Homework 1: shell

This assignment will make you more familiar with the Unix system call interface and the shell by implementing several features in a small shell, which we will refer to as the 238P shell. You can do this assignment on any operating system that supports the Unix API (Linux andromeda-XX.ics.uci.edu machines, your laptop that runs Linux or Linux VM, and even MacOS, etc.). Submit your 238P shell through Canvas Canvas HW1 Shell see instructions at the bottom of this page.

First, you have to read the Chapter 0 of the xv6 book.

If you are not familiar with what a shell does, do the Unix hands-on from 6.033 class at MIT (this is optional and will not be graded in 238P).

Download the 238P shell, and look it over. The 238P shell contains two main parts: parsing shell commands and implementing them. The parser recognizes only simple shell commands such as the following:

ls > y
cat < y | sort | uniq | wc > y1
cat y1
rm y1
ls |  sort | uniq | wc
rm y

To compile sh.c, you need a C compiler, such as gcc. On andromeda-XX.ics.uci.edu (Openlab machines), you can compile the skeleton shell as follows:

$ gcc sh.c

$ ./a.out < t.sh

Alternatively you can pass an additional option to gcc to give a more meaningful name to the compiled binary, like

$gcc sh.c -o sh238P

Here gcc will compile your shell as sh238P.

Executing simple commands

Now, you're ready to work on the homework itself. First, extend your shell to implement simple commands, such as executing external programs, for example ls:

$ ls

Here you tell the shell to execute ls.

In the sh.c, the parser already builds an execcmd for you, so the only code you have to write is for the ' ' case in runcmd. At a high level you should understand a typical UNIX interface that we've discussed in class (the functions to cone processes, i.e., fork(), executing new processes, i.e., exec(), working with file descriptors (close(), dup(), open(), wait(), etc.). Combine these functions to implement various shell features.

You might find it useful to look at the manual page for exec, for example, type

$man 3 exec

To test your program, compile and run the resulting a.out:

$./a.out

238P$ ls

Your shell may print an error message (unless there is a program named ls in your working directory or you are using a version of exec that searches PATH, i.e., execlp(), execvp(), or execvpe()). Now type the following:

238P$ /bin/ls

This should execute the program /bin/ls, which should print out the file names in your working directory. You can stop the 238P shell by typing ctrl-d, which should put you back in your computer's shell.

You may want to change the 238P shell to always try /bin, if the program doesn't exist in the current working directory, so that below you don't have to type "/bin" for each program, or (which is better) use one of the exec functions that search the PATH variable.

I/O redirection

Implement I/O redirection commands so that you can run:

echo "238P is cool" > x.txt
cat < x.txt

The parser already recognizes ">" and "<", and builds a redircmd for you, so your job is just filling out the missing code in runcmd for those symbols. You might find the man pages for open and close useful.

Note that the mode field in redircmd contains access modes (e.g., O_RDONLY), which you should pass in the flags argument to open; see parseredirs for the mode values that the shell is using and the manual page for open for the flags argument.

Make sure you print an error message if one of the system calls you are using fails.

Make sure your implementation runs correctly with the above test input. A common error is to forget to specify the permission with which the file must be created (i.e., the 3rd argument to open).

Implement pipes

Implement pipes so that you can run command pipelines such as:

$ ls | sort | uniq | wc

The parser already recognizes "|", and builds a pipecmd for you, so the only code you must write is for the '|' case in runcmd. You might find the man pages for pipe, fork, close, and dup useful.

Test that you can run the above pipeline. The sort program may be in the directory /usr/bin/ and in that case you can type the absolute pathname /usr/bin/sort to run sort. (In your computer's shell you can type which sort to find out which directory in the shell's search path has an executable named "sort".)

From one of the andromeda machines you should be able to run the following command correctly (here a.out is your 238P shell):

$ a.out < t.sh

Don't forget to submit your solution through Canvas Canvas HW1 Shell (as a source file "sh.c"). If you decide to submit a challenge exercise submit two files ("sh.c" and "sh-extra.c", and a shell script "extra.sh" that contains an example extra command that your shell can handle as a single tar or zip archive). Please write us a comment at the top of "sh-extra.c" explaining which extra features you decided to handle.

Challenge exercises (extra 21%, 7% each)

You can add any feature of your choice to your shell. But, you may want to consider the following as a start:

• Implement lists of commands, separated by ";"
• Implement sub shells by implementing "(" and ")"
• Implement running commands in the background by supporting "&" and "wait"