В документации модуля рубин IO , я полагаю.
Mode | Meaning
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"r" | Read-only, starts at beginning of file (default mode).
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"r+" | Read-write, starts at beginning of file.
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"w" | Write-only, truncates existing file
| to zero length or creates a new file for writing.
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"w+" | Read-write, truncates existing file to zero length
| or creates a new file for reading and writing.
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"a" | Write-only, starts at end of file if file exists,
| otherwise creates a new file for writing.
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"a+" | Read-write, starts at end of file if file exists,
| otherwise creates a new file for reading and
| writing.
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"b" | Binary file mode (may appear with
| any of the key letters listed above).
| Suppresses EOL <-> CRLF conversion on Windows. And
| sets external encoding to ASCII-8BIT unless explicitly
| specified.
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"t" | Text file mode (may appear with
| any of the key letters listed above except "b").