Project 1 – A Simplified Linux Shell Program

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COMP3511 Operating System
Project 1 – A Simplified Linux Shell Program

Introduction
The aim of this project is to help students understand process management and interprocess communication in Linux. Upon completion of the project, students should be able
to implement a useful system program using related Linux system calls.
Overview
In this assignment, you need to implement a non-interactive command line interpreter that
supports multi-level pipes. The non-interactive command line interpreter is named as cmd.
Suppose there are 4 files in the current working directory:

cmd cmd.c in.txt out.txt
Here is a sample usage of the non-interactive command line interpreter.
$> ./cmd < in.txt > out.txt
$> represents the shell prompt.
The contents of in.txt and out.txt are as follows:
Content of in.txt Content in out.txt
ls cmd
cmd.c
in.txt
out.txt
Restrictions
In this assignment, you CANNOT use system function defined in the C Standard library
// Should not use this function
int system (const char* command);
The purpose of the project assignment is to help students understand process management
and inter-process communication. It is meaningless to directly use the system function to
process the whole command. You should use related Linux system calls such as pipe and
dup2. In addition, POSIX file operations such as read, open, write, close should
be used, but not using fread, fopen, fwrite, fclose from C standard library.
Please review the lab materials. The grader TA will check your submitted source codes.
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Development Environment
CS Lab 2 is the development environment. Please use one of the following machines
(csl2wkXX.cse.ust.hk), where XX=01…50. The grader TA will use the same platform
to grade all submissions. Please note that different gcc compilers and Linux platforms may
produce different results.
In other words, “my program works on my own laptop/desktop computer, but not in one of
the CS Lab 2 machines” is an invalid appeal reason. Please test your program on our
development environment (not on your own desktop/laptop) thoughtfully before your final
submission, even you are running your own Linux OS. Remote login is supported on all CS
Lab 2 machines, so you are not required to be physically present in CS Lab 2.
Starting Point
cmd_given.c is a starting point. To start your work, please rename the file as cmd.c
Read carefully the documentation in the base code. You are not required to start from
scratch as the base file already provides you some useful features (e.g. command line
parsing). Necessary programming concepts will also be introduced during the labs.
Please note that C programming language (instead of C++) must be used to complete this
assignment. C is not the same as C++. Here is the command to compile and run cmd.c
$> ls
cmd.c in.txt
$> gcc -o cmd cmd.c
$> ./cmd < in.txt > out.txt
Multi-level Pipes
In C programming in Linux, a process creates a pipe by:
int ps[2];
pipe(ps);
After the pipe function call, ps[0]will be assigned to a file pointer to the read end of the
pipe, and ps[1] will be assigned to the write end of the pipe. In a shell program, a pipe
symbol (|) is used to connect the output of the first command to the input of the second
command. For example,
$> ls | sort
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The ls command lists the contents of the current directory. As the output of ls is already
connected to sort, it won’t print out the content to the screen. After the output of ls has
been sorted by sort, the sorted list of files appears on the screen.
In this project, you are required to support multiple-level pipes. We assume that there exists
at most 16 pipe segments. Please note that each command segment may have zero to
many input parameters. For example, the following is a pipe command with 3 segments:
$> ls -l | sort | wc -l
Sample Test Cases
The grade TA will use the following pattern to grade your submission
$> ./cmd < [Input file name] > [output filename]
The input test cases (inX.txt, where X=1-7) are provided. You can assume that each
input file stores a single line of command. You can assume that the input format is valid.
Input Expected output
ls ls displays the filenames of the
current working directory
ls -l -h ls -l -h displays the filenames
of the current working directory
using a better format. In this input,
several empty space characters (i.e.
“\t\r\n\v\f” and “ “) are used
as delimiters
ls -lh It displays the same output as
above
ls | sort ls displays the filenames of the
current working directory, and the
filenames are sorted in an
ascending order
ls | sort | sort | sort | sort | sort |
sort | sort | sort | sort | sort | sort |
sort | sort | sort | sort
Same as above. This input contains
16 pipe segments. This pattern is
useful to test the upper bound of
the number of pipe segment
ls -l | sort | wc -l It displays the number of lines
printed on the screen after running
ls -l
curl
https://www.ceo.gov.hk/images/carrie02.jpg
--output carrie.jpg | ls carrie.jpg
The picture of the chief executive of
HKSAR will be downloaded by
curl. The filename
“carrie.jpg” will be shown if
the download is successful
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Marking Scheme
1. (30%) Explanation of process_cmd within the comment block. You should use
point form to clearly explain how you implement this function
2. (70%) The given test cases. Please note that the grader TA will check the source
codes and may shuffle the order of test cases. Thus, students who hardcoding the
results cannot get any mark.
Plagiarism: Both parties (i.e. students providing the codes and students copying the codes)
will receive 0 marks. A plagiarism detection software will be used to identify the cheating
cases.
Submission
File to submit: cmd.c
Please check carefully you submit the correct file. In the past semesters, some students
submitted the executable file instead of the source file. Zero marks will be given as the
grader TA cannot grade the executable file.
You are not required to submit other files such as the input test cases.
You only need to submit the file via CASS on /before the due day mentioned on the course
web page.