CSC60- Lab 10 Solved

Write your own UNIX Shell, part 2. 

Student will work with process management and some basic system calls. Important note:  please use sp1, sp2, sp3, or atoz servers for this lab.

UNIX Shell

In Lab9 we did the 3 built-in commands:  cd, pwd, exit.

Now we need to implement: a fork, an exec, and code to handle redirection.

 FILES TO COPY-   Different    Instructions this time :

To get the file you need, first move to your class folder by typing:  cd csc60 Type: cp  -R  /gaia/home/faculty/bielr/files_csc60/lab10   .

Spaces needed:  (1) After the cp                                                ↑ Don’t miss the space & dot.

After the -R
After the directory name at the end & before the dot.You have now created a lab10 directory and copied in three sample files:
execvp.c, redir.c, waitpid.c

Stay in directory csc60, and you need to:

type:  chmod 755 lab10 type:  cp  lab9/lab9.c  lab10/lab10.c

We have copied lab9 code and renamed it to lab10.c for you to start work on it.

Next move to lab10 directory by typing: cd lab10                and type:  chmod  644  * This will set permissions on all the files.

Your new lab10 directory should now contain:  lab10.c, waitpid.c, redir.c, and execvp.c The files: waitpid.c, redir.c, and execvp.c, contain code examples that may help you in completing Lab10.

A lot of code to be used in Lab10 is currently commented out.

Use the file lab9-10 RemoveCommentsGuide.docx (on Canvas) to guide you to properly remove the extra comments…without removing Every Comment!

Pseudo Code  (Yellow highlight indicates the code from Lab9.)

/*----------------------------------------------------------*/ int main (void)  

{

     while (TRUE)

    {

int childPid; char *cmdLine;

             print the prompt();      /* i.e. csc60mshell > , Use printf*/

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fgets(cmdline, MAXLINE, stdin);

/* You have to write the call. The function itself is provided: function parseline */ Call the function parseline, sending in cmdline & argv, getting back argc

/* code to print out the argc and the agrv list to make sure it all came in. Required.*/

Print a line. Ex: “Argc = %i” loop starting at zero, thru less than agrc, increment by one.       print each argv[loop counter] [("Argv %i = %s \n", i, argv[i]);]

/* Start processing the built-in commands */ if ( argc compare equal to zero)

       /* a command was not entered, might have been just Enter or a space&Enter */        continue to end of while(TRUE)-loop

// next deal with the built-in commands

//   Use strcmp to do the test

//   after each command, do a continue to end of while(TRUE)-loop if (“exit”)       issue an exit call else if (“pwd”)

      declare a char variable array of size MAX_PATH_LENGTH to hold the path       do a getcwd       print the path else if (“cd”)

     declare a char variable dir as a pointer (with an *)       if the argc is 1

use the getenv call with “HOME” and return the value from the call to variable dir

      else
 
variable dir gets assigned the value of argv[1]
      execute a call to chdir(dir) with error checking. Message = “error changing directory”

void process_input (int argc, char **argv)                            // Child Process

{          

call handle_redir passing it argc and argv

call execvp passing in argv[0] and argv and return a value to an integer variable

                (Example:  ret)

if (ret == -1) error check and do  _exit(EXIT_FAILURE)

}

void handle_redir(int argc, char *agrv[])                              // Child Process

{  

You need two integer variables to keep track of the location in the string of the redirection symbols, (one for out_loc (>), one for in_loc (<) ). Initialize them to zero.

for loop from 0 to < argc

                if ( “>” == 0)                 // use strcmp function

if out_loc not equal 0

Cannot output to more than one file.  fprint error. _exit failure.

else if loop_counter compares equal 0

No command entered. print error. _exit failure.

set out_loc to the current loop_counter.

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else if (“<” == 0)          // use strcmp function if (in_loc not equal 0)

Cannot input from more than one file.  print error. _exit failure.

else if loop_counter compares equal 0

No command entered. print error. _exit failure.

set in_loc to the current loop_counter.

// end of the if // end of the for loop

if(out_loc != 0)  if argv (indexed by out_loc +1) contains a NULL

 There is no file, so print an error, and _exit in failure.

 Open the file using name from argv, indexed by out_loc+1,                           and assign returned value to fd.  [See 9-Unix, slides 6-10]                 use flags:  to read/write; to create file if needed;             to truncate existing file to zero length

                             use permission bits for:  user-read; user-write

              Error check the open. perror &  _exit

 Call dup2 to switch standard-out to the value of the file descriptor.

              Close the file

 Set things up for the future exec call by setting argv[out_loc] to NULL        // end of if(out_loc != 0)

if(in_loc != 0) if argv (indexed by in_loc +1) contains a NULL

There is no file, so print an error, and _exit in failure.

Open the file using name from argv, indexed by in_loc+1                  and assign returned value to fd.  use flags; for read only

              Error check the open. perror & _exit                

 Call dup2 to switch standard-in to the value of the file descriptor.

              Close the file

 Set things up for the future exec call by setting argv[in_loc] to NULL

       //end of if(in_loc != 0)

Word of warning:  In the past many students have duplicated the code for OpenOutputFile to be used for OpenInputFile.  If you do that, lots of little things need to be changed.  Be careful.

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Resources

Useful Unix System Calls:    See PowerPoint Slides file named Lab10 Slides C Library functions:

#include <string.h> String compare:

     int strcmp(const char *s1, const char *s2); //Function prototype from string.h

    

     if(strcmp(argv[0], "exit") == 0)         //Sample. One line completed.      strcmp(argv[0],"pwd")      strcmp(argv[0],"cd")      strcmp(….,">")      strcmp(….,"<")

print a system error message:

     perror("Shell Program error \n");
Compilation & Building your program

The use of gcc is just fine.  If you want to have the executable with a different name than a.out, type: gcc –o  name-of-executable  name-of-source-code

or gcc  name-of-source-code  –o  name-of-executable 

Partnership

Students may form a group of 2 students (maximum) to work on this lab.  As usual, please always contact your instructor for questions or clarification.  Your partner does not have to attend the same section.

All code files should include both names.

Using vim, create a small name file with both of your names in it. When you start your script file, cat that name file so both names show up in the script file.

You must BOTH submit your effort.  As both of your names occur on everything, when I or another grader find the first submission, we will then give the same grade to the second student.

Marks Distribution

Lab 10 is worth 76 points. 

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Hints

Our compiler does not like:  for (int i = 0; …..)

You will receive the following errors:

test_loopcounter.c:6: error: ‘for’ loop initial declarations are only allowed in C99 mode test_loopcounter.c:6: note: use option -std=c99 or -std=gnu99 to compile your code These errors imply that on every “gcc” line, you must add:  -std=c99   OR  -std=gnu99.

It does like it on two lines:

int i;

for (i = 0; ……)

 

Keep versions of your code. This is in case you need to go back to your older version due to an unforeseen bug/issue.

A lot of code to be used in Lab10 was commented out.

Use the file lab9-10 RemoveCommentsGuide.docx to guide you to remove a set of the extra comments…without deleting Every Comment.