[SOLVED] Csci 4100 – assignment 5 – writing a simple shell
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Required Reading
Linux – Chapter 5For this programming assignment you are going to implement a simple shell called turtle in C. This shell
will not be able to implement pipes or redirects, but it will be able to execute any Linux command with any
number of command line arguments.The shell should repeatedly prompt the user for a command then execute that command, unless the user
types exit in which case the shell should exit sucessfully (using exit(0)), or the user enters only whitespace
in which case the shell should prompt the user again.If the user does not type exit or a blank line the shell should break up the line of text the user entered into
an array of C-strings.
Here is a sample of what running your shell might look like:
[colemann@apmycs2 ~]$ ./turtle
> ls
colemanAssign5.c turtle
> ls -l
total 32
-rw-r–r– 1 colemann staff 2146 Sep 14 16:04 colemanAssign5.c
-rwxr-xr-x 1 colemann staff 9076 Sep 14 16:05 turtle
>
> fnord
turtle: command fnord not found.
> exit
[exiting turtle shell]
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I have provided the following functions to get you started:
• read_line(line) – reads a line of text from input and stores it in a string.
• is_blank(str) – returns true (1) if a string contains only whitespace, false (0) otherwise.
• parse_args(cmd, argv) – takes a command line and breaks it up into a sequence of strings containing
the command and the arguments, then stores the strings in the array provided.
I have also defined the following constants using the #define preprocessor statement:• MAX_LINE – the maximum number of characters allowed in a single line. Use this as the size when you
declare a character array to represent the string to pass to read_line.
• MAX_ARGS – the maximum number of arguments (including the command) that may be used in a
command line. Use this as the size when you declare an array of C-strings to represent the array of
arguments to pass to parse_args.To execute a command you must do the following:
• Use the fork function to fork off a child process. The fork function takes no arguments and returns a
value of type pid_t that can be treated like an integer value. WARNING: be very careful with using
fork in a loop. If you are not careful you will fork too many processes for your shell to handle.• Check to see if the current process is the child or the parent by checking the return value of fork. If
the return value is 0, the current process is the child process. Otherwise, it is the parent process.
• The parent process should call the wait function with a single argument of NULL to wait for the child
to complete.
• The child process should call the execvp function with two arguments:
– The name of the command being executed (argument 0 of the array.)
– The entire array of command line arguments.If execvp succeeds, the process will no longer be running the shell code. This means that the next line is
executed only when execvp fails. If this is the case, display an error message and terminate the child process
by calling exit(1).
Displaying Output to the Console
In previous assignments I suggested using puts to display output to the console.
puts ( ” Hello World ! n ” );
If you want to insert a C-string into your output you can use the printf function with the special format
code %s and provide the string as an extra argument:
printf ( ” Hello % s ! n ” , name );
We also used the fputs function with the stream stderr to write to standard error:
fputs ( stderr , ” Something went wrong . n ” );
If you want to insert a C-string into your error message you can use fprintf:
fprintf ( stderr , ” Something went wrong with % s . n ” , str );
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String Comparison
When you are checking to see if the user has entered the exit command it may be tempting to do something
like this:
i f ( line == ” exit ” )
// display message and exit program
This will not work because the == operator compares the addresses of the strings, not the contents of the
strings.
Instead you will need to use the strcmp function. This function takes two strings as arguments and returns
zero if they are the same:
i f ( strcmp ( str1 , str2 ) == 0)
puts ( ” The strings are the same ” );
Compiling Your Code
Your source file should be called yourlastnameAssign5.c except with your actual last name. To compile
this code you should use the gcc compiler on the Linux server. To create an executable file called turtle
use the following command:
gcc -o turtle yourlastnameAssign5.c
What to Hand In
Your source file should have comments at the top listing your name, CSCI 4100, Assignment 5, and a brief
explanation of what the program does. Download the source file to your local machine, then upload it to
D2L in the dropbox called Assignment 5.
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