Shell

My mum asked me to write a shell 😱

Features

1. Write a working read/parse/execute loop and an exit command; [5]

2. Handle single commands without arguments (e.g. ls); [5]

3. Support commands with arguments (e.g. apt-get update or pkgin update); [5]

4. File I/O redirection: [5+5+5+2]

   4.1. Output redirection by overwriting a file (e.g. echo 123 > 1.txt);

   4.2. Output redirection by appending to a file (e.g. echo 465 >> 1.txt);

   4.3. Input redirection (e.g. cat < 1.txt);

   4.4. Combine 4.1 and 4.2 with 4.3;

5. Support for bash style redirection syntax (e.g. cat < 1.txt 2.txt > 3.txt 4.txt); [8]

6. Pipes: [5+5+5+10]

   6.1. Basic pipe support (e.g. echo 123 | grep 1);

   6.2. Run all ‘stages’ of piped process in parallel. (e.g. yes ve482 | grep 482);

   6.3. Extend 6.2 to support requirements 4. and 5. (e.g. cat < 1.txt 2.txt | grep 1 > 3.txt);

   6.4. Extend 6.3 to support arbitrarily deep “cascade pipes” (e.g. echo 123 | grep 1 | grep 1 | grep 1)

   Note: the sub-processes must be reaped in order to be awarded the marks.

7. Support CTRL-D (similar to bash, when there is no/an unfinished command); [5]

8. Internal commands: [5+5+5]

   8.1. Implement pwd as a built-in command;

   8.2. Allow changing working directory using cd;

   8.3. Allow pwd to be piped or redirected as specified in requirement 4.;

9. Support CTRL-C: [5+3+2+10]

   9.1. Properly handle CTRL-C in the case of requirement 4.;

   9.2. Extend 9.1 to support subtasks 6.1 to 6.3;

   9.3. Extend 9.2 to support requirement 7., especially on an incomplete input;

   9.4. Extend 9.3 to support requirement 6.;

10. Support quotes: [5+2+3+5]

    10.1. Handle single and double quotes (e.gm. echo "de'f' ghi" '123"a"bc' a b c);

    10.2. Extend 10.1 to support requirement 4. and subtasks 6.1 to 6.3;

    10.3. Extend 10.2 in the case of incomplete quotes (e.g. Input echo "de, hit enter and input cd");

    10.4. Extend 10.3 to support requirements 4. and 6., together with subtask 9.3;

11. Wait for the command to be completed when encountering >, <, or |: [3+2]

    11.1. Support requirements 3. and 4. together with subtasks 6.1 to 6.3;

    11.2. Extend 11.1 to support requirement 10.;

12. Handle errors for all supported features. [10]

    Note: a list of test cases will be published at a later stage. Marks will be awarded based on the number of cases that are correctly handled, i.e. if only if:

    • A precise error message is displayed (e.g. simply saying “error happened!” is not enough);

    • The program continues executing normally after the error is identified and handled;

13. A command ending with an & should be run in background. [10]

    13.1. For any background job, the shell should print out the command line, prepended with the job ID and the process ID (e.g. if the two lines /bin/ls & and /bin/ls | cat & are input the output could be the two lines [1] (32757) /bin/ls & and [2] (32758) (32759) /bin/ls | cat & );

    13.2. Implement the command jobs which prints a list of background tasks together with their running states (e.g. in the previous case output the two lines [1] done /bin/ls & and [2] running /bin/ls | cat &);

For those who take ve482:

This shell fails to execute cd -, which leads to a 4-point deduction.