10. 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.
(2) After the -R
(3) 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 Thefiles: waitpid.c,redir.c,andexecvp.c,containcodeexamplesthatmayhelpyouin 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)
{
> , Use printf*/ more on next page
while (TRUE)
{
int childPid;
char *cmdLine;
|
print the prompt(); /* |
i.e. |
csc60mshell |
Page1of7.
CSC 60. Fall 2019. Lab 10. Write your own UNIX Shell, part 2.
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 *)
iftheargcis1
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” |
|
else |
/* fork off a process. This section was commented out for lab9. */ |
{
pid = fork();
switch(pid)
{
case -1:
perror(“Shell Program fork error”);
exit(1);
more on next page Page2of7.
CSC 60. Fall 2019. Lab 10. Write your own UNIX Shell, part 2.
case 0:
/* I am child process.
* I will execute the command, call: execvp */
process_input(argc, argv);
break;
default:
/* I am parent process */
if (wait(&status) == -1)
perror(“Shell Program error”);
else
printf(“Child returned status: %d\n”,status);
break;
} /* end of the switch */
} /* end of if-else-if that starts with EXIT
}
/* end of the while(TRUE)-loop
/* end of main
}
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
if (ret == -1)
error check and do _exit(EXIT_FAILURE)
}
(Example: ret)
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.
more pseudo code on next page
Page3of7.
CSC 60. Fall 2019. Lab 10. Write your own UNIX Shell, part 2.
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.
//endoftheif
// 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.
more on next page
Page4of7.
CSC 60. Fall 2019. Lab 10. Write your own UNIX Shell, part 2. Resources
Useful Unix System Calls: See PowerPoint Slides file named Lab10 Slides C Library functions:
if(strcmp(argv[0], “exit”) == 0)
strcmp(argv[0],”pwd”) strcmp(argv[0],”cd”) strcmp(….,”>”) strcmp(….,”<“)
print a system error message:
perror(“Shell Program error \n”);
//Sample. One line completed.
#include <string.h>
String compare:
int strcmp(const char *s1, const char *s2); //Function prototype from string.h
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
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.
gcc name-of-source-code –o name-of-executable
Marks Distribution
Lab 10 is worth 76 points.
more on next page Page5of7.
CSC 60. Fall 2019. Lab 10. Write your own UNIX Shell, part 2. 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.
Deliverables
Submit two files to Canvas:
1. lab10.c
2. YourName_lab10.txt
- Your program’s output test (with various test cases).
- Please use the UNIX script command to capture your program’s output.
- Details below. (Do not include lab10.c in this file)
More on next page.
Page6of7.
CSC 60. Fall 2019. Lab 10. Write your own UNIX Shell, part 2.
Preparing your script file:
Be located in csc60/lab10 directory.
When all is well and correct, type: script StudentName_lab10.txt
Attheprompt,type: gcclab10.c-Wall tocompilethecode type: a.out toruntheprogram
Enter in sequence:
1. 2. 3.
4. 5. 6. 7.
If you are on a team, cat your name file here.
ls > lsout cat lsout
ls > lsout > file1 cat foo.txt
> lsout
< lsout
// should work with output going to file // display the contents of the output file
// should produce an error // should produce an error // should produce an error // should produce an error
/* wc prints newline, word, and byte counts for each file */ //outputwillgotothescreen. //outputwillgotoafile
//displaytheoutput
//shouldproduceanerror
//movetolab1directory //showthattheexecworks //showoutputoflab1 //(exitfromtheshell) //(exitfromthescript)
wc<lsout wc<lsout>wcout
8.
9.
10. catwcout
11. wc<lsout<wcout
12. cd../lab1 13. gcclab1.c 14. a.out
15. exit
16. exit
When finished, submit your two files (the C file and the Script file) to Canvas.
(The script file will NOT contain the contents of lab10.c). No zip files please.
