ruby File.open options
#"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 or creates a new file for writing.
#"w+" Read-write, truncates existing file to zero length
# or creates a new file for reading and writing.
#"a" Write-only, each write call appends data at end of file.
# Creates a new file for writing if file does not exist.
#"a+" Read-write, each write call appends data at end of file.
# Creates a new file for reading and writing if file does
# not exist.