Description
In this project you will be creating your own version of a UNIX shell. It will not be as sophisticated as
bash, the shell you have been using in lab, but it will perform similar functions. A UNIX shell is simply an
interactive interface between the OS and the user. It repeatedly accepts input from the user and initiates
processes on the OS based on that input.
2.1 Requirements
1. Your shell should accept a -p option on the command line when it is initiated. The
option should become the user prompt. If this option is not specified, the default prompt
of “308sh> ” should be used. Read section 3.1 on command line options for more information.
2. Your shell must run an infinite loop accepting input from the user and running commands until the
user requests to exit.
3. Each line of user input should be treated as either a builtin command, or a command to be executed.
See section 2.2 for a list of the builtin commands your shell should support.
4. For each executed command, the shell should spawn a child process to run the command. The
shell should try to run the command exactly as typed. This will require the user to know the full
path (absolute /some/dir/some/exec or relative ../../some/exec) to the executable, unless the
executable they wish to run is in one of the directories listed in the PATH environment variable.
HINT: execvp() will search PATH for you.
5. The shell should notify the user if the requested command is not found and cannot be run.
6. For each spawned child, print the process ID (PID) before executing the specified command. It
should only be printed once and it must be printed before any output from the command. You can
include other information (name of program, command number, etc) if you find it useful.
7. By default, the shell should block (wait for the child to exit) for each command. Thus the prompt
will not be available for additional user input until the command has completed. Your shell should
only wake up when the most recently executed child process completes. See waitpid.
8. The shell should evaluate the exit status of the child process and print the conditions under which it
exited (identify which process exited by its PID). Read the man page for wait to see a list of macros
that provide this information (man 2 wait). Two examples of exit status are normal exit and killed
(signaled).
9. If the last character in the user input is an ampersand (&), the child should be run in the background
(the shell will not wait for the child to exit before prompting the user for further input). Remove
the & when passing the parameters to exec. When your background process does exit, you must
print its status like you do for foreground processes.
An example is, if the user entered “/usr/bin/emacs” then the shell would wait for the process to
complete before returning to the prompt. However, if the user entered “/usr/bin/emacs &” then
the prompt would return as soon as the process was initiated.
HINT: to evaluate and print the exit status of a background child process call waitpid with -1 for
the pid and WNOHANG set in the options. This checks all children and doesn’t block – see the
man page. To do this periodically, a check can be done every time the user enters a command.
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2.2 Builtin Commands
The following commands are special commands (also called built-in commands) that are not to be treated
as programs to be launched. No child process should be spawned when these are entered.
• exit – the shell should terminate and accept no further input from the user
• pid – the shell should print its process ID
• ppid – the shell should print the process ID of its parent
• cd
– change the working directory. With no arguments, change to the user’s home directory
(which is stored in the environment variable HOME)
• pwd – print the current working directory
• set – sets an environment variable (which is visible in all future child processes).
If there is only one argument, clears the variable.
• get – prints the current value of an environment variable
2.3 Extra Credit (5 pts)
• Keep track of the names of your child processes and output them with their PIDs (i.e. “process pid
(procnam) ”). For this, you need a data structure which can maintain a list of currently executing
background tasks – we suggest using a list. Also, now that you are maintaining a list of background
tasks, add another built- in command “jobs” that outputs the name and PID of each task running
in the background.
• Sometimes it’s nice to have your program dump its output to a file rather than the console. In bash
(and most shells) if you do cmd arg1 arg2 argN > filename then the output from cmd will go the
file filename. If the file doesn’t exist, it is created. If it does exist, it is truncated (removes all its
data, allowing you to effectively overwrite it). Add this feature to your shell.
• Make sure you clearly describe how your added feature works in your lab report.
3 Useful Information
3.1 Command line options
Command line options are options that are passed to a program when it is initiated. For example, to get
ls to display the size of files as well as their names the -l option is passed to it. (e.g. ls {l /home/me).
In this example, /home/me is also a command line option which specifies the desired directory for which
the contents should be listed. From within a C program, command line options are handled using the
parameters argc and argv which are passed into main.
int main(int argc, char **argv)
The parameter argc is a value that contains the number of command line arguments. This value is always
at least 1, as the name of the executable is always the first parameter. The parameter argv is an array of
string values that contain the command line options. The first option, as mentioned above, is the name
of the executed program. The program below simply prints each command line option passed in on its
own line.
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int main(int argc, char **argv) {
int i;
for(i=0; i<argc; i++) printf(“Option %d is \”%s\”\n”, i, argv[i]); return 0; } If you are unfamiliar with how command line options work, enter the above program and try it with different values. (e.g. ./myprog I guess these are options) 3.2 Useful system and library calls The following system calls will be useful in this project. Read the corresponding man pages for those you are unfamiliar with. • fork – create a child process • execvp – replace the current process with that of the specified program • waitpid – wait for a child to exit (or get exit status) • exit – force the current process to exit, with the given return value • chdir – change working directory • getcwd – get current working directory • getenv/setenv – retrieve and set environment variables. • perror – display error messages based on the value of errno • strcmp, strcpy, strcat – string manipulation. • open, dup2 – for the extra credit 4 Example and Test Cases This is an example test case and output for your progrem. Your output does not have to look exactly like this; it is simply meant to give you some idea of how the shell should work and how output looks. A $ prompt indicates a bash prompt, used to execute your shell, and 308sh> indicates your shell prompt.
Also, you may want to prefix all your output messages with some sort of identifier, such as “>>>”, to
more easily determine which lines are yours and which came from the executed program.
You should test all of the builtin commands in your shell, in addition to several commands that are
not builtin. You should test any error handling you use. You do not have to turn in any test code that
you use, but don’t expect to find errors without testing!
Note that you do not have to output the process name on the “exit” line unless you are doing the
extra credit; just the PID and status is sufficient.
Shell command line:
$ ./shell -p “hello> ”
hello> exit
$
Simple execution:
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308sh> /bin/ls
[977801] ls
shell.c shell.o shell
[977801] ls Exit 0
308sh> pwd
/home/danield/308/
308sh> cd
308sh> pwd
/home/danield/
308sh> cd 308
308sh> pwd
/home/danield/308/
308sh> ps
[977819] ps
PID TTY TIME CMD
3590 pts/2 00:00:01 bash
977797 pts/2 00:00:00 shell
977819 pts/2 00:00:00 ps
[977819] ps Exit 0
308sh>
308sh> nonexistant
[977850] nonexistant
Cannot exec nonexistant: No such file or directory
[977850] nonexistant Exit 255
308sh> test 2 = 3
[978229] test
[978229] test Exit 1
308sh>
Background processes:
308sh> sleep 1 &
[977999] sleep
308sh>
[977999] sleep Exit 0
308sh>
308sh> sleep 1 &
[977864] sleep
308sh> sleep 2
[977865] sleep
[977865] sleep Exit 0
[977864] sleep Exit 0
308sh>
308sh> sleep 10 &
[977943] sleep
308sh> kill 977943
[977945] kill
[977945] kill Exit 0
[977943] sleep Killed (15)
308sh>
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