commit 6c17f630901eec2a4b54b70748d7fbc9b47eecd8
from: Russ Cox <rsc@swtch.com>
date: Tue Jan 14 00:20:34 2020 UTC

mk: treat X= as empty list in rc shell

This brings mk's behavior when using rc in line with Plan 9's.
The existing code is for Unix environment data structures but
also was assuming Unix shell semantics where empty and missing
variables are mostly equivalent.

The Plan 9 code (/sys/src/cmd/mk/plan9.c in the distribution)
explicitly removes /env/name (creating an empty list) when the
value is missing or an empty string.

Fixes #255.

commit - 9962d916e88f66014f1008d4356a2d395ae8d31b
commit + 6c17f630901eec2a4b54b70748d7fbc9b47eecd8
blob - 37f05b717f5f54e3cfbc4d5e9fe750352f0b2531
blob + 66bdb1ffbffe4fa87fa951ae6216cb7907d29c0b
--- src/cmd/mk/unix.c
+++ src/cmd/mk/unix.c
@@ -53,20 +53,26 @@ readenv(void)
 void
 exportenv(Envy *e, Shell *sh)
 {
-	int i;
+	int w, n;
 	char **p;
+	Envy *e1;
 	static char buf[16384];
 
-	p = 0;
-	for(i = 0; e->name; e++, i++) {
-		p = (char**) Realloc(p, (i+2)*sizeof(char*));
+	n = 0;
+	for(e1 = e; e1->name; e1++)
+		n++;
+	p = Malloc((n+1)*sizeof(char*));
+	w = 0;
+	for(; e->name; e++) {
+		if(sh == &rcshell && (e->values == 0 || e->values->s == 0 || e->values->s[0] == 0))
+			continue; /* do not write empty string for empty list */
 		if(e->values)
 			snprint(buf, sizeof buf, "%s=%s", e->name,  wtos(e->values, sh->iws));
 		else
 			snprint(buf, sizeof buf, "%s=", e->name);
-		p[i] = strdup(buf);
+		p[w++] = strdup(buf);
 	}
-	p[i] = 0;
+	p[w] = 0;
 	environ = p;
 }