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Milestone 1
Memory leaks and undefined behaviors are tested here but not counted into grade.

Tasks
1. Write a working read/parse/execute loop and an exit command; [5]
Case: 1
Don't forget to print "exit" when exiting your shell
2. Handle single commands without arguments (e.g. ls); [5]
Case: 2-4
One of test command is exactly "ls"
3. Support commands with arguments (e.g. apt-get update or pkgin update); [5]
Case: 5-6
One of test command is exactly "ls -al"
4. File I/O redirection: [5+5+5+2]
4.1 Output redirection by overwriting a file (e.g. echo 123 > 1.txt); [5]
Case: 7-8
4.2 Output redirection by appending to a file (e.g. echo 465 >> 1.txt); [5]
Case: 9-10
4.3 Input redirection (e.g. cat < 1.txt) [5]
Case: 11-12
Should complete task 4.1 before this task
4.4 Combine 4.1 and 4.2 with 4.3; [2]
Case: 13-14
Should complete task 4.1 - 4.3 before this task
5 Support for bash style redirection syntax (e.g. cat < 1.txt 2.txt > 3.txt 4.txt); [8]
Case: 15-18
Space are not needed between <,>,>> and other characters (16)
The position of <,>,>> can be arbitrary (17)