diff --git a/Makefile b/Makefile
index 8086e48..d096122 100644
--- a/Makefile
+++ b/Makefile
@@ -76,6 +76,9 @@ lowercase: sysexits src/lowercase.c
 nonzero: src/nonzero.c
 	$(CC) $(CFLAGS) -o bin/nonzero src/nonzero.c
 
+nutshell: src/nutshell.c src/nutshell.h src/nutshell_builtins.c
+	$(CC) $(CFLAGS) -o bin/nutshell src/nutshell.c
+
 runlength: noargvzero sysexits src/runlength.c
 	$(CC) $(CFLAGS) -o bin/runlength src/runlength.c
 
diff --git a/src/nutshell.c b/src/nutshell.c
new file mode 100644
index 0000000..612b5cd
--- /dev/null
+++ b/src/nutshell.c
@@ -0,0 +1,115 @@
+#include "nutshell.h"
+#include "nutshell_builtins.c"
+
+int
+fdputd(int fd, int d){
+	if(d < 0){
+		putc('-', stdout);
+		d *= -1;
+	}
+	return printf("%d\n", d);
+}
+
+/* Probably not buggy. */
+int
+fdputs(int fd, char *s){
+	int r;
+
+	r = write(1, s, strlen(s));
+	r += write(1, "\n", 1);
+
+	return r;
+}
+
+static int
+parse_argv(char *buffer, char **argv, int argc_max){
+	int i;
+	int j;
+	int s;
+
+	for(i = 0, j = 0, s = 1; buffer[i] != '\0' && j < argc_max; ++i){
+		while(isspace(buffer[i])){
+			s = 1;
+			buffer[i++] = '\0';
+		}
+		if(s)
+			argv[j++] = buffer + i;
+	}
+
+	return j;
+}
+
+static char *
+read_buffer(char *buf, size_t buf_s){
+	int i;
+
+	for(i = 0; ; ++i){
+		/* ..., '\0', buf_s */
+		/* This isn't perfect but is simple and doesn't matter for
+		 * longer buffers that much. */
+		if(i == buf_s - 1 - 1){
+			buf[i] = '\0';
+			break;
+		}
+
+		buf[i] = getc(stdin);
+
+		if(buf[i] == EOF)
+			return NULL;
+
+		else if(buf[i] == '\n'){
+			buf[i + 1] = '\0';
+			break;
+		}
+	}
+
+	return buf;
+}
+
+static int
+run(char *buf){
+	int argc;
+	static char *argv[ARGV_MAX];
+	int i;
+
+	/* get argv and argc */
+	for(i = 0; i < ARRAYLEN(argv); ++i){
+		if(argv[i] == NULL)
+			break;
+		argv[i] = NULL;
+	}
+
+	argc = parse_argv(buf, argv, ARRAYLEN(argv));
+
+	/* builtins get priority */
+	for(i = 0; i < ARRAYLEN(builtins); ++i)
+		if(strcmp(builtins[i].name, argv[0]) == 0)
+			return builtins[i].f(argc, argv);
+
+	if(state.jailed)
+		goto runerr;
+
+runerr:
+	/* This is what busybox ash(1) does. */
+	return 127;
+}
+
+int main(int argc, char *argv[]){
+	static char buf[BUF_MAX];
+
+	srand(time(NULL));
+	builtin_init(0, empty_argv);
+
+	for(;;){
+		write(1, prompt, ARRAYLEN(prompt));
+		if(read_buffer(buf, ARRAYLEN(buf)) == NULL){
+			write(1, "EOF\n", 4);
+			state.status = 0;
+			break;
+		}
+
+		state.status = run(buf);
+	}
+
+	return state.status;
+}
diff --git a/src/nutshell.h b/src/nutshell.h
new file mode 100644
index 0000000..62349bd
--- /dev/null
+++ b/src/nutshell.h
@@ -0,0 +1,49 @@
+#include <ctype.h> /*
+	isspace(3)
+*/
+#include <stddef.h> /*
+	NULL
+*/
+#include <stdio.h> /*
+	getc(3)
+*/
+#include <stdlib.h> /*
+	getenv(3)
+	rand(3)
+	srand(3)
+*/
+#include <string.h> /*
+	strcmp(3)
+	strlen(3)
+*/
+#include <sys/wait.h> /*
+	wait(2)
+*/
+#include <time.h> /*
+	time(3)
+*/
+#include <unistd.h> /*
+	fork(2)
+	write(2)
+*/
+
+#define ARRAYLEN(a) sizeof(a) / sizeof(*(a))
+
+/* conservative defaults */
+#define ARGV_MAX 10
+#define BUF_MAX 100
+
+char *empty_argv[1] = { NULL };
+char *nethack_name = "nethack";
+char prompt[] = "^_^?: ";
+
+struct {
+	int jailed;
+	int status;
+}state = {
+	0,
+	0
+};
+
+int fdputd(int fd, int d);
+int fdputs(int fd, char *s);
diff --git a/src/nutshell_builtins.c b/src/nutshell_builtins.c
new file mode 100644
index 0000000..3d1d96c
--- /dev/null
+++ b/src/nutshell_builtins.c
@@ -0,0 +1,72 @@
+int builtin_escape	(int, char **);
+int builtin_init	(int, char **);
+int builtin_jail	(int, char **);
+int builtin_nethack	(int, char **);
+int builtin_reassure	(int, char **);
+int builtin_status	(int, char **);
+
+struct {
+	char *name;
+	int (*f)(int, char **);
+}builtins[] = {
+	{"escape", builtin_escape},
+	{"init", builtin_init},
+	{"jail", builtin_jail},
+	{"nethack", builtin_nethack},
+	{"status", builtin_status}
+};
+
+int
+builtin_escape(int argc, char **argv){
+	char *s;
+
+	if((s = getenv("SHELL")) == NULL){
+		fdputs(1, "$SHELL seems to be empty. This is an OS error.\n");
+	}else if(fork() == 0)
+		execl(s, s, NULL);
+	else
+		wait(NULL);
+
+	return 0;
+}
+
+int
+builtin_init(int argc, char **argv){
+	fdputs(1,"\
+\
+	A voice whispers to you. \"Play NetHack...\" (\"nethack\" [enter]).\n\
+	What would you like to do?"
+	);
+	return 0;
+}
+
+int
+builtin_jail(int argc, char **argv){
+	printf("%s\n", argv[1]);
+	if(argv[1] == NULL)
+		fdputd(1, state.jailed);
+	else if(strcmp(argv[1], "break") == 0 || strcmp(argv[1], "build") == 0)
+		state.jailed = (argv[1][1] - 'r') / ('u' - 'r'); /* cheap */
+	else
+		return 1;
+	return 0;
+}
+
+int
+builtin_nethack(int argc, char **argv){
+	char *a[2];
+	if(fork() == 0){
+		a[0] = nethack_name;
+		a[1] = NULL;
+		execvp("nethack", a);
+	}else
+		wait(NULL);
+	
+	return 0;
+}
+
+int
+builtin_status(int argc, char **argv){
+	fdputd(1, state.status);
+	return 0;
+}