open(*args) public

With no associated block, open is a synonym for IO.new. If the optional code block is given, it will be passed io as an argument, and the IO object will automatically be closed when the block terminates. In this instance, IO.open returns the value of the block.

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January 14, 2009
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File open permissions

Usage: File.open path, flags, [permissions]

Flags (bitmasks)

Access:

File::RDONLY

Read-only

File::WRONLY

Write-only

File::RDWR

Read and write

If the file exists:

File::TRUNC

Truncate

File::APPEND

Append

File::EXCL

Fail

If the file doesn’t exist:

File::CREAT

Create

Flags (strings)

r

File::RDONLY

r+

File::RDWR

w

File::WRONLY|File::TRUNC|File::CREAT

a

File::WRONLY|File::APPEND|File::CREAT

Examples

File.open path, File::RDONLY
File.open path, 'w'
File.open path, File::WRONLY|File::TRUNC|File::CREAT
File.open path, File::WRONLY|File::TRUNC|File::CREAT, '0666'
February 12, 2009
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Binary files

Another real important flag is b when dealing with binary files. For example to download an mp3 from the internet you need to pass the b flag or the data will be screwed up:

# Downloads a binary file from the internet
require 'open-uri'
url = "http://fubar/song.mp3"
open(url, 'rb') do |mp3|
  File.open("local.mp3", 'wb') do |file|
    file.write(mp3.read)
  end
end

Don’t say you haven’t been warned. :)

April 21, 2010
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Errors Raised

Non IO errors (IOError) are contained in the Errno module. They are the same as those given in open(2), see:

http://www.kernel.org/doc/man-pages/online/pages/man2/open.2.html#ERRORS

Common Errors

  • Errno::ENOENT: No such file or directory

  • Errno::EACCES: Permission denied

  • Errno::EEXIST: File exists (i.e. IO::EXCL | IO::CREAT)

April 21, 2010
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Mode Flags

RDONLY, TRUNC, etc… are defined in the File::Constants module which is include'd by IO and File.

IO.open fd, IO::RDONLY
File.open path, File::RDONLY

Though as pointed out above, they are interchangeable.