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Languages :: Java :: Unix shell
By: collegeBoy U.S.A.  Date: 05/09/2003 00:00:00  English  Points: 300 Status: Answered
Quality : Excellent
Im doing an assignment, programming in C, in Unix.
The task is, stimulating a shell.
It should accept any command entered.

I have done it, and so far it could run commands entered from the prompt.

However, the problems are:

1. No error returning back when an invalid command entered.

2. Another task is, the pipe operator. I dont know how to do this. Any1 know
how to make it accept pipes like in normal shell? For example:

command1 paratemeters | command 2 ....

Thanks for reading.

Alan.




-----------------------------------------
Here's the code at the moment:
-----------------------------------------

#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

int main();
void parent(int pid);
void child();

char command[100];

int main() {
int ret;

do {
printf("alan_ass2$");
fgets(command,100,stdin);

if ( strncmp(command,"logout\n",8) == 0 )
exit(1);
if ( strncmp(command,"\n",1) == 0 )
continue;

if( (ret=fork()) > 0 )
parent(ret);
else if ( ret == 0 )
child();
else
perror("fork() failed");
} while (1);

}

void parent(int pid)
{
int childs_exit_val;

/*
** The parent only invokes the wait() if the command is a foreground
command,
** i.e. there is no '&' at the end of the line
*/
}

void child()
{
char *argv[100]; /* 100 is arbitrarily long enough for this assignment
*/
char *tmp;
int cnt,i;

cnt = -1;
printf("\n");
tmp=strtok(command, "\n");
tmp=strtok(tmp, " ");
while ( tmp != NULL )
{
argv[++cnt]= tmp;
tmp=strtok(NULL," ");
}

if ((strncmp(argv[0],"cd",2)==0)&&(strlen(argv[0])==2))
argv[0]="chdir";

for (i=0; i<=cnt; i++) {
printf("argv[%d] = '%s'\n", i, argv);
}

argv[cnt+1]=NULL;

execvp(argv[0], argv);

/*
** If we arrive here, an error must have occurred otherwise a different
** program image would be active.
*/
perror("execvp");
exit(1); /* must be careful to exit a child process */
}
By: VGR Date: 06/09/2003 18:26:00 English  Type : Answer
1) I guess you coud test the return value of the FUNCTION call to execvp(argv[0], argv);

2) I suggest tokenizing/explode()-ing on charactr '|' and recursively calling your execution function by passing the previous stdout as stdin...
By: grg99 Date: 06/09/2003 23:57:00 English  Type : Comment
Funny thing about execvp-- on many a Unix system it's an undocumented quirk, the parameters have to be accessible even after you do the execvp!

That is, you can't return and deallocate your argv array. No idea why, except MAYBE the kernel just scheduls the exec, then returns control to your task (or exits it). So maybe try putting your argv array someplace where its going to b around for a while, like in static memory.

As for the vertical bar operator, that's a bit tricky, you have to fork off both programs, but with the stdout of #1 passed to the stdin of #2. See the pipe() and dup() man pages. Better yet, google for some example code, it's a tricky thing to get right, and hard to debug too!

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