files-and-dirs.md

Files and Dirs

Ruby has built in classes for reading and writing to files and directories on your computer.

File

The File class provides ways to read and write to a file. Typically the same method is used to read or write, which is .open with a filename as an argument and an additional argument clarifying if you want to read or write. "r" for read "w" for write. To create a new file, we can do:

file = File.open("test.txt", "w")
# Note that a new file must be in write mode

Once we .open a file, we can read or write to it, to write we can use the .write method:

file = File.open("test.txt", "w")
file.write "This is a test.\n"
file << "This can also be used to add text to a file\n"
file.close

Note that files need to be closed after they are opened, just like in Atom if you don't explicitly close a file, it will stay open forever

If the file already exists (or if we've just created it), we can use "r" as the second argument to open the file in read mode.

file = File.open("test.txt", "r")
file.read
file.close

The second argument to .open (the mode) sets what permissions Ruby has to the file when it is opened. This defaults to r (read only). Possible values are:

Mode	Meaning
"r"	Read-only, starts at beginning of file (default mode)
"r+"	Read-write, starts at beginning of file.
"w"	Write-only, truncates existing file to zero length
"w+"	Read-write, truncates existing file to zero length.
"a"	Write-only, starts at end of file if file exists.
"a+"	Read-write, starts at end of file if file exists.
"b"	Binary file mode
"t"	Text file mode

Most modes will create a new file if one doesn't already exist with the given name.

If you pass a block to open, Ruby will open the file, execute the code within the block (the file contents is given to the block as a local variable), and then close the file.

File.open("test.txt", 'w') do |file|
  file << "Some text for our file"
end

The .read method requires one arguement, the filename.

File.read "test.txt"
# => "Some text for our file"

Dirs

The ruby class Dir is typically used to find files within a directory, just like ls in unix. In ruby we can use a method called .glob which is like ls but allows for pattern matching, so it would be easy to only list .rb files or only .html files. Often in pattern matching * means to match all, same with glob:

Dir.glob("*")

This should list all of the files and directories within your current directory (wherever you started irb from).

These file names could then be used in the File class to .open and read or write (although, you probably don't want to systematically write to all your files.)