Question: the tee command reads its standard input until endoffile writing...

The tee command reads its standard input until end-of-file, writing a copy of the input to standard output and to the file named in its command-line argument. Implement tee using I/O system calls. By default, tee overwrites any existing file with the given name. Implement the -a command-line option (tee -a file), which causes tee to append text to the end of a file if it already exists. (Use the getopt() function or write your own to parse command-line options.)