ECSE427/COMP310 Programming Assignment #1: A Simple Shell


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In this assignment you are required to create a C program that implements a shell interface that
accepts user commands and executes each command in a separate process. Your shell program
provides a command prompt, where the user inputs a line of command. The shell is responsible for
executing the command. The shell program assumes that the first string of the line gives the name of
the executable file. The rest of the strings in the line are considered as arguments for the command.
Consider the following example.
sh > cat prog.c
The cat is the command that is executed with prog.c as its argument. Effectively, the user
displays the contents of the file prog.c on the display terminal. If the file prog.c is not present, the
cat program displays an appropriate error message. The shell is not responsible for such error
One technique for implementing a shell interface is to have the parent process first read what the
user enters on the command line (i.e., cat prog.c), and then create a separate child process that
performs the command. Unless otherwise specified, 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. By re-entering the
above command as follows the parent and child processes can run concurrently.
sh > cat prog.c &
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 (see textbook or man exec for more
Simple Shell
A C program that provides the basic operations of a command line shell is supplied below. This
program is composed of three functions: main(), getcmd(), and freecmd(). The getcmd()
function reads in the user’s next command, and then parses it into separate tokens that are used to fill
the argument vector for the command to be executed. (If the command is to be run in the background,
it will end with ‘&’, and getcmd() will update the parameter background so the main() function
can act accordingly. The program terminates when the user enters ; getcmd()
invokes exit().
The main() invokes getcmd(), which waits for the user to enter a command. The contents of
the command entered by the user are loaded into the args array. For example, if the user enters ls –
l at the command prompt, args[0] becomes equal to the string ls and args[1] is set to the string
to –l. (By “string,” we mean a null-terminated, C-style string variable.)
This programming assignment is organized into three parts: (1) creating the child process and
executing the command in the child, (2) modifying the shell to allow a history feature, and (3)
additional commands.
Creating a Child Process
The first part of this programming assignment is to modify the main() function in the figure
below so that upon returning from getcmd(), child process is forked and it executes the command
specified by the user.
int getcmd(char *prompt, char *args[], int *background)
int length, i = 0;
char *token, *loc;
char *line;
size_t linecap = 0;
printf(“%s”, prompt);
length = getline(&line, &linecap, stdin);
if (length <= 0) { exit(-1); } // Check if background is specified.. if ((loc = index(line, '&')) != NULL) { *background = 1; *loc = ' '; } else *background = 0; while ((token = strsep(&line, " \t\n")) != NULL) { for (int j = 0; j < strlen(token); j++) if (token[j] <= 32) token[j] = '\0'; if (strlen(token) > 0)
args[i++] = token;
return i;
int main(void)
char *args[20];
int bg;
while(1) {
bg = 0;
int cnt = getcmd(“\n>> “, args, &bg);
/* the steps are:
(1) fork a child process using fork()
(2) the child process will invoke execvp()
(3) if background == 0, the parent will wait,
otherwise gets the next command… */
// you need to implement this one. Might need some modifications to
// getcmd() provided with the assignment
As noted above, the getcmd() function loads the contents of the args array with the command
line given by the user. This args array will be passed to the execvp() function, which has the
following interface:
execvp(char *command, char *params[]);
Where command represents the file to be executed and params store the parameters to be supplied
to this command. Be sure to check the value of background to determine if the parent process is to wait
for the child to exit or not. You can use the waitpid() function to wait on the newly created child
process. Check the man page for the actual usage of the waitpid() or similar functions that you can
Creating a History Feature
The next task is to modify the above program so that it provides a history feature that allows the
user access up to the 10 most recently entered commands. These commands will be numbered starting
at 1 and will continue to grow larger even past 10, e.g. if the user has entered 35 commands, the 10
most recent commands should be numbered 26 to 35. With this list, the user can run any of the
previous 10 commands by entering r x where ‘x’ is the first letter of that command. If more than
one command starts with ‘x’, execute the most recent one. Also, the user should be able to run the
most recent command again by just entering ‘r’. You can assume that only one space will separate
the ‘r’ and the first letter and that the letter will be followed by ‘\n’. Again, ‘r’ alone will be used
immediately followed by the ‘\n’ character if it is wished to execute the most recent command.
Any command that is executed in this fashion should be echoed on the user’s screen and the
command is also placed in the history buffer as the next command. (r x does not go into the history
list; the actual command that is specifies does.)
If the user attempts to use this history facility to run a command and the command is detected to
be erroneous, an error message should be given to the user and the command not entered into the
history list, and the execvp() function should not be called. (It would be nice to know about
improperly formed commands that are handed off to execvp() that appear to look valid and are not,
and not include them in the history as well, but that is beyond the capabilities of this simple shell
program.) You could modify getcmd() as part of the history implementation.
Built-in Commands
The history command is a built-in command because the functionality is completely built into
the shell. On the other hand, the process forking mechanism was used to execute outside commands. In
addition to the history command, implement the cd (change directory), pwd (present working
directory), and exit (leave shell) commands. The cd command could be implemented using the
chdir() system call, the pwd could be implemented using the getcwd() library routine, and the
exit command is necessary to quit the shell. Other built-in commands to be implemented include fg
and jobs. The command jobs should list all the jobs that are running in the background at any given
time. These are jobs that are put into the background by giving the command with & as the last one in
the command line. Each line in the list provided by the jobs should have a number identifier that can be
used by the fg command to bring the job to the foreground.
Turn-in and Marking Scheme
The programming assignment should be submitted via My Courses. Other submissions (including
email) are not acceptable.
Useful Information for the Assignment
You need to know how process management is performed in Linux/Unix to complete this
assignment. Here is a brief overview of the important actions.
(a) Process creation: The fork() system call allows a process to create a new process (child of the
creating process). The new process is an exact copy of the parent with a new process ID and its
own process control block. The name “fork” comes from the idea that parent process is
dividing to yield two copies of itself. The newly created child is initially running the exact
same program as the parent – which is pretty useless. So we use the execvp() system call to
change the program that is associated with the newly created child process.
(b) The exit() system call terminates a process and makes all resources available for subsequent
reallocation by the kernel. The exit(status) provides status as an integer to denote the exiting
condition. The parent could use waitpid() system call to retrieve the status returned by the child
and also wait for it.