File Handling in Ruby

Working with files is an essential part of software development, as sometimes one needs to write some important information to a file and read it later.

Ruby provides many different ways to work with files.

Let’s first check the number of supported file modes.

Mode Description
r Read only mode. File pointer is positioned at start of file.
r+ Read and write mode. File pointer is positioned at start of file.
w Write only mode. File pointer is positioned at start of file.
w+ Read and write mode. File pointer is positioned at start of file.
a Append mode. File pointer is at end of file. Append the data if file exists else create a new file.
a+ Read and append mode. File pointer is at end of file. Append the data if file exists else create a new file.

Opening a file

If you want to open an existing file for reading, then it can be done in following way:

file_object = File.open("some_file.txt")  # read mode('r') is default
# use file_object for processing

file_object = File.open("some_file.txt", "w+")  # read mode('r') is default

Creating a file

We can create a new file using File.new() method for reading, writing, or, for both, as per the r+, or a+ mode mentioned above. Also, we need to use file_object.close() method to close that file so that system resources can be released.

file_object = File.new("new_file.txt", "w")
# use file_object for processing

Write to a file

To write content to the file, we can use file_object.syswrite() method. The file should be opened in write mode for this method.

file_object.syswrite("content");

# or
file_object.write("content");

# or
file_object.puts("content");

NOTE: The methods write and puts will write the content to the buffer, while syswrite will directly write to the disk. The method puts will add a newline, unless there is already a newline character at the end of the content, after the content while write and syswrite just write the specified content only. The addition of a newline character acts the same as when we use it for an instance of String.

Closing the file

It is important to close the file handle after the required operations are done. It will help to release the resources like memory to be released for other purposes.

file_object.close()

Example

file = File.new("foo.txt", "w")
file.syswrite "this is first line\n"
file.close

file = File.open("foo.txt", "w")
file.syswrite "this is second line\n"
file.close

file = File.open("foo.txt", "w+")
file.syswrite "this is third line\n"
file.close

file = File.open("foo.txt", "a")
file.puts "this is fourth line"
file.close

file = File.open("foo.txt", "a+")
file.puts "this is fifth line"
file.close

# output
this is third line
this is fourth line
this is fifth line

NOTE: Notice above that the file has been overwritten on mode w and w+.

Using a block

It is recommended to use block style while reading and writing files as it handles closing the file for you when the block is finished executing.

File.open("some_file.txt", "w") do |file|
  file.puts "some content"
end

File reading variants

A file can be read in many different ways. We will see few reading options we have in Ruby.

In the following examples, we will consider a file named foo.txt with following content:

this is first line
this is second line
this is third line

Read at once

We can read the whole file at once.

# let's say some_file.txt has "some content as file content"
File.open("foo.txt", "r") do |file|
  puts file.read
end

# output
this is first line
this is second line
this is third line

Read line by line using File#readlines

We can read the file content line by line through FILE#readlines method too. In such case, each line of the file will be treated like an element in the array.

FILE.readlines('foo.txt').each do |line|
  puts line
end

# output
this is first line
this is second line
this is third line

Read line by line using IO#readlines

We can read the file content line by line through IO#readlines method too. In such case, each line of the file will be treated like an element in the array.

IO.readlines('foo.txt').each do |line|
  puts line
end

# output
this is first line
this is second line
this is third line

Read line by line using IO#foreach

We can read the file content line by line through IO#foreach method, too. The difference between the method foreach and the method readlines is that the method foreach is yielding each line from the block.

IO.foreach('foo.txt') do |line|
  puts line
end

# output
this is first line
this is second line
this is third line

Getting Information about File

Sometimes it is necessary to find information about a file before performing any operations. Ruby provides many different methods for this very purpose.

We will see few common methods below:

  • To see if a file already exists or not

    File.exists?("foo.txt") # return true of false
    
  • Check whether a file is a file or directory

    File.file?("foo.txt") # true
    File.directory?("some_directory") # true
    
  • Check whether a file is readable, writable or executable?

    File.readable?("foo.txt") # true
    
    File.writable?("foo.txt") # true
    
    File.executable?("foo.txt") # false
    
  • Check size of a file

    File.size("foo.txt") # 58
    
  • Check if a file is empty or not

    File.zero?("foo.txt") # false
    
  • Find the extension of a file

    File.extname("foo.txt") # .txt
    

You can find all of the file related methods in File official documentation.


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