c - shell后台进程

Question

我想实现自己的 shell。我坚持执行后台进程。实际上，我写了一些代码，但我不确定它是否有效。

我的代码:

#include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>

 #define MAX_LINE 80 /* 80 chars per line, per command, should be enough. */
  #define HISTORY_SIZE 5 /*keep track of 5 most recent commands*/

 int cmd_count; /*global to keep track of most recent commands entered*/
 char history[HISTORY_SIZE][MAX_LINE]; /* global so it can be         accessed     in interrupt handler. */


 void viewHistory()
{
int i;

if (cmd_count < 1)
    printf("No command history to show. \n");
else {
    printf("\n\n");
    for (i = (cmd_count >= HISTORY_SIZE) ? cmd_count - HISTORY_SIZE:0;
         i < cmd_count; i++)
        printf("%d: %s\n",i+1,history[i%HISTORY_SIZE]);
}
//printf("SystemsIIShell->");
  }



  int setup(char inputBuffer[], char *args[],int *background)
   {
int length, /* # of characters in the command line */
i,      /* loop index for accessing inputBuffer array */
start,  /* index where beginning of next command parameter is */
ct;     /* index of where to place the next parameter into args[] */

int temp;

ct = 0;

/* read what the user enters on the command line */
length = read(STDIN_FILENO, inputBuffer, MAX_LINE);

start = -1;
if (length == 0)
    exit(0);            /* ^d was entered, end of user command stream */
if (length < 0){
    perror("error reading the command");
    exit(-1);           /* terminate with error code of -1 */
}else{
    inputBuffer[length]='\0';
    if(inputBuffer[0]=='r'){
        if(inputBuffer[1]=='r'){
            if(cmd_count==0){
                printf("No recent command can be found in the history. \n");
                return 0;
            }
            strcpy(inputBuffer,history[(cmd_count)% HISTORY_SIZE]);
        }else{
            temp = atoi(&inputBuffer[1]);
            if(temp < 1 || temp > cmd_count || temp <= cmd_count -HISTORY_SIZE){
                printf("Command number cannot be found. \n");
                return 0;

            }
            strcpy(inputBuffer,history[(temp-1)%HISTORY_SIZE]);

        }
        length = strlen(inputBuffer);

    }
    cmd_count++;
    strcpy(history[(cmd_count-1)%HISTORY_SIZE], inputBuffer);
    for (i = 0; i < length; i++) {
        if (inputBuffer[i] == '&') {
            inputBuffer[i] = '\0';
            *background = 1;
            --length;
            break;
        }

    }
}

/* examine every character in the inputBuffer */
for (i = 0; i < length; i++) {
    switch (inputBuffer[i]){
        case ' ':
        case '\t' :               /* argument separators */
            if(start != -1){
                args[ct] = &inputBuffer[start];    /* set up pointer */
                ct++;
            }
            inputBuffer[i] = '\0'; /* add a null char; make a C string */
            start = -1;
            break;

        case '\n':                 /* should be the final char examined */
            if (start != -1){
                args[ct] = &inputBuffer[start];
                ct++;
            }
            inputBuffer[i] = '\0';
            args[ct] = NULL; /* no more arguments to this command */
            break;

        case '&':
            *background = 1;
            inputBuffer[i] = '\0';
            break;

        default :             /* some other character */
            if (start == -1)
                start = i;
    }
}
args[ct] = NULL; /* just in case the input line was > 80 */
  }

   int main(void)
   {
char inputBuffer[MAX_LINE]; /* buffer to hold the command entered */
int background;             /* equals 1 if a command is followed by '&' */
char *args[MAX_LINE/2+1];/* command line (of 80) has max of 40 arguments */

while (1){            /* Program terminates normally inside setup */
    background = 0;
    printf("SystemsIIShell->");
    fflush(0);
    setup(inputBuffer, args, &background);       /* get next command */

    pid_t child; /* process id for child */
    int status; /* status for execvp */

    child = fork(); /* create a child process*/

    if(child < 0){ /* if the child process didn't return 0, the fork is failed */
        printf("Fork failed! \n");

    }else if(child==0){ /* child process */
        if(inputBuffer[0]=='history' || inputBuffer[0] =='h'){
            viewHistory();
            return 0;
        }
        status = execvp(args[0],args);
        if(status !=0){
            printf("%s: command not found. \n", args[0]);
        }

    }else{ /* parent process */
        if(background == 0)
            waitpid(child,&background,0);

    }

    /* the steps are:
     (1) fork a child process using fork()
     (2) the child process will invoke execvp()
     (3) if background == 0, the parent will wait,
     otherwise returns to the setup() function. */

}return 0;

我没有添加完整的代码，但其他的都是真的。我调用 execv 并且它有效。当我在控制台上写的时候:

输出端:

$ gedit  ------->it works correctly because it is a foreground


$ gedit & -----> it opens a gedit file which name is "&"
$ firefox ---> it works correctly
$ firefox & ---> it opens a firefox window which url is www.&.com 

如何解决？有什么建议吗？

编辑部分:https://github.com/iqbalhasnan/CSE2431-System-II/blob/master/lab2/lab2.c --> 我用这段代码作为引用

Answer 1

你不是在执行后台进程，你是在尝试使用计算机上已经实现的 shell 的语法启动后台进程(但老实说，很难说出缩进是怎么回事。那是真的很糟糕。你能让它变得可读吗？)。 '&' 字符由您的 shell 识别，而不是由 execvp 识别。看看this similar looking question这是针对您的问题的谷歌搜索中的第一个匹配项。