/* mush.c
 * by Douglas Jones
 * a Minimally Usable SHell
 * augmented with use of $PATH as the search path
 * augmented with redirection of standard input and output
 * 
 * comments added only in augmented code.
 */

/* Note:  the logic for envp was not immediately apparent and is
 * outside the scope of this assignment.  Getting it right is
 * needed to use this shell to launch cc and other utilities that
 * need to access the environment.
 */

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

char command[100];
char *argv[100];
char *infile;    /* pointer to input file name, if redirected */
char *outfile;   /* pointer to output file name, if redirected */

char *opath;     /* pointer to $PATH (the original) */
char *path;      /* a copy of the path */
char *patha[100];/* NULL terminated array of pointers to path components */

char **myenvp;   /* the environment passed to the main program */
		 /* as documented in the execve man page */

void getpath(){  /* initialize patha from $PATH */
	/* the logic here is cribbed from parseargv */
	int i = 0; /* patha index */
	int j = 0; /* text index */
	opath = getenv( "PATH" );
	path = strcpy( malloc( strlen( opath ) + 1 ), opath );
	for (;;) {
		/* no need to skip blanks */
		if (path[j] == '\000') break;
		patha[i] = &path[j];
		i++;
		while ((path[j] != ':') /* path delimiter is colon */
		     &&(path[j] != '\000')) {
			j++;
		}
		if (path[j] == '\000') break;
		path[j] = '\000';
		j++;
	}
	patha[i] = NULL;
}

void getcommand(){
	char ch;
	int i = 0;
	putchar('>');
	do {
		ch = getchar();
		command[i] = ch;
		i++;
	} while (ch != '\n');
	command[i-1] = '\000';
}
void parseargv(){
	/* parse command arguments and find redirected file names, if any */
	int i = 0; /* argv index */
	int j = 0; /* text index */
	infile = NULL;    /* no redirection is the default */
	outfile = NULL;   /* no redirection is the default */
	for (;;) {
		while (command[j] == ' ') j++;
		if (command[j] == '\000') break;
		if (command[j] == '>' ) { /* found output redirection */
			j++; /* skip over the > and any blanks */
			while (command[j] == ' ') j++;
			if (command[j] != '\000') outfile = &command[j];

		} else if (command[j] == '<' ) { /* found input redirection */
			j++; /* skip over the < and any blanks */
			while (command[j] == ' ') j++;
			if (command[j] != '\000') infile = &command[j];

		} else { /* found a command line argument */
			argv[i] = &command[j];
			i++;
		}
		while ((command[j] != ' ')
		     &&(command[j] != '\000')) {
			j++;
		}
		if (command[j] == '\000') break;
		command[j] = '\000';
		j++;
	}
	argv[i] = NULL;
}

void launch(){
	if (fork() == 0) { /*child*/
		int i = 0; /* patha index */

		if (infile != NULL) { /* redirect input */
			int fd; /* file descriptor returned by open */
			close( 0 ); /* close standard input for reopening */
			fd = open( infile, O_RDONLY ); /* try to reopen */
			if (fd != 0) { /* gripe if redirect failed */
				fputs( "can't open ", stderr );
				fputs( infile, stderr );
				fputs( " for input\n", stderr );
				exit( EXIT_FAILURE );
			}
		}
		if (outfile != NULL) { /* redirect input */
			int fd; /* file descriptor returned by open */
			close( 1 ); /* close standard input for reopening */
			fd = open( outfile, /* try to reopen */
				   O_RDWR | O_CREAT | O_TRUNC,
				   S_IRUSR | S_IWUSR |
				   S_IRGRP | S_IWGRP | /* on file creation, */
				   S_IROTH | S_IWOTH   /* it is read/write */
				 );
			if (fd != 1) { /* gripe if redirect failed */
				fputs( "can't open ", stderr );
				fputs( outfile, stderr );
				fputs( " for output\n", stderr );
				exit( EXIT_FAILURE );
			}
		}
		/* note:  control only reaches this point if standard
		 * input and output remain well defined.  Failed redirection
		 * causes the child process to terminate
		 */
	
		execve( argv[0], argv, myenvp ); /* first, try it as itself */

		while (patha[i] != NULL) { /* try successive path prefixes */
			char filename[100];
			strncpy( filename, patha[i], 99 );
			filename[99] = '\000'; /* insurance */
			strncat( filename, "/", 99 - strlen( filename ) );
			strncat( filename, argv[0], 99 - strlen( filename ) );
			execve( filename, argv, myenvp ); /* try one prefix */
			i = i + 1;
		}

		printf("no such command\n");
		exit( EXIT_FAILURE ); /* a minor bug fix */
	} else { /*parent*/
		wait( NULL );
	}
}

main( int argc, char *argv[], char *envp[] ){
	myenvp = envp;
	getpath();
	for (;;) {
		getcommand();
		parseargv();
		launch();
	}
}