/* ----------------------------------------------------------------- */
/* PROGRAM shell.c */
/* This program reads in an input line, parses the input line */
/* into tokens, and use execvp() to execute the command. */
/* ----------------------------------------------------------------- */
#include <stdio.h>
#include <sys/types.h>
/* ----------------------------------------------------------------- */
/* FUNCTION parse: */
/* This function takes an input line and parse it into tokens. */
/* It first replaces all white spaces with zeros until it hits a */
/* non-white space character which indicates the beginning of an */
/* argument. It saves the address to argv[], and then skips all */
/* non-white spaces which constitute the argument. */
/* ----------------------------------------------------------------- */
void parse(char *line, char **argv)
{
while (*line != '\0') { /* if not the end of line ....... */
while (*line == ' ' || *line == '\t' || *line == '\n')
*line++ = '\0'; /* replace white spaces with 0 */
*argv++ = line; /* save the argument position */
while (*line != '\0' && *line != ' ' &&
*line != '\t' && *line != '\n')
line++; /* skip the argument until ... */
}
*argv = '\0'; /* mark the end of argument list */
}
/* ----------------------------------------------------------------- */
/* FUNCTION execute: */
/* This function receives a commend line argument list with the */
/* first one being a file name followed by its arguments. Then, */
/* this function forks a child process to execute the command using */
/* system call execvp(). */
/* ----------------------------------------------------------------- */
void execute(char **argv)
{
pid_t pid;
int status;
if ((pid = fork()) < 0) { /* fork a child process */
printf("*** ERROR: forking child process failed\n");
exit(1);
}
else if (pid == 0) { /* for the child process: */
if (execvp(*argv, argv) < 0) { /* execute the command */
printf("*** ERROR: exec failed\n");
exit(1);
}
}
else { /* for the parent: */
while (wait(&status) != pid) /* wait for completion */
;
}
}
/* ----------------------------------------------------------------- */
/* The main program starts here */
/* ----------------------------------------------------------------- */
void main(void)
{
char line[1024]; /* the input line */
char *argv[64]; /* the command line argument */
while (1) { /* repeat until done .... */
printf("Shell -> "); /* display a prompt */
gets(line); /* read in the command line */
printf("\n");
parse(line, argv); /* parse the line */
if (strcmp(argv[0], "exit") == 0) /* is it an "exit"? */
exit(0); /* exit if it is */
execute(argv); /* otherwise, execute the command */
}
}
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