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2021/2/8 SFU CMPT 300 - Assignment 1
https://systems.cs.sfu.ca/cmpt300/spring21/assignments/a1/a1.html 1/9
CMPT 300 Assignment 1: Shell Commands and Processes
Total marks: 100 + 15 bonus
Overall percentage: 12% + 1.8% bonus
Due: Feb 15 (10am Pacic)
This assignment may be completed idividually or in a group (up to three people).
1. Overview
In this assignment, you will develop a simple Linux shell. The shell accepts user commands and then
executes each command in a separate process. The shell provides the user a prompt at which the next
command is entered. One technique for implementing a shell interface is to have the parent process
rst
read what the user enters on the command line and then create a separate child process that
performs the command. Unless otherwise specied,
the parent process waits for the child to exit before
continuing. However, UNIX shells typically also allow the child process to run in the background - or
concurrently - as well by specifying the ampersand (&) at the end of the command. The separate child
process is created using the fork() system call and the user's command is executed by using one of
the system calls in the exec() family.
Note: As usual, all code must be written in C and run on a Linux machine. We will grade your code on a
Linux machine. You should create a directory for this assignment, such as ~/cmpt300/a1/ and put all
les
related to this assignment in it.
2. A Simple Shell
A C program that provides the basic operation of a command line shell is given below. The main()
function rst
calls read_command(), which reads a full command from the user and tokenizes it into
separate words (arguments). These tokens can be passed directly to execvp() in the child process. If
the user enters an "&" as the nal
argument, read_command() will set the in_background parameter to
true (and remove the "&" from the array of tokens). For example, if the user enters "ls -l" at the '$' prompt,
tokens[0] will contain "ls", tokens[1] will contain (or point to) the string "-l", and tokens[2] will be a
NULL pointer indicating the end of the arguments. (Each of these strings is a NULL terminated C-style
string). Note that the character array buff will contain the text that the user entered; however, it will not
be one single NULL terminated string but rather a bunch of NULL terminated strings, each of which is a
token pointed to by the tokens array.
#include
#include
#include
#include
#include
2021/2/8 SFU CMPT 300 - Assignment 1
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#define COMMAND_LENGTH 1024
#define NUM_TOKENS (COMMAND_LENGTH / 2 + 1)
/**
* Read a command from the keyboard into the buffer 'buff' and tokenize it
* such that 'tokens[i]' points into 'buff' to the i'th token in the command.
* buff: Buffer allocated by the calling code. Must be at least
* COMMAND_LENGTH bytes long.
* tokens[]: Array of character pointers which point into 'buff'. Must be at
* least NUM_TOKENS long. Will strip out up to one final '&' token.
* 'tokens' will be NULL terminated.
* in_background: pointer to a boolean variable. Set to true if user entered
* an & as their last token; otherwise set to false.
*/
void read_command(char *buff, char *tokens[], _Bool *in_background)
{
// ... Full code available in shell.c...
}
/**
* Main and Execute Commands
*/
int main(int argc, char* argv[])
{
char input_buffer[COMMAND_LENGTH];
char *tokens[NUM_TOKENS];
while (true) {
// Get command
// Use write because we need to use read()/write() to work with
// signals, and they are incompatible with printf().
write(STDOUT_FILENO, "$ ", strlen("$ "));
_Bool in_background = false;
read_command(input_buffer, tokens, &in_background);
/**
* Steps For Basic Shell:
* 1. Fork a child process
* 2. Child process invokes execvp() using results in token array.
* 3. If in_background is false, parent waits for
* child to finish. Otherwise, parent loops back to
* read_command() again immediately.
*/
}
2021/2/8 SFU CMPT 300 - Assignment 1
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return 0;
}
3. Problems
There are three mandatory problems (100 points) that you will need to solve:
Create the child process and executing the command in the child.
Implement some internal commands.
Implement a history feature.
There is also one optional problem for a 20-point bonus, but you must attempt the mandatory problems
rst,
before attempting the bonus problem. Attempting the bonus problem without work done in the
mandatory problems will not get you any points for the bonus problem.
Problem 1. Creating Child Process [30 points]
First, modify main() so that upon returning form read_command(), a child is forked and executes the
command specied
by the user. As noted above, read_command() loads the contents of the tokens
array with the command specied
by the user. This tokens array will be passed to the execvp()
function, which has the following interface:
execvp(char *command, char * params[]);
where command represents the command to be performed and params stores the parameters to this
command. For this project, the execvp() function should be invoked as
execvp(tokens[0], tokens);
Be sure to check the value of in_background to determine if the parent process is to wait for the child
exit or not. Hint: use waitpid() instead of wait() because you want to wait on the child you just
launched. If you only use wait() and have previously launched any child processes in the background
that have terminated, wait() will immediately return having "consumed" the previous zombie process,
and your current process incorrectly acts as though it was run in the background. Note that we won't be
testing with interactive command-line processes run in the background (think vim), or test using signals
while running a command in the background. If execvp() returns an error (see man execvp) then
display an error message. Note that using printf() may not work well for this assignment and that you
should use write() instead (look up more with man write). The issue is that we need to use the
read() function for getting the user's command and use write() when working with signals (later).
And, it turns out that printf() and read()/write() don't always work well together. Therefore, when
printing to the screen, use the write() command. For common things, such as displaying a simple
string, or writing a number to the screen, you may want to make your own functions which make it
easier. You can convert an integer to a string using sprintf().
Waiting Aside: When a process in Linux nishes,
it still exists in the kernel with some status information
until the parent process waits on that child. These un-waited-on terminated child processes are known
2021/2/8 SFU CMPT 300 - Assignment 1
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zombies. For this assignment, you won't lose any marks if you don't correctly wait() on zombie
processes from background tasks; however, it's a good habit to correctly cleanup the zombies on your
system! Above it is suggested that you use waitpid() to wait on the correct child. However, this will
leave any background process as zombie processes (having exited, the parent process will never
wait() on the child). You can correct this by occasionally trying a non-blocking wait to handle any
zombie child processes. We can pass the WNOHANG option to waitpid() to be non-blocking, and setting
the PID to -1 will wait on any child process. For example, have the following code run after every user
command is processed:
// Cleanup any previously exited background child processes
// (The zombies)
while (waitpid(-1, NULL, WNOHANG) > 0)
; // do nothing.
Problem 2. Shell Prompt and Internal Commands [30 points]
First, make sure your shell prompt always shows the current working directory. For example, if in the
/home/cmpt300 folder, the prompt should be:
/home/cmpt300$
Next, let's implement some internal commands. Internal commands are built-in features of the shell
itself, as opposed to a separate program that is executed. Implement the commands listed below. Note
that for these you need not fork a new process as they can be handled directly in the parent process. All
the commands here are case-sensitive.
exit: Exit the shell program. If the user provided any argument, abort the operation (i.e., command
not executed) and display an error message.
pwd: Display the current working directory. Use the getcwd() function. Run man getcwd for more.
Again, abort the operation and display an error message if the user provided any argument.
cd: Change the current working directory. Use the chdir() function. Pass chdir() the rst
argument
the user enters (it will accept absolute and relative paths). If the user passed in more than one
argument, abort the operation and display an error message. If chdir() returns an error, display an
error message.
help: Display help information on internal commands.
If the rst
argument is one of our internal commands, print " is a builtin command" plus
a brief description on what the command does. For example, if argument is 'cd', the output should
be:
'cd' is a builtin command for changing the current working directory.
If the rst
argument is not an internal command, this command prints " is an external
command or application". For example, if argument is 'ls', the output must be:
'ls' is an external command or application
2021/2/8 SFU CMPT 300 - Assignment 1
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If there is more than one argument, display an error message
If there is no argument provided, list all the supported internal commands. For each command,
include a short summary on what it does.
Problem 3. History Feature [40 points]
The next task is to modify your shell to provide a history feature that allows the user access up to the 10
most recently entered commands. Start numbering the user's commands at 0 and increment for each
command entered. These numbers will grow past 9. For example, if the user has entered 35 commands,
then the most recent 10 will be numbered 15 through 34.
History Commands: First implement an internal command "history" which displays the 10 most recent
commands executed in the shell. If there are not yet 10 commands entered, display all the commands
entered so far.
Display the commands in descending order (sorted by its command number).
The output should include both external application commands and internal commands.
Display the command number on the left, and the command (with all its arguments) on the right.
Hint: Print a tab between the two outputs to have them line up easily.
If the command is run in the background using &, it must be added to the history with the &.
A sample output of the history command is shown below:
/home/cmpt300$ history
30 history
29 cd /home/cmpt300
28 cd /proc
27 cat uptime
26 cd /usr
25 ls
24 man pthread_create
23 ls
22 echo "Hello World from my shell!"
21 ls -la
/home/cmpt300$
Next, implement the ! commands which allows users to run commands directly from the history list:
Command "!n" runs command number n, such as "!22" will re-run the 23rd command entered this
session. In the above example, this will re-run the echo command.
If n is not a number, or an invalid value (not one of the previous 10 command numbers) then display
an error.
You may treat any command starting with ! as a history command. For example, if the user types
"!hi", just display an error. Note that atoi("hi") returns 0, which should not be treated as a valid
command.
Command "!!" runs the previous command.
If there is no previous command, display an error message.
2021/2/8 SFU CMPT 300 - Assignment 1
https://systems.cs.sfu.ca/cmpt300/spring21/assignments/a1/a1.html 6/9
When running a command using "!n" or "!!", display the command from history to the screen so the
user can see what command they are actually running.
Neither the "!!" nor the "!n" commands are to be added to the history list themselves, but rather the
command being executed from the history must be added. Here is an example.
/home/cmpt300$ echo test
test
/home/cmpt300$ !!
echo test
test
/home/cmpt300$ history
15 history
14 echo test
13 echo test
12 ls
11 man pthread_create
10 cd /home/cmpt300
9 ls
8 ls -la
7 echo Hello World from my shell!
6 history
/home/cmpt300$
Suggestions
Implement history as a global two dimensional array:
#define HISTORY_DEPTH 10
char history[HISTORY_DEPTH][COMMAND_LENGTH];
Rather than having all your code directly access the history array, write some functions which
encapsulate accesses to this array. Suggested functions would include: add command to history,
retrieve command (copy into buffer, likely), printing the last 10 commands to the screen.
Signals: Change your shell program to display the help information when the user presses ctrl-c (which
is the SIGINT signal). A guide on using signals is available here.
In main(), register a custom signal handler for the SIGINT signal.
Have the signal handler display the help information (same as the help command).
Then re-display the command-prompt before returning.
Note that when another child process is running, ctrl-c will likely cancel/kill that process. Therefore
displaying the help information with ctrl-c will only be tested when there are no child processes
running.
Suggestions
2021/2/8 SFU CMPT 300 - Assignment 1
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To implement this, you will also need to change read_command() a little bit.
The signal handler will do nothing but displaying the help information and then display the prompt
again. The signal will interrupt the read() system call and discard all data already read for this
command.
When read() fails, it returns -1. You can check why read fails: if it returns -1 and the environment
variable errno equals EINTR it means that it was interrupted by a signal. If the return value is -1 and
errno is any other value, it means read just failed and the program should exit.
To correctly check read()'s return value you can change the code that you now have:
if (length < 0) {
perror("Unable to read command. Terminating.\n");
exit(-1); /* terminate with error */
}
to the following:
if ((length < 0) && (errno !=EINTR)) {
perror("Unable to read command. Terminating.\n");
exit(-1); /* terminate with error */
}
Problem 4 (Optional). Better cd Command [15 bonus points]
Note: You must nish
Problems 1, 2 and 3 before attempting Problem 4. Submitting Problem 4 alone
without solutions to Problems 1, 2 or 3 will leave you with 0 mark for Problem 4.
Implement the following features that are often supported by the cd command in modern shells (e.g.,
bash).
Change to the home directory if no argument is provided. [4 points]
Support ~ for the home folder. [6 points]
For example, cd ~/cmpt300 should change to the "cmpt300" directory under the current user's home
directory. Issuing cd ~ will switch to the home directory.
Support - for changing back to the previous directory. [5 points]
For example, suppose that the current working directory is /home and you issued cd / to change to
the root directory. Then, cd - will switch back to the /home directory.
Hints: You may nd
the getuid and getpwuid functions useful. They allow you to gather useful
information about the current user.
3. Notes
You do not need to support either > or | from the terminal. These are features of the normal Linux
terminal that we are not implementing. Your code must not have any memory leaks or memory access
errors (using Valgrind). Your shell must free all memory before it exits. However, your child processes
2021/2/8 SFU CMPT 300 - Assignment 1
https://systems.cs.sfu.ca/cmpt300/spring21/assignments/a1/a1.html 8/9
may exit without freeing all their memory (if exec() fails then the child process will have a copy of all the
memory that the parent holds; freeing this memory would be unnecessarily time consuming for the OS).
Therefore, you may ignore any Valgrind messages when a child process terminates to the effect of
memory being held when program terminated.
4. Resources
Sample code: shell.c, Makele
Guide to Signals
5. Submission
You need to submit a compressed archive named a1.tar.gz to CourSys of your code and a make le
(sample code: shell.c, Makele).
If you did this assignment in a group, include also a simple readme text
le
in your package that details each group member's contributions; each of you should contribute to a
reasonable share of the assignment, and every group member will receive the same marks. We will build
your code using your make le,
and run it using the command: ./shell. You may use more than one
.c/.h le
in your solution if you like. If so, your Makefile must correctly build your project. Please
remember that all submissions will automatically be compared for unexplainable similarities.
Following the steps below to prepare your submission:
Make sure that your les
are stored in a directory as called a1
Change to each of your folders and issue the command make clean. This will remove all object les
as well as all output and temporary les
Change to your a1 folder:
$ cd ~/cmpt300/a1
Then, issue:
$ tar cvf a1.tar *
which creates a tar ball (i.e., a single le)
that contains the contents of the folder.
Compress your le
using gzip:
$ gzip a1.tar
Submit via CourSys by the deadline posted there.
Grading Policies
Make sure you are familiar with the course policies. Especially, we do not accept late submissions, so
please submit on time by the deadline. Your code must compile and run on Linux; you will receive a 0
2021/2/8 SFU CMPT 300 - Assignment 1
https://systems.cs.sfu.ca/cmpt300/spring21/assignments/a1/a1.html 9/9
mark if your code does not compile. Sample solutions will not be provided for assignments. Your code
should also not leak any memory; we will check using Valgrind. Any memory leak will lead to 10 marks
overall deduction. We will test your code using more complex and hidden test cases, so you are
encouraged to vary as many parameters as possible to test your code.
* Created by Mohamed Hefeeda, modied
by 436 Brian Fraser, Keval Vora and Tianzheng Wang.

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