[英]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));
}
}
运行shell程序时,输入ls或cat之类的命令,该命令将正确执行。 如果我在那条命令之后运行另一条命令,该命令将根本不会执行。 它甚至可能与我输入的第一个命令相同,并且仍然不会第二次运行。 没有给出错误信息。 它只是不输出命令调用它要输出的内容。 我当前正在使用execvp,并且我相信当前正在传递参数,因此我不确定从这里开始。 有人能给我一些有关为什么发生的信息吗?
您认为合适的地方(在主循环中对我有用,尽管我将设计权留给您):
numTokens = 0;
您无需重置在tokenize函数中增加的令牌数。 使用全局变量可以轻松错过此类小事情。
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