hw2-piping

adding redirection to your shell

coding requirements

Extend your rshell program so that it properly handles input redirection <, output redirection > and >>, and piping |. This will require using the Unix functions dup and pipe. You can find help on how to use these functions in the man pages and the additional resources section below.

As an example, after this assignment, your program should be able to successfully handle the following command:

$ cat < existingInputFile | tr A-Z a-z | tee newOutputFile1 | tr a-z A-Z > newOutputFile2

IMPORTANT: This is a necessary but not sufficient test case. You must come up with others on your own.

Bash has an extensive syntax for redirection, and you are not required to implement all of it. But if you're curious, see the linux documentation project's bash io-redirection tutorial for details.

submission instructions

You will add this code to your rshell project on github. Create a branch called redirect and do all of your work under this branch. When finished, merge with the master branch and create a tag called hw2.

To download and grade your homework, the TA will run the following commands:

$ git clone  http://github.com/yourusername/rshell.git
$ cd rshell
$ git checkout hw2
$ make
$ bin/rshell

You should run them as well to verify that you've submitted your code successfully.

project structure

There are no changes to your project structure.

testing

Again, the tests you choose will be the most important part of your grade.

You should carefully consider: which redirections can be legally combined together, and which cannot? Does order matter? Also make sure to test that you are parsing the command correctly.

As with your previous assignments: Your tests directory will contain a file called piping.script that contains all of the test cases you tried. You will generate the file using the script command, and it must be succinct (i.e. it cannot have unnecessary commands in it). You should use comments in your script to document what you are testing with each test case.

IMPORTANT: If you are unsure if your test cases are sufficient, ask one of the instructors to review them before the deadline.

collaboration policy

You MAY NOT look at the source code of any other student.

You MAY discuss with other students in general terms how to use the unix functions.

You are ENCOURAGED to talk with other students about test cases. You are allowed to freely share ideas in this regard.

You are ENCOURAGED to look at bash's source code for inspiration.

grading

25 points for input redirection <

25 points for output redirection > and >>

50 points for piping |

extra credit 1

The bash shell has an additional form of input redirection that lets you redirect from a string instead of a file. For example, these two commands will give us the same output:

$ echo extra credit rocks | cat
$ cat <<< "extra credit rocks"

You can receive up to 20 points extra credit for implementing this functionality.

extra credit 2

The bash shell lets you perform output redirection on whatever file descriptors you want by placing a number before the > command. For example,

$ g++ main.cpp 2> errors

will direct stderr (which is where g++ prints error messages) to the file errors. If you implement this syntax for any file descriptor and both > and >>, you will get up to 20 points extra credit.

additional resources

Here is a complete list of resources created by previous cs100 students that might help with this assignment:

• how to use syscalls

• video: how to duplicate file descriptors

• video: how to use pipes

• video: how to use dup() syscall cartoon