diff --git a/src/cmd/idiff.c b/src/cmd/idiff.c
new file mode 100644
index 0000000..4732678
--- /dev/null
+++ b/src/cmd/idiff.c
@@ -0,0 +1,335 @@
+/*
+ * interactive diff, inspired/stolen from
+ * kernighan and pike, _unix programming environment_.
+ */
+
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+
+int diffbflag;
+int diffwflag;
+
+void copy(Biobuf*, char*, Biobuf*, char*);
+void idiff(Biobuf*, char*, Biobuf*, char*, Biobuf*, char*, Biobuf*, char*);
+int opentemp(char*, int, long);
+void rundiff(char*, char*, int);
+
+void
+usage(void)
+{
+	fprint(2, "usage: idiff [-bw] file1 file2\n");
+	exits("usage");
+}
+
+void
+main(int argc, char **argv)
+{
+	int fd, ofd;
+	char diffout[40], idiffout[40];
+	Biobuf *b1, *b2, bdiff, bout, bstdout;
+	Dir *d;
+
+	ARGBEGIN{
+	default:
+		usage();
+	case 'b':
+		diffbflag++;
+		break;
+	case 'w':
+		diffwflag++;
+		break;
+	}ARGEND
+
+	if(argc != 2)
+		usage();
+
+	if((d = dirstat(argv[0])) == nil)
+		sysfatal("stat %s: %r", argv[0]);
+	if(d->mode&DMDIR)
+		sysfatal("%s is a directory", argv[0]);
+	free(d);
+	if((d = dirstat(argv[1])) == nil)
+		sysfatal("stat %s: %r", argv[1]);
+	if(d->mode&DMDIR)
+		sysfatal("%s is a directory", argv[1]);
+	free(d);
+
+	if((b1 = Bopen(argv[0], OREAD)) == nil)
+		sysfatal("open %s: %r", argv[0]);
+	if((b2 = Bopen(argv[1], OREAD)) == nil)
+		sysfatal("open %s: %r", argv[1]);
+
+	strcpy(diffout, "/tmp/idiff.XXXXXX");
+	fd = opentemp(diffout, ORDWR|ORCLOSE, 0);
+	strcpy(idiffout, "/tmp/idiff.XXXXXX");
+	ofd = opentemp(idiffout, ORDWR|ORCLOSE, 0);
+	rundiff(argv[0], argv[1], fd);
+	seek(fd, 0, 0);
+	Binit(&bdiff, fd, OREAD);
+	Binit(&bout, ofd, OWRITE);
+	idiff(b1, argv[0], b2, argv[1], &bdiff, diffout, &bout, idiffout);
+	Bterm(&bdiff);
+	Bflush(&bout);
+	seek(ofd, 0, 0);
+	Binit(&bout, ofd, OREAD);
+	Binit(&bstdout, 1, OWRITE);
+	copy(&bout, idiffout, &bstdout, "<stdout>");
+	exits(nil);
+}
+
+int
+opentemp(char *template, int mode, long perm)
+{
+	int fd, i;
+	char *p;	
+
+	p = strdup(template);
+	if(p == nil)
+		sysfatal("strdup out of memory");
+	fd = -1;
+	for(i=0; i<10; i++){
+		mktemp(p);
+		if(access(p, 0) < 0 && (fd=create(p, mode, perm)) >= 0)
+			break;
+		strcpy(p, template);
+	}
+	if(fd < 0)
+		sysfatal("could not create temporary file");
+	strcpy(template, p);
+	free(p);
+
+	return fd;
+}
+
+void
+rundiff(char *arg1, char *arg2, int outfd)
+{
+	char *arg[10], *p;
+	int narg, pid;
+	Waitmsg *w;
+
+	narg = 0;
+	arg[narg++] = "/bin/diff";
+	arg[narg++] = "-n";
+	if(diffbflag)
+		arg[narg++] = "-b";
+	if(diffwflag)
+		arg[narg++] = "-w";
+	arg[narg++] = arg1;
+	arg[narg++] = arg2;
+	arg[narg] = nil;
+
+	switch(pid = fork()){
+	case -1:
+		sysfatal("fork: %r");
+
+	case 0:
+		dup(outfd, 1);
+		close(0);
+		exec("/bin/diff", arg);
+		sysfatal("exec: %r");
+
+	default:
+		w = wait();
+		if(w==nil)
+			sysfatal("wait: %r");
+		if(w->pid != pid)
+			sysfatal("wait got unexpected pid %d", w->pid);
+		if((p = strchr(w->msg, ':')) && strcmp(p, ": some") != 0)
+			sysfatal("%s", w->msg);
+		free(w);
+	}
+}
+
+void
+runcmd(char *cmd)
+{
+	char *arg[10];
+	int narg, pid, wpid;
+
+	narg = 0;
+	arg[narg++] = "/bin/rc";
+	arg[narg++] = "-c";
+	arg[narg++] = cmd;
+	arg[narg] = nil;
+
+	switch(pid = fork()){
+	case -1:
+		sysfatal("fork: %r");
+
+	case 0:
+		exec("/bin/rc", arg);
+		sysfatal("exec: %r");
+
+	default:
+		wpid = waitpid();
+		if(wpid < 0)
+			sysfatal("wait: %r");
+		if(wpid != pid)
+			sysfatal("wait got unexpected pid %d", wpid);
+	}
+}
+
+void
+parse(char *s, int *pfrom1, int *pto1, int *pcmd, int *pfrom2, int *pto2)
+{
+	*pfrom1 = *pto1 = *pfrom2 = *pto2 = 0;
+
+	s = strchr(s, ':');
+	if(s == nil)
+		sysfatal("bad diff output0");
+	s++;
+	*pfrom1 = strtol(s, &s, 10);
+	if(*s == ','){
+		s++;
+		*pto1 = strtol(s, &s, 10);
+	}else
+		*pto1 = *pfrom1;
+	if(*s++ != ' ')
+		sysfatal("bad diff output1");
+	*pcmd = *s++;
+	if(*s++ != ' ')
+		sysfatal("bad diff output2");
+	s = strchr(s, ':');
+	if(s == nil)
+		sysfatal("bad diff output3");
+	s++;
+	*pfrom2 = strtol(s, &s, 10);
+	if(*s == ','){
+		s++;
+		*pto2 = strtol(s, &s, 10);
+	}else
+		*pto2 = *pfrom2;
+}
+
+void
+skiplines(Biobuf *b, char *name, int n)
+{
+	int i;
+
+	for(i=0; i<n; i++){
+		while(Brdline(b, '\n')==nil){
+			if(Blinelen(b) <= 0)
+				sysfatal("early end of file on %s", name);
+			Bseek(b, Blinelen(b), 1);
+		}
+	}
+}
+
+void
+copylines(Biobuf *bin, char *nin, Biobuf *bout, char *nout, int n)
+{
+	char buf[4096], *p;
+	int i, m;
+
+	for(i=0; i<n; i++){
+		while((p=Brdline(bin, '\n'))==nil){
+			if(Blinelen(bin) <= 0)
+				sysfatal("early end of file on %s", nin);
+			m = Blinelen(bin);
+			if(m > sizeof buf)
+				m = sizeof buf;
+			m = Bread(bin, buf, m);
+			if(Bwrite(bout, buf, m) != m)
+				sysfatal("error writing %s: %r", nout);
+		}
+		if(Bwrite(bout, p, Blinelen(bin)) != Blinelen(bin))
+			sysfatal("error writing %s: %r", nout);
+	}
+}
+
+void
+copy(Biobuf *bin, char *nin, Biobuf *bout, char *nout)
+{
+	char buf[4096];
+	int m;
+
+	USED(nin);
+	while((m = Bread(bin, buf, sizeof buf)) > 0)
+		if(Bwrite(bout, buf, m) != m)
+			sysfatal("error writing %s: %r", nout);
+}
+
+void
+idiff(Biobuf *b1, char *name1, Biobuf *b2, char *name2, Biobuf *bdiff, char *namediff, Biobuf *bout, char *nameout)
+{
+	char buf[256], *p;
+	int interactive, defaultanswer, cmd, diffoffset;
+	int n, from1, to1, from2, to2, nf1, nf2;
+	Biobuf berr;
+
+	nf1 = 1;
+	nf2 = 1;
+	interactive = 1;
+	defaultanswer = 0;
+	Binit(&berr, 2, OWRITE);
+	while(diffoffset = Boffset(bdiff), p = Brdline(bdiff, '\n')){
+		p[Blinelen(bdiff)-1] = '\0';
+		parse(p, &from1, &to1, &cmd, &from2, &to2);
+		p[Blinelen(bdiff)-1] = '\n';
+		n = to1-from1 + to2-from2 + 1;	/* #lines from diff */
+		if(cmd == 'c')
+			n += 2;
+		else if(cmd == 'a')
+			from1++;
+		else if(cmd == 'd')
+			from2++;
+		to1++;	/* make half-open intervals */
+		to2++;
+		if(interactive){
+			p[Blinelen(bdiff)-1] = '\0';
+			fprint(2, "%s\n", p);
+			p[Blinelen(bdiff)-1] = '\n';
+			copylines(bdiff, namediff, &berr, "<stderr>", n);
+			Bflush(&berr);
+		}else
+			skiplines(bdiff, namediff, n);
+		do{
+			if(interactive){
+				fprint(2, "? ");
+				memset(buf, 0, sizeof buf);
+				if(read(0, buf, sizeof buf - 1) < 0)
+					sysfatal("read console: %r");
+			}else
+				buf[0] = defaultanswer;
+
+			switch(buf[0]){
+			case '>':
+				copylines(b1, name1, bout, nameout, from1-nf1);
+				skiplines(b1, name1, to1-from1);
+				skiplines(b2, name2, from2-nf2);
+				copylines(b2, name2, bout, nameout, to2-from2);
+				break;
+			case '<':
+				copylines(b1, name1, bout, nameout, to1-nf1);
+				skiplines(b2, name2, to2-nf2);
+				break;
+			case '=':
+				copylines(b1, name1, bout, nameout, from1-nf1);
+				skiplines(b1, name1, to1-from1);
+				skiplines(b2, name2, to2-nf2);
+				if(Bseek(bdiff, diffoffset, 0) != diffoffset)
+					sysfatal("seek in diff output: %r");
+				copylines(bdiff, namediff, bout, nameout, n+1);
+				break;
+			case '!':
+				runcmd(buf+1);
+				break;
+			case 'q':
+				if(buf[1]=='<' || buf[1]=='>' || buf[1]=='='){
+					interactive = 0;
+					defaultanswer = buf[1];
+				}else
+					fprint(2, "must be q<, q>, or q=\n");
+				break;
+			default:
+				fprint(2, "expect: <, >, =, q<, q>, q=, !cmd\n");
+				break;
+			}
+		}while(buf[0] != '<' && buf[0] != '>' && buf[0] != '=');
+		nf1 = to1;
+		nf2 = to2;
+	}
+	copy(b1, name1, bout, nameout);
+}