16a17,22
> void eval_bg(int nwords, char *xword[]);
> void eval_builtin(int nwords, char *xword[]);
> void eval_echo(char *xword[]);
> void eval_quit(char *xword[]);
> void eval_nonbuiltin(int nwords, char *xword[]);
> void eval_wait(char *xword[]);
17a24
> void init (void);
19a27,33
> 
> struct ltstr {
>   bool operator()(const char *s1, const char *s2) const {
>     return strcmp(s1, s2) < 0;
>   }
> };
> 
23a38
> 			{ "foo",  0},
29a45
> 			{ "ps",    "-l",    0},
34a51,54
> pid_t	bg_pid;			// $! (internal format)
> char	bg_pid_str[10];		// $! (string format)
> Bool	is_done = NO;		// set to YES by quit command
> int	quit_status;		// exit status of shell (quit arg)
52a73,77
> static inline Bool is_bg(int nwords, char *xword[]) {	// YES if bg command
>   if (strcmp(xword[0], "&") == 0)	return YES;
>   else					return NO;
> }
> 
60a86,90
> static inline Bool
> is_defined(char *name) {	// YES if var name is "!"
>   return ( ( (strcmp(name, "!") == 0) ) ? YES : NO );
> }
> 
67d96
< 
78c107
<   for (int i=0; cmnd[i][0] != NULL; i++) {
---
>   for (int i=0; (cmnd[i][0] != NULL) && (!is_done); i++) {
80a110,111
>     if( debug )	fprintf(stderr, "========================== %s\n", cmnd[i][0] );
>     for (int j=0; cmnd[i][j] != 0; j++)	++nwords;
92c123
<       // ... eval builtin command ...
---
>       eval_builtin(nwords, xword);
95c126,127
<       // ... eval non-builtin command
---
>       if (is_bg(nwords, xword))	eval_bg(nwords, xword);
>       else			eval_nonbuiltin(nwords, xword);
101c133
<   return 0;
---
>   return quit_status;
105d136
< // --| Get command-line args |--
131d161
< 
142a173
>     char	*var_name;
144,145c175,183
<       DEBUG( (stderr, "\t\t+++ do_subst:  word %d is variable %s\n",
<     							i, word[i]) );
---
>       var_name = &(word[i][1]);
>       if (is_defined(var_name)) {	// lookup value
> 	DEBUG( (stderr, "\t\t+++ do_subst:  %s = <%s>\n",
> 							var_name, bg_pid_str) );
>         xword[i] = strdup(bg_pid_str);
>       } else {				// undefined var = ""
> 	DEBUG( (stderr, "\t\t+++ do_subst:  %s = UNDEF\n", var_name) );
>         xword[i] = strdup("");
>       }
148a187
>       xword[i] = strdup(word[i]);
150d188
<     xword[i] = strdup(word[i]);
172a211,303
> // --| evaluate non-builtin commands |--
> void
> eval_nonbuiltin(int nwords, char *xword[]) {
>   pid_t	pid;
>   int	status;
> 
>   DEBUG( (stderr, "*** eval_nonbuiltin called with:  ") );
>   DebugWords(xword);
> 
>   pid = Fork();
>   if( debug )	fprintf( stderr, "*** Syscall from pid %d:  Fork returned %d\n",
>   							getpid(), pid );
>   if (pid == 0) {		// child
>     Execvp(xword[0], xword);
>     exit(1);
>   } else {
>     DEBUG( (stderr, "*** eval_nonbuiltin:  child pid = %d\n", pid) );
>     Waitpid(pid, &status, 0);
>   }
> }
> 
> // --| evaluate backgrounded command |--
> void
> eval_bg(int nwords, char *xword[]) {
>   int	status;
> 
>   DEBUG( (stderr, "\t\t+++ eval_bg called\n") );
>   DebugWords(xword);
> 
>   bg_pid = Fork();
>   if( debug )	fprintf( stderr, "*** Syscall from pid %d:  Fork returned %d\n",
>   							getpid(), bg_pid );
>   if (bg_pid == 0) {		// child
>     Execvp(xword[1], &xword[1]);
>     exit(1);
>   }
>   sprintf(bg_pid_str, "%d", bg_pid);
> 
>   DEBUG( (stderr, "*** eval_bg:  child pid = %d (str: %s)\n",
>   							bg_pid, bg_pid_str) );
> }
> 
> // --| evaluate builtin commands |--
> 
> void
> eval_builtin(int nwords, char *xword[]) {
>   DEBUG( (stderr, "*** eval_builtin called with:  ") );
>   DebugWords(xword);
> 
>   if (strcmp(xword[0], "quit") == 0) {
>     eval_quit(xword);
>     is_done = YES;
>   } else if (strcmp(xword[0], "echo") == 0)	eval_echo(xword);
>   else if (strcmp(xword[0], "wait") == 0)	eval_wait(xword);
>   else {
>     // nothing for now
>   }
> }
> 
> void
> eval_echo(char *xword[]) {			// --| echo W ... |--
>   DEBUG( (stderr, "*** eval_echo called with:  ") );
>   DebugWords(xword);
>   show_words(&(xword[1]));
> }
> 
> void
> eval_quit(char *xword[]) {			// --| quit; quit W |--
>   DEBUG( (stderr, "*** eval_quit called with:  ") );
>   DebugWords(xword);
> 
>   if (xword[1] != NULL)	quit_status = atoi(xword[1]);
>   else			quit_status = 0;
> }
> 
> void
> eval_wait(char *xword[]) {			// --| wait W |--
>   int	status;
>   pid_t	pid = atoi(xword[1]);
>   int	rc;
> 
>   DEBUG( (stderr, "*** eval_wait called with:  ") );
>   DebugWords(xword);
> 
>   rc = waitpid(pid, &status, 0);
>   if (rc != pid) {	// bad pid
>     fprintf(stderr, "*** eval_wait:  waitpid(%d,...) returned %d\n", pid, rc);
>     exit(1);
>   } else if( debug ) {
>     fprintf(stderr, "*** eval_wait:  waitpid(%d,...) returned %d\n", pid, rc);
>   }
> }
>