Thanks to John Cummings.

src/cmd/upas/common/aux.c

@@ -0,0 +1,148 @@
+#include "common.h"
+
+/* expand a path relative to some `.' */
+extern String *
+abspath(char *path, char *dot, String *to)
+{
+	if (*path == '/') {
+		to = s_append(to, path);
+	} else {
+		to = s_append(to, dot);
+		to = s_append(to, "/");
+		to = s_append(to, path);
+	}
+	return to;
+}
+
+/* return a pointer to the base component of a pathname */
+extern char *
+basename(char *path)
+{
+	char *cp;
+
+	cp = strrchr(path, '/');
+	return cp==0 ? path : cp+1;
+}
+
+/* append a sub-expression match onto a String */
+extern void
+append_match(Resub *subexp, String *sp, int se)
+{
+	char *cp, *ep;
+
+	cp = subexp[se].s.sp;    /* jpc .sp -> .s.sp */
+	ep = subexp[se].e.ep;    /* jpc .ep -> .e.ep */
+	for (; cp < ep; cp++)
+		s_putc(sp, *cp);
+	s_terminate(sp);
+}
+
+/*
+ *  check for shell characters in a String
+ */
+static char *illegalchars = "\r\n";
+
+extern int
+shellchars(char *cp)
+{
+	char *sp;
+
+	for(sp=illegalchars; *sp; sp++)
+		if(strchr(cp, *sp))
+			return 1;
+	return 0;
+}
+
+static char *specialchars = " ()<>{};=\\'\`^&|";
+static char *escape = "%%";
+
+int
+hexchar(int x)
+{
+	x &= 0xf;
+	if(x < 10)
+		return '0' + x;
+	else
+		return 'A' + x - 10;
+}
+
+/*
+ *  rewrite a string to escape shell characters
+ */
+extern String*
+escapespecial(String *s)
+{
+	String *ns;
+	char *sp;
+
+	for(sp = specialchars; *sp; sp++)
+		if(strchr(s_to_c(s), *sp))
+			break;
+	if(*sp == 0)
+		return s;
+
+	ns = s_new();
+	for(sp = s_to_c(s); *sp; sp++){
+		if(strchr(specialchars, *sp)){
+			s_append(ns, escape);
+			s_putc(ns, hexchar(*sp>>4));
+			s_putc(ns, hexchar(*sp));
+		} else
+			s_putc(ns, *sp);
+	}
+	s_terminate(ns);
+	s_free(s);
+	return ns;
+}
+
+uint
+hex2uint(char x)
+{
+	if(x >= '0' && x <= '9')
+		return x - '0';
+	if(x >= 'A' && x <= 'F')
+		return (x - 'A') + 10;
+	if(x >= 'a' && x <= 'f')
+		return (x - 'a') + 10;
+	return -512;
+}
+
+/*
+ *  rewrite a string to remove shell characters escapes
+ */
+extern String*
+unescapespecial(String *s)
+{
+	String *ns;
+	char *sp;
+	uint c, n;
+
+	if(strstr(s_to_c(s), escape) == 0)
+		return s;
+	n = strlen(escape);
+
+	ns = s_new();
+	for(sp = s_to_c(s); *sp; sp++){
+		if(strncmp(sp, escape, n) == 0){
+			c = (hex2uint(sp[n])<<4) + hex2uint(sp[n+1]);
+			if(c < 0)
+				s_putc(ns, *sp);
+			else {
+				s_putc(ns, c);
+				sp += n+2-1;
+			}
+		} else
+			s_putc(ns, *sp);
+	}
+	s_terminate(ns);
+	s_free(s);
+	return ns;
+
+}
+
+int
+returnable(char *path)
+{
+
+	return strcmp(path, "/dev/null") != 0;
+}