Sherbs Sherbs - 7 months ago 44
Bash Question

Why does execvp only work for the first command entered in my shell?

#include <stdio.h>
#include <string.h>
#include <cstring>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>

char input[100];
#define DELIMITERS " \t\n"
char * tokens[100];
int numTokens = 0;
int i = 0;
int n = 0;
char * cmd;

// function prototype
void nonBuiltin(char * inputs[]);
void handlePipe(char * cmds[]);

// this breaks the input into tokens and returns them
char * tokenize(char input[100])
{
n = 0;
// grabs input and separates by delimiters until the char is null
for(cmd = strtok(input, DELIMITERS); cmd; cmd = strtok(NULL, DELIMITERS))
{
if(n >= 100)
{
break;
}
tokens[n++] = cmd;
numTokens++;
}

for(size_t i = 0; i != n; i++)
{
printf("Tokens %zu is %s\n", i, tokens[i]);
}
return * tokens;
}

// handles the commands you enter
// checks if builtin or non builtin command
// if builtin, execute command
// else, send command to nonbuiltin function
void handleCommands(char * inputs)
{
// changes directory
if(strcmp(tokens[0], "cd") == 0)
{
int change = chdir(tokens[1]);
if(change == 0)
{
chdir(tokens[1]);
}
else
{
perror("Cannot find path specified...\n");
}
}
// prints working directory
else if(strcmp(tokens[0], "pwd") == 0)
{
printf("Your current working directory is: %s\n", getenv("PWD"));
}
// implements set command
else if(strcmp(tokens[0], "set") == 0)
{
// do this
}
// exits program
else if(strcmp(tokens[0], "exit") == 0)
{
printf("Now exiting...\n");
exit(3);
}
// input contains a nonbuiltin command
// sends input to nonbuiltin function
else
{
printf("Your command is a nonbuilt-in command\n");
nonBuiltin(tokens);
}
}

void nonBuiltin(char * inputs[])
{
int fp;
int status;

// create a new process
pid_t pid;
pid = fork();
// process creation was unsuccessful
if(pid < 0)
{
perror("Fork unsuccessful...\n");
exit(-1);
}
// child process
else if(pid == 0)
{
// printf("Fork successful...\n");
if(numTokens >= 3)
{
for(int i = 0; i < numTokens; i++)
{
if(strcmp(tokens[i], "<") == 0)
{
tokens[i] = tokens[i+1];
fp = open(tokens[i], O_RDONLY, 0);
dup2(fp, 0);
execvp(tokens[0], tokens);
}
else if(strcmp(tokens[i], ">") == 0)
{
tokens[i] = tokens[i+1];
fp = open(tokens[i], O_WRONLY | O_CREAT | O_TRUNC, S_IWUSR | S_IRUSR | S_IRGRP | S_IWGRP);
dup2(fp, 1);
close(fp);
tokens[i]= NULL;
execvp(tokens[0], tokens);

}
else if(strcmp(tokens[i], "|") == 0)
{
tokens[i] = tokens[i+1];
handlePipe(tokens);
}
}
}

else
{
// printf("Nothing special as far as commands go\n");
execvp(tokens[0], tokens);


}
}
// wait for child process to end
else
{
waitpid(-1, &status, WUNTRACED);
printf("the child process has now terminated\n");
// exit(0);
}

}


void handlePipe(char * cmds[])
{
printf("Initiate piping\n");
// file descriptors
int fd[2];
// fd[0] is the read end of pipe
// fd[1] is the write end of pipe
pipe(fd);

if(!fork())
{
// first close the write end
close(1);
// fd[1] can now take stdout
dup(fd[1]);
close(fd[0]);
execvp(cmds[0], cmds);

}
else
{
// close read end
close(0);
// fd[1] can now take stdin
dup(fd[0]);
close(fd[1]);
execvp(cmds[1], cmds);
}


}

// starts the program and asks the user to enter input
// loops until the user enters 'exit'
int main()
{
while(1)
{
printf("Enter a command...\n");
fgets(input, 100, stdin);
handleCommands(tokenize(input));
}
}



When I run my shell program, I enter in a command, like ls or cat, and
the command executes properly. If I run another command after that
one, the command will not execute at all. It could even be the same
command as the first one I entered, and it will still not run for the
second time. No error messages are given. It just doesn't output what
the command calls it to output. I'm currently using execvp, and I believe I am passing in the arguments currently, so I'm not sure where to go from here. Could someone give me some input on
why this is happening?

Answer

Wherever you feel is appropriate (it worked for me in the main loop, although I'll leave the design up to you):

numTokens = 0;

You aren't resetting the number of tokens which you increment in the tokenize function. Using a global variable makes it easy to miss little things like this.

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