mysh.c

#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#define MAX_LINE 80 /* 80 chars per line, per command, should be enough. */
void setup(char[], char *[], int *); 
/** 
*  * setup() reads in the next command line, separating it into distinct tokens 
*  * using whitespace as delimiters. setup() sets the args parameter as a 
*  * null-terminated string. 
*  */ 
void 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[] */
    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 */
    } 
    /* 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; 
        default:             /* some other character */
            if (start == -1) 
                start = i; 
            if (inputBuffer[i] == '&'){ 
                *background = 1; 
                inputBuffer[i] = '\0'; 
                start = -1; 
            } 
        } 
    } 
    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,pid, staus;             /* equals 1 if a command is followed by '&' */
    char *args[MAX_LINE / +1];/* command line (of 80) has max of 40 arguments */ 

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

        if (pid == 0) 
        { 
            execvp(args[0], &args[0]);
        } 
        if (background == 0) {
            waitpid(pid, &staus, 0); 
        } else { 
            setup(inputBuffer, args, &background); 
        } 
         
        if (args == exit) 
            exit(1); 
    } 
}