/* mp3.c */

/*************************************************
 * A minimally usable shell                      *
 * Author:  Douglas W. Jones                     *
 * Revised by: Douglas W. Jones                  *
 *     as mp2.c, to conform to manual of C style *
 *     as mp3.c, to support the following:       *
 *         a) quotation marks around parameters  *
 *         b) pound is comment delimiter         *
 *         c) dollar sign environment variables  *
 *         d) builtin commands exit and setenv   *
 *         e) pass environment to launched app   *
 *************************************************/

#include <stdio.h>
#include <stdlib.h>
extern char **environ;		            /* e */

/* string terminator character */
#define NUL '\000'

/* maximum line length */
#define COMMANDLEN 100

/* maximum number of arguments, worst case: one char each separted by blanks */
#define ARGVLEN (COMMANDLEN / 2)

/* the command line, a character string, later a collection of strings
     initialized by getcommand
     modified by parseargv
     indirectly used by launch
*/
char command[COMMANDLEN];

/* the argument vector, an array of strings
     initialized by parseargv
   c modified by dollarsubs 
     used by launch
*/
char *argv[ARGVLEN];

void getcommand()
/* read one command line from standard input
   note: result returned in the global variable command
*/
{
	char ch;   /* one character from standard input */
	int i = 0; /* a count of the number of characters on the line */

	/* prompt for input */
	putchar('>');

	/* collect the input line, one character at a time */
	do {
		ch = getchar();
		if (i < COMMANDLEN) {
			command[i] = ch;
			i++;
		}
	} while (ch != '\n');

	/* replace the newline at the end with a string termiantor */
	command[i - 1] = NUL;
}

void parseargv()
/* parse the command line into a sequence of arguments
   note: result returned in the global variable argv
         chops the string stored in the global variable command into substrings
*/
{
	int i = 0; /* index of the current argument in argv */
	int j = 0; /* index of the current character in the command */

	for (;;) { /* iterate once per argument */

		/* look for the start of an argument */
		while (command[j] == ' ') j++;
		if (command[j] == NUL) break;
/* b */		if (command[j] == '#') break;

/* a */		if ((command[j] != '\'')
/* a */		&&  (command[j] != '"')) { /* regular parameter */
			/* mark the start of the argument */
			if (i < (ARGVLEN - 1)) {
				argv[i] = &command[j];
				i++;
			}

			/* find the end of the argument */
			while ((command[j] != ' ')
			     &&(command[j] != NUL)
/* b */			     &&(command[j] != '#')) {
				j++;
			}
			if (command[j] == NUL) break;
/* b */			if (command[j] == '#') {
/* b */				command[j] = NUL;
/* b */				break;
/* b */			}

/* a */		} else { /* quoted parameter */
/* a */			char quotemark = command[j];
/* a */			j++; /* skip over opening quote */
/* a */
/* a */			/* mark the start of the argument */
/* a */			if (i < (ARGVLEN - 1)) {
/* a */				argv[i] = &command[j];
/* a */				i++;
/* a */			}
/* a */
/* a */			/* find the end of the argument */
/* a */			while (command[j] != quotemark) {
/* a */				if (command[j] == NUL) {
/* a */					printf( "missing closing quote\n" );
/* a */					break;
/* a */				}
/* a */				j++;
/* a */			}
/* a */		}

		/* mark end of command */
		command[j] = NUL;
		j++;
	}

	/* mark the end of argv */
	argv[i] = NULL;
}

void dollarsubs()
/* handle dollar sign substitution in the global variable argv */
{
/* c */	int i = 0;
/* c */	char * arg;
/* c */	for (;;) { /* for each argument */
/* c */		arg = argv[i];
/* c */		if (arg == NULL) break;
/* c */
/* c */		if (arg[0] == '$') { /* argument begins with $ */
/* c */			char * v = getenv( &arg[1] );
/* c */			if (v != NULL) { /* there is such a variable */
/* c */				argv[i] = v;
/* c */			} else {
/* c */				printf( "no such environment variable\n" );
/* c */			}
/* c */		}
/* c */
/* c */		i++;
/* c */	}
}

void launch()
/* launch the application required to execute the command
   note:  uses the global variable argv for the command name and arguments
*/
{
/* d */	char * command = argv[0];
/* d */
/* d */	if ( argv[0] == NULL ) return; /* do nothing for blank lines */
/* d */
/* d */	if ( strcmp( command, "exit" ) == 0 ) { /* handle built in exit */
/* d */		if (argv[1] != NULL) {
/* d */			printf( "too many arguments\n" );
/* d */		}
/* d */		exit( EXIT_SUCCESS );
/* d */	}
/* d */
/* d */	if ( strcmp( command, "setenv" ) == 0 ) { /* handle built in setenv */
/* d */		char * name = argv[1];
/* d */		char * value = argv[2];
/* d */		if ((name == NULL) || (value == NULL)) {
/* d */			printf( "missing arguments\n" );
/* d */		} else {
/* d */			setenv( name, value, 1 );
/* d */		}
/* d */		return;
/* d */	}
/* d */
	if (fork() == 0) { /*child*/

		/* try to launch the application */
/* e */		execve( command, argv, environ );

		/* if launch fails, error message */
		printf( "no such command\n" );
		exit( EXIT_FAILURE );

	} else { /*parent*/

		/* wait for command to finish */
		wait( NULL );
	}
}

int main()
/* repeatedly read, parse and execute commands from standard input
   note:  communication between subsidiary routines is through global variables
*/
{
	for (;;) {
		getcommand();
		parseargv();
/* c */		dollarsubs();
		launch();
	}
}