Check out example codes for "ruby File.open options". It will help you in understanding the concepts better.

Code Example 1

#"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.

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