our land.\n"
>> fout.write(line2)
=> 24

When you are done writing, you should close the file.

>> fout.close
=> nil

If you don’t close the file, it gets closed for you when the program ends.

Format operator

The arguments of write has to be a string, otherwise they are converted to string using to_s method.

>> x = 52
=> 52
>> x.to_s
=> "52"

An alternative is to use the format operator, %. When applied to integers, % is the modulus operator. But when the first operand is a string, % is the format operator.

The first operand is the format string, which contains one or more format sequences, which specify how the second operand is formatted. The result is a string.

For example, the format sequence '%d' means that the second operand should be formatted as a decimal integer:

>> camels = 42
=> 42
>> '%d' % camels
=> "42"

The result is the string '42', which is not to be confused with the integer value 42.

A format sequence can appear anywhere in the string, so you can embed a value in a sentence:

>> 'I have spotted %d camels.' % camels
=> "I have spotted 42 camels."

If there is more than one format sequence in the string, the second argument has to be an array for positional arguments or hash for named arguments. For array, each format sequence is matched with an element, in order.

The following example uses '%d' to format an integer, '%f' to format a floating-point number, and '%s' to format a string:

>> 'In %d years I have spotted %.2f %s.' % [3, 22.0 / 7, 'camels']
=> "In 3 years I have spotted 3.14 camels."

The number of elements in the array has to be at least the number of format sequences in the string. Also, the types of the elements have to match the format sequences:

>> '%d' % [1, 2]
=> "1"

>> '%d %d %d' % [1, 2]
ArgumentError (too few arguments)

>> '%d' % 'dollars'
ArgumentError (invalid value for Integer(): "dollars")

In the second example, there aren’t enough elements; in the third, the element is the wrong type.

For more information on the format operator, see https://ruby-doc.org/core-2.5.0/Kernel.html#method-i-sprintf.

Filenames and paths

Files are organized into directories (also called “folders”). Every running program has a “current directory”, which is the default directory for most operations. For example, when you open a file for reading, Ruby looks for it in the current directory.

The Dir and File classes provide methods for working with files and directories. Dir.pwd returns the name of the current directory:

>> cwd = Dir.pwd
=> "/home/dinsdale"

pwd method will give “path of current working directory”. The result in this example is /home/dinsdale, which is the home directory of a user named dinsdale.

A string like '/home/dinsdale' that identifies a file or directory is called a path.

A simple filename, like memo.txt is also considered a path, but it is a relative path because it relates to the current directory. If the current directory is /home/dinsdale, the filename memo.txt would refer to /home/dinsdale/memo.txt.

A path that begins with / does not depend on the current directory; it is called an absolute path. To find the absolute path to a file, you can use File.absolute_path:

>> File.absolute_path('memo.txt')
=> "/home/dinsdale/memo.txt"

File.exist? checks whether a file or directory exists:

>> File.exist?('memo.txt')
=> true

If it exists, Dir.exist? checks whether it’s a directory:

>> Dir.exist?('memo.txt')
=> false
>> Dir.exist?('/home/dinsdale')
=> true

Similarly, File.file? checks whether it’s a file.

Dir.children returns a list of the files (and other directories) in the given directory:

>> Dir.children(cwd)
=> ["music", "photos", "memo.txt"]

To demonstrate these methods, the following example “walks” through a directory, prints the names of all the files, and calls itself recursively on all the directories.

def walk(dirname)
  Dir.each_child(dirname) do |name|
    path = File.join(dirname, name)
    if File.file?(path)
      puts path
    else
      walk(path)
    end
  end
end

Dir.each_child helps to iterate over list of files and directories in the given directory. File.join takes list of strings and joins them into a path using / as separator.

The Dir.glob method works similar to this one but more versatile. As an exercise, read the documentation and use it to print the names of the files in a given directory and its subdirectories.

Catching exceptions

A lot of things can go wrong when you try to read and write files. If you try to open a file that doesn’t exist, you get an exception:

>> fin = File.open('bad_file')
Errno::ENOENT (No such file or directory @ rb_sysopen - bad_file)

Here Errno::ENOENT is the name of the exception. If you don’t have permission to access a file:

>> fout = File.open('/etc/passwd', 'w')
Errno::EACCES (Permission denied @ rb_sysopen - /etc/passwd)

And if you try to open a directory for reading, you get

>> fin = File.open('/home')
=> #<File:/home>
>> fin.readline
Errno::EISDIR (Is a directory @ io_fillbuf - fd:7 /home)

To avoid these errors, you could use methods like File.exist? and File.file?, but it would take a lot of time and code to check all the possibilities (if “Errno” is any indication, there are many things that can go wrong).

It is better to go ahead—and deal with problems if they happen—which is exactly what the rescue clause does inside a begin...end block. The syntax is similar to an if...else statement:

begin
  fin = File.open('bad_file')
rescue
  puts 'Something went wrong.'
end

Ruby starts by executing the begin clause. If all goes well, it skips the rescue clause and proceeds. If an exception occurs, it jumps out of the begin clause and runs the rescue clause.

Handling an exception with a begin statement is called catching an exception. In the above example, the rescue clause prints an error message that is not very helpful.

rescue => e
  puts e
  puts e.inspect

By assigning the exception object to a variable, you can deal with the issue better, for example displaying the error message:

No such file or directory @ rb_sysopen - bad_file
#<Errno::ENOENT: No such file or directory @ rb_sysopen - bad_file>

rescue also accepts list of exceptions to specifically rescue only those exceptions. In general, catching an exception gives you a chance to fix the problem, or try again, or at least end the program gracefully.

Databases

A database is a file that is organized for storing data. Many databases are organized like a hash in the sense that they map from keys to values. The biggest difference between a database and a hash is that the database is on disk (or other permanent storage), so it persists after the program ends.

The DBM class provides an interface for creating and updating database files. As an example, I’ll create a database that contains captions for image files.

Opening a database is similar to opening other files:

require 'dbm'
db = DBM.open('captions', 0666, DBM::WRCREAT)

The mode 0666 is octal file permission value, same notation as the chmod unix command. The flag DBM::WRCREAT means that the database should be created if it doesn’t already exist. The result is a database object that can be used (for most operations) like a hash.

When you create a new item, DBM updates the database file.

>> db['cleese.png'] = 'Photo of John Cleese.'
"Photo of John Cleese."

When you access one of the items, DBM reads the file:

>> db['cleese.png']
"Photo of John Cleese."

If you make another assignment to an existing key, DBM replaces the old value:

>> db['cleese.png'] = 'Photo of John Cleese doing a silly walk.'
"Photo of John Cleese doing a silly walk."

Similar to hash, use each method to iterate over key-value pairs:

db.each { |k, v| puts "#{k} #{v}" }

As with other files, you should close the database when you are done:

db.close

It is a good idea to read the documentation https://ruby-doc.org/stdlib-2.5.0/libdoc/dbm/rdoc/DBM.html to know about the caveats of using DBM.

Marshaling

A limitation of dbm is that the keys and values have to be strings. If you try to use any other type, they get converted to string during assignment and you can specify only string keys while retrieving.

The Marshal module can help. It translates almost any type of object into a string suitable for storage in a database, and then translates strings back into objects.

Marshal.dump takes an object as a parameter and returns a string representation (dump is short for “dump string”):

>> t = [1, 2, 3]
=> [1, 2, 3]
>> Marshal.dump(t)
=> "\x04\b[\bi\x06i\ai\b"

The format isn’t obvious to human readers; it is meant to be easy for Marshal to interpret. Marshal.load (“load string”) reconstitutes the object:

>> t1 = [1, 2, 3]
=> [1, 2, 3]
>> s = Marshal.dump(t1)
=> "\x04\b[\bi\x06i\ai\b"
>> t2 = Marshal.load(s)
=> [1, 2, 3]

Although the new object has the same value as the old, it is not (in general) the same object:

>> t1 == t2
=> true
>> t1.equal?(t2)
=> false

In other words, marshaling and then unmarshaling has the same effect as copying the object.

As with DBM, read the documentation for caveats - https://ruby-doc.org/core-2.5.0/Marshal.html

Pipes

Most operating systems provide a command-line interface, also known as a shell. Shells usually provide commands to navigate the file system and launch applications. For example, in Unix you can change directories with cd, display the contents of a directory with ls, and launch a web browser by typing (for example) firefox.

Any program that you can launch from the shell can also be launched from Ruby using a pipe object, which represents a running program.

For example, the Unix command ls -l normally displays the contents of the current directory in long format. You can launch ls with IO.popen (File is a subclass of IO class):

cmd = 'ls -l'
fp = IO.popen(cmd)

The argument is a string that contains a shell command. The return value is an object that behaves like an open file. You can read the output from the ls process one line at a time with readline or get the whole thing at once with read:

res = fp.read

When you are done, you close the pipe like a file:

>> stat = fp.close
=> nil

nil means that it ended normally (with no errors). The exit status of the process can be obtained from global variable $?

For example, most Unix systems provide a command called md5sum that reads the contents of a file and computes a “checksum”. You can read about MD5 at https://en.wikipedia.org/wiki/Md5. This command provides an efficient way to check whether two files have the same contents. The probability that different contents yield the same checksum is very small (that is, unlikely to happen before the universe collapses).

You can use a pipe to run md5sum from Ruby and get the result:

>> filename = 'words.txt'
=> "words.txt"
>> cmd = 'md5sum ' + filename
=> "md5sum words.txt"
>> fp = IO.popen(cmd)
=> #<IO:fd 7>
>> res = fp.read
=> "e58eb7b851c2e78770b20c715d8f8d7b  words.txt\n"
>> stat = fp.close
=> nil

Writing modules

Any file that contains Ruby code can be inserted inside another script using require statement. For example, suppose you have a file named wc.rb with the following code:

def linecount(filename)
  count = 0
  File.foreach(filename) { count += 1 }
  return count
end

puts linecount('wc.rb')

If you run this script, it reads itself and prints the number of lines in the file, which is 7. You can also use it in another script by specifying its file path like this:

>> require './wc'
7
=> true
>> linecount('wc.rb')
7

The only problem with this example is that when you load the script it runs the test code at the bottom.

In such cases, you will often see the following idiom:

if __FILE__ == $0
  puts linecount('wc.rb')
end

__FILE__ is a built-in keyword that contains name of current file. $0 is a global variable that contains name of script being executed. So, if these two match, it means the file is being used as executable and not loaded as a library in another script.

As an exercise, type this example into a file named wc.rb and run it as a script. Then run the Ruby interpreter and use require statement. What is the value of $0?

To write a collection of methods like the Math module, enclose the methods inside module statement:

module Wc
  def self.linecount(filename)
    count = 0
    File.foreach(filename) { count += 1 }
    return count
  end
end

The module name has to start with an uppercase alphabet. Also note that the method name is prefixed with self. which we’ll cover in chapters dealing with classes.

>> require './wc_module'
=> true
>> Wc.linecount('wc_module.rb')
=> 7

Debugging

When you are reading and writing files, you might run into problems with whitespace. These errors can be hard to debug because spaces, tabs and newlines are normally invisible:

>> s = "1 2\t 3\n 4"
=> "1 2\t 3\n 4"
>> puts s
1 2  3
 4

The inspect method can help, we’ve already seen a few examples before. It returns a string representation of the object. For strings, it represents whitespace characters with backslash sequences:

>> puts s.inspect
"1 2\t 3\n 4"

This can be helpful for debugging.

One other problem you might run into is that different systems use different characters to indicate the end of a line. Some systems use a newline, represented \n. Others use a return character, represented \r. Some use both. If you move files between different systems, these inconsistencies can cause problems.

For most systems, there are applications to convert from one format to another. You can find them (and read more about this issue) at https://en.wikipedia.org/wiki/Newline. Or, of course, you could write one yourself.

Glossary

• persistent:
  Pertaining to a program that runs indefinitely and keeps at least some of its data in permanent storage.

• format operator:
  An operator, %, that takes a format string and an array or hash and generates a string that includes the elements formatted as specified by the format string.

• format string:
  A string, used with the format operator, that contains format sequences.

• format sequence:
  A sequence of characters in a format string, like %d, that specifies how a value should be formatted.

• text file:
  A sequence of characters stored in permanent storage like a hard drive.

• directory:
  A named collection of files, also called a folder.

• path:
  A string that identifies a file.

• relative path:
  A path that starts from the current directory.

• absolute path:
  A path that starts from the topmost directory in the file system.

• catch:
  To prevent an exception from terminating a program using the begin and rescue statements.

• database:
  A file whose contents are organized like a hash with keys that correspond to values.

• shell:
  A program that allows users to type commands and then executes them by starting other programs.

• pipe object:
  An object that represents a running program, allowing a Ruby program to run commands and read the results.

Exercises

Exercise 1
Write a method called sed that takes as arguments a pattern string, a replacement string, and two filenames; it should read the first file and write the contents into the second file (creating it if necessary). If the pattern string appears anywhere in the file, it should be replaced with the replacement string.

If an error occurs while opening, reading, writing or closing files, your program should catch the exception, print an error message, and exit.

Exercise 2
In a large collection of MP3 files, there may be more than one copy of the same song, stored in different directories or with different file names. The goal of this exercise is to search for duplicates.

1. Write a program that searches a directory and all of its subdirectories, recursively, and returns an array of complete paths for all files with a given suffix (like .mp3). Hint: Dir and File classes provide several useful methods for manipulating file and path names.

2. To recognize duplicates, you can use md5sum to compute a “checksum” for each files. If two files have the same checksum, they probably have the same contents.

3. To double-check, you can use the Unix commands diff or cmp.