Homework 1: OS interface and shell

This assignment will make you more familiar with the Unix system call interface and the shell by implementing several simple programs and several features in a small shell, which we will refer to as the 143A 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 programs and the shell through Canvas (see instructions at the bottom of this page).

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

Part 1: Simple UNIX programs

Download the main.c, and look it over. This is a skeleton for a simple UNIX program.

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

 $ gcc main.c 

 $ ./a.out 

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

 $gcc sh.c -o foobar143a

Here gcc will compile your program as foobar143A. In the rest of this part of the assignment you will convert main.c into several simple UNIX programs.

File copy command (cp143a)

Use the main.c template as a starting point for a simple file copy command that you should implement. First copy the main.c into main-cp143a.c (you will need to use main.c for other programs later, so lets keep it around).

The file copy command should take two arguments: names of the input and the output files and copy the input file into the output file. Here is an example invocation which copies main.c into main-out.c (assuming you call your executable cp143a).

cp143a main.c main-out.c

int main(int argc, char *argv[])

Note: You might find it useful to look at the manual page for read(), write(), and other system calls. For example, type

 $man read 

#include <unistd.h>

 ssize_t read(int fd, void *buf, size_t count); 

Note that when the manual list a function like open(2) it means that it's described in the 2nd section of the manual and to get to the specific section you have to invoke man with an additional argument like this:

man 2 open

man man

man -k open

man open

Simple I/O redirection (lsy143a)

Use the main.c template again as a starting point for another simple program that starts ls command but redirects its output into a y file. I.e., your program should do an equivalent of this shell command

ls > y

First copy the main.c into main-lsy143a.c (again you will need to use main.c for other programs later).

Here is how an example invocation of your program should look (assuming you call your executable lsy143a).

lsy143a

exec()

Simple pipes (pipe143a)

Use the main.c template again as a starting point for another simple program that starts ls command but redirects its output into the grep "main" program, which itself redirects its output to the wc. I.e., your program should internally start three programs connected into with pipes that produce output equivalent of this shell command

ls |grep "main" |wc

Copy the main.c into main-pipe143a.c. Here is an example invocation of your program (assuming you call your executable pipe143a).

pipe143a

Part 2: Building a shell

Download the 143A shell, and look it over. The 143A 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 sh143A

Here gcc will compile your shell as sh143A.

The output of ls executed on shell may differ from the one executed inside the script. This is because ls is usally aliased to include some options on most shells.

alias ls='ls -F --color=auto --human-readable --almost-all'

Especially, -F option appends * to the executable file names, adding a few bytes to the wc's input. In order to make outputs consistent, run the following command on shell.

$ unalias ls

You can restore the ls alias with the following command.

$ source ~/.bashrc

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

143A$ 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:

143A$ /bin/ls

This should execute the program /bin/ls, which should print out the file names in your working directory. You can stop the 143A shell by typing ctrl-d, which should put you back in your computer's shell.

You may want to change the 143A 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 "143A 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 143A shell):

$ a.out < t.sh

Don't forget to submit your solution through Canvas Canvas HW1 OS Interface and Shell (as a collection of source files "main-cp143a.c", "main-lsy143a.c", "main-pipe143a.c", and "sh.c"). If you decide to submit a challenge exercise submit an extra file "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"