rc: implement 9atoms `sep{command} syntax

After a bunch of experience using this, I
find that it  makes a good deal of code
far simpler to write. For example,

	nl='
	'

	for(f in `$nl{find . -type f})
		munge $f

ends up quite a bit more pleasant to use
for handling file output line by line,
since it doesn't cause scoping issues
for rc functions, and is more explicit
about what is being split on.

man/man1/rc.1

@@ -228,6 +228,8 @@ The value is a single string containing the components of the named variable
 separated by spaces.  A variable with zero elements yields the empty string.
 .HP
 .BI `{ command }
+.HP
+.BI ` "split " { command }
 .br
 .I rc
 executes the
@@ -240,6 +242,8 @@ If
 .B $ifs
 is not otherwise set, its value is
 .BR "'\ \et\en'" .
+In the second form of the command, split is used instead of
+.BR $ifs .
 .HP
 .BI <{ command }
 .HP

src/cmd/rc/code.c

@@ -141,10 +141,20 @@ outcode(tree *t, int eflag)
 		emitf(Xconc);
 		break;
 	case '`':
+		emitf(Xmark);
+		if(c0){
+			outcode(c0, 0);
+			emitf(Xglob);
+		}else{
+			emitf(Xmark);
+			emitf(Xword);
+			emits(strdup("ifs"));
+			emitf(Xdol);
+		}
 		emitf(Xbackq);
 		if(havefork){
 			p = emiti(0);
-			outcode(c0, 0);
+			outcode(c1, 0);
 			emitf(Xexit);
 			stuffdot(p);
 		} else

src/cmd/rc/exec.c

@@ -200,7 +200,7 @@ main(int argc, char *argv[])
  * Xappend(file)[fd]			open file to append
  * Xassign(name, val)			assign val to name
  * Xasync{... Xexit}			make thread for {}, no wait
- * Xbackq{... Xreturn}			make thread for {}, push stdout
+ * Xbackq(split){... Xreturn}		make thread for {}, push stdout
  * Xbang				complement condition
  * Xcase(pat, value){...}		exec code on match, leave (value) on
  * 					stack

src/cmd/rc/havefork.c

@@ -115,12 +115,14 @@ Xbackq(void)
 	int c, n;
 	char *s, *ewd=&wd[8192], *stop, *q;
 	struct io *f;
-	var *ifs = vlook("ifs");
 	word *v, *nextv;
 	int pfd[2];
 	int pid;
 	Rune r;
-	stop = ifs->val?ifs->val->word:"";
+
+	stop = "";
+	if(runq->argv && runq->argv->words)
+		stop = runq->argv->words->word;
 	if(pipe(pfd)<0){
 		Xerror("can't make pipe");
 		return;
@@ -168,6 +170,7 @@ Xbackq(void)
 		}
 		closeio(f);
 		Waitfor(pid, 0);
+		poplist();	/* ditch split in "stop" */
 		/* v points to reversed arglist -- reverse it onto argv */
 		while(v){
 			nextv = v->next;

src/cmd/rc/parse.c

@@ -501,8 +501,13 @@ word1(int tok, int *ptok)
 		return tree1(COUNT, word1(yylex(), ptok));
 
 	case '`':
-		// |	'`' brace		{$$=tree1('`', $2);}
-		t = tree1('`', brace(yylex()));
+		// |	'`' brace		{$$=tree1('`', nil, $2);}
+		// |	'`' word brace		{$$=tree1('`', $2, $3);}
+		w = nil;
+		tok = yylex();
+		if(tok != '{')
+			w = yyword(tok, &tok, 1);
+		t = tree2('`', w, brace(tok));
 		*ptok = yylex();
 		return t;
 

src/cmd/rc/pcmd.c

@@ -32,7 +32,7 @@ pcmd(io *f, tree *t)
 	break;
 	case '^':	pfmt(f, "%t^%t", c0, c1);
 	break;
-	case '`':	pfmt(f, "`%t", c0);
+	case '`':	pfmt(f, "`%t%t", c0, c1);
 	break;
 	case ANDAND:	pfmt(f, "%t && %t", c0, c1);
 	break;

src/cmd/rc/syn.y

@@ -83,7 +83,8 @@ comword: '$' word		{$$=tree1('$', $2);}
 |	'"' word		{$$=tree1('"', $2);}
 |	COUNT word		{$$=tree1(COUNT, $2);}
 |	WORD
-|	'`' brace		{$$=tree1('`', $2);}
+|	'`' brace		{$$=tree2('`', (struct tree*)0, $2);}
+|	'`' word brace		{$$=tree2('`', $2, $3);}
 |	'(' words ')'		{$$=tree1(PAREN, $2);}
 |	REDIRW brace		{$$=mung1($1, $2); $$->type=PIPEFD;}
 keyword: FOR|IN|WHILE|IF|NOT|TWIDDLE|BANG|SUBSHELL|SWITCH|FN