Description
Building a Simple Shell (Based on Nutt, Kernel Projects for Linux,
Addison-Wesley, 2001, Lab #2), or Simple Programming with Files, Processes
and Pipes.
For this assignment you will implement your own Shell or Command Line
Interpreter (e.g. to replace /bin/bash for simple interactions with the Linux
Kernel). Your shell will be character-oriented, and will fork off processes to execute
user commands. Your shell should read lines of user input into a 256-byte buffer,
then parse and execute the commands (be sure to clear the buffer between
successive commands!) It should be possible for the user to specify the command to
execute by giving an absolute path to the file containing the executable (e.g. ./hw1);
or to use path expansion to locate the file containing the executable by using the
environment PATH variable to construct a series of absolute paths and executing
the first file found in this way (note that the execvp()command performs this
processing automatically, you do not need to program this yourself!) Your code
should parse the input string and separate it into a collection of sub-strings (stored
in myargv[]) along with a count of the number of strings encountered (stored in
myargc). Note that piped commands will require multiple argc/argv instances!
Your shell should support the following functions:
• Execute a single command with up to four command line arguments
(including command line arguments with associated flags). For example:
Myshell> ./ls –l
Myshell> cat myfile
Myshell> ls –al /usr/src/linux
• Execute a command in background. For example:
Myshell> ./ls -l &
Myshell> ls –al /usr/src/linux &
• Redirect the standard output of a command to a file. For example:
Myshell> ./ls -l > outfile
Myshell> ./ls -l >> outfile
Myshell> ls –al /usr/src/linux > outfile2
Myshell> ls –al /usr/src/linux >>outfile2
• Redirect the standard input of a command to come from a file. For example:
Myshell> grep disk < outfile Myshell> grep linux < outfile2 • Execute multiple commands connected by a single shell pipe. For example: Myshell> ls –al /usr/src/linux | grep linux
Suggested implementation strategy to implement a shell with multiple command
line arguments (using iterative refinement):
1. Implement shell0 to initialize variables and then go into an infinite loop
until stdin detects EOF (i.e. the user enters CTL-D). Each iteration through
theloop, the shell program should prompt the user for input, read the input &
echo it back to the user and then print out another prompt.
2. Implement shell1 by extending shell0 to parse the user’s line of input into a
series of strings that are stored into a char myargv ** array, along with a
count of the number of strings in an int myargc variable. Print out the
contents of myargv and myargc after you have parsed the comment. Allow
the user to enter an ‘exit’ command to terminate the shell (instead of typing
CTL-D and sending a kill signal to terminate execution).
3. Implement shell2 by extending shell1 to create a child process to execute the
command (passing in myargv/myargc as arguments) and then wait for the
child process to complete execution.
Submit all work done to your given repo. This assignment will be given a maximum
of 150 homework points.
Extra Credit: Implement your shell so that any combination of the shell functions
above can be used in a single command line. Please document the extra credit if you
choose to do this. This can be done inside the readme.txt.
