(https://pdos.csail.mit.edu/6.828/2012/homework/xv6-shell.html). The MIT assignment asks the students to implement
the pipe "|" and IO indirection ">" operators and somewhat more in a very simple shell. We ask you to implement the sequence command execution operator ";" and the parallel execution operator "&" and more
1) After your shell has started, it will give a prompt to the user. You should use "F19" for the prompt.
2) If the user types cmd1;cmd2 after the prompt, cmd1 will get executed, followed by cmd2. After cmd2 is executed, your shell will give a new prompt in a new line, ready to execute the next command line.
3) If the user types cmd1&cmd2, both commands will get executed in parallel. After both terminate, your shell will give a new prompt in a new line, ready to execute the next command line.
4) The commands cmd1 etc. are real commands supported by xv6 and they take arguments. An example is the command echo "A".
5) Your shell shall support a command line with a string of 3 or more commands connected by the ";" and "&" operators. The operators are of equal rank and they will be executed from left to right. The string "cmd1&cmd2;cmd3" will see cmd1 and cmd2 executed in parallel. Cmd3 after they both terminate.
6) A command string terminated by a "&" is illegal; but it is fine with ";"'
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