CS342 – Project #1 A Simple Shell Program


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In this project you will develop a simple shell program, a command line interpreter,
called bilshell. It will work in two modes: interactive mode and batch mode.
In interactive mode your shell will provide a prompt string, like bilshell-$:, to
the user where the user will type a command name, i.e., a program name, with
zero or more parameter strings and your shell will execute the command. In batch
mode your shell will read the commands from a file and will execute them one after
another. Your shell will be invoked as follows:
bilshell N inputf ilename
The inputf ilename parameter is the name of an ascii text file containing commands. There is one command per line. The inputf ilename parameter is optional.
If omitted, the shell will run in interactive mode; if specified it will run in batch
mode. The parameter N is the number of characters to receive in each read operation and will be used for compound command execution that will be explained
later. An example invocation of the program can be:
bilshell 1 infile.txt
A command to execute will include a command name, i.e., a program name, and
zero or more parameters. An example command can be “cp file1.txt file2.txt”.
When such a command is entered by a user or read as a line from an input
file, your shell will divide it into arguments, i.e., strings, where argument 0 is the
command name, argument 1, if any of course, is the first parameter to the command,
argument 2 is the second parameter to the command, and so on.
When started, your shell will run as a process, i.e., main parent process, and will
wait for an input command line. When user gives an input line, the line will be separated into arguments and a child process will be created to execute the command.
For this the main process will use the fork() system call. In the child process,
exec() system call will be used to finally execute the program. There are various
exec related library functions. You can use execv() or execvp() for example. It
takes the pathname of a program to execute and an array of argument strings.
Please read the man page of exec system call. After creating the child process, the
parent process will wait. When command is executed and child process terminates,
the parent process will return from waiting and will provide another prompt string
to the user so that the user can type another command line; or if commands are
taken from an input file, the parent will read another command line from the input
file and will execute that command again in another child process.
Your shell will also support composition of two commands where the output of
one command will be given to another command as input. For example, there is
“ps aux” program in Linux that is listing the current processes, and there is “sort”
program in Linux that is sorting a text file. When we write “ps aux | sort” in
Linux shell, it prints to the screen the sorted list of processes. Similarly, when
we would write such a command line in your shell, it should also print a listing
of processes in sorted order. Such a command line consists of two commands, with
possible parameters, separated with | symbol. The symbol | is called the pipe symbol.
Your shell will support use of only one pipe symbol in a command line, hence the
compound execution of two commands in a command line.
When a command line with pipe symbol is entered in your shell or read from the
input file, the main shell process with create two child processes. Two fork calls are
needed to do this. In each child we need to use the exec system call to execute the
respective program. The output of one program, that would normally go to screen,
i.e., to standard output, will now go as input to the second program, which would
normally receive the input from a user or from a file. This requires communication
(IPC) between these processes.
Your shell will provide communication between two child processes executing two
programs by use of Linux unnamed pipes. But unlike a normal Linux shell program
that creates a single pipe between two child processes directly so that the output of
one child process is fed directly as input to the other, your shell will use two pipes.
For this, your main process will create two pipes that can be called as pipe1 and
pipe2, for example. The output of the first child process will be directed to the first
pipe, from where your main process will read the incoming stream of characters.
Your main process will write those characters to the second pipe from where the
second child process will take the input. This is illustrated in Figure 1. Your main
process will read from pipe1 and write to pipe2 N characters at a time using the
read and write system calls. N is the value that is given as an argument to your
shell program when it is started. N can be between 1 and 4096.
Main process will create the pipes using the pipe() system call. Main process
will then create the child processes using the fork system call. Then exec system call
will be called in each child to run the respective program. Before calling exec, in each
child, I/O redirection will be done to direct the output or input. For the first child,
the output will not go to standard output anymore but will be redirected to pipe1.
For the second child, the input will not come from standard input anymore but will
be directed from the second pipe. This can be done by use of the dup2() system call.
Figure 1: Compound command execution and communication of child processes
through the parent.
It is used to duplicate a file descriptor. For the first child, the write end of the pipe1
will be duplicated to file descriptor 1. In this way whenever child 1 program would
access file descriptor 1, i.e., standard out descriptor, it would access the write end
of pipe1. Integer 1 is always the file descriptor corresponding to standard output.
This can be done by the statement dup2 (pipe1[1], 1) (see Figure 2, showing the
open file tables of parent and child processes before and after dup2 call). For the
second child, the read end of pipe2 will be duplicated to file descriptor 0. Integer 0
is always the file descriptor corresponding to standard input. This can be done by
the statement dup2 (pipe2[0], 0).
Figure 2: Effect of dup2 call.
After creating child processes, the main process will read from pipe1 and will
write to pipe2. Do not forget to close the unused ends of the pipes at the main
process. When a child terminates, the ends of the pipes that are used by the child
are closed automatically as well.
For compound command execution, since the main process is on the data flow
path from one chid process to the other, it can keep some statics about the trans3
ferred data. Count the number of bytes transferred through the pipes for compound
commands. Count also the number of read/write operations performed from and to
pipes. Print the count to screen after compound command execution has finished.
The output format should be as in the following example:
character-count: 15000
read-call-count: 15000
Now do the following simple experiment. Assume we are wondering about the effect
of N on the performance of a compound command execution. Write two simple
programs “producer M” and “consumer M” as commands to be compounded. When
separately executed, the producer will just print M random alphanumeric characters
to screen one by one or N characters at a time. You can do this again by using
the write system call where file descriptor is standard out, i.e., 1. And consumer
will just read M characters from standard input one by one or N characters at
time. Now run these programs in a compound fashion in your shell and measure
the time it takes. That means execute the following command, for example, from
an input file in your shell: “producer 1000000 | consumer 1000000”. Using the time
command, measure the running time of your shell executing just this compound
command from a file. Repeat this for various values of N and for a big enough M.
Report the results in a report.pdf file and try to comment on them.
Submit a pdf file as your report discussing your experiments. Your report will
include the results, your interpretations and conclusions, and also the code of the
simple programs that your have written. Put your report.pdf file and all other files
(bilshell.c and a Makefile and a README.txt file) into a directory named with your
ID. Then tar and gzip the directory. For example a student with ID 21404312 will
create a directory named 21404312 and will put the files there. Then he will tar the
directory (package the directory) as follows:
tar cvf 21404312.tar 21404312
Then he will gzip the tar file as follows:
gzip 21404312.tar
In this way he will obtain a file called 21404312.tar.gz. Then he will upload this file
in Moodle.
Late submission will not be accepted (no exception). A late submission will get
0 automatically (you will not be able to argue it). Make sure you make a submission
one day before the deadline. You can then overwrite it.
Tips and Clarification
• Will be posted to course website and can be sent via piazza as well.