diff --git a/c/builtins.h b/c/builtins.h
index 0db1c81..904ce65 100644
--- a/c/builtins.h
+++ b/c/builtins.h
@@ -7,6 +7,8 @@ value_t builtin_user_effective_uid(value_t *args, uint32_t nargs);
 value_t builtin_user_real_gid(value_t *args, uint32_t nargs);
 value_t builtin_user_real_uid(value_t *args, uint32_t nargs);
 
+value_t builtin_spawn(value_t *args, uint32_t nargs);
+
 value_t builtin_read_ini_file(value_t *args, uint32_t nargs);
 
 value_t builtin_color_name_to_rgb24(value_t *args, uint32_t nargs);
diff --git a/c/libraries.c b/c/libraries.c
index c539025..9305717 100644
--- a/c/libraries.c
+++ b/c/libraries.c
@@ -97,6 +97,8 @@ static struct builtin_procedure builtin_procedures[] = {
     { "user-real-gid", builtin_user_real_gid, SRFI_170 | UP_2019 },
     { "user-real-uid", builtin_user_real_uid, SRFI_170 | UP_2019 },
 
+    { "spawn", builtin_spawn, SRFI_170 | UP_2019 },
+
     { "color-name->rgb24", builtin_color_name_to_rgb24, UP_2019 },
 
     { 0, 0, 0 },
diff --git a/c/os_unix_process.c b/c/os_unix_process.c
new file mode 100644
index 0000000..8b53404
--- /dev/null
+++ b/c/os_unix_process.c
@@ -0,0 +1,146 @@
+#include <sys/wait.h>
+
+#include <assert.h>
+#include <ctype.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <math.h>
+#include <poll.h>
+#include <setjmp.h>
+#include <stdarg.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "dtypes.h"
+#include "utils.h"
+#include "utf8.h"
+#include "ios.h"
+#include "socket.h"
+#include "timefuncs.h"
+#include "hashing.h"
+#include "htable.h"
+#include "htableh_inc.h"
+#include "bitvector.h"
+#include "os.h"
+#include "random.h"
+#include "llt.h"
+
+#include "flisp.h"
+
+#include "argcount.h"
+#include "os.h"
+
+static void warn(const char *msg) { fprintf(stderr, "%s\n", msg); }
+
+static void warnsys(const char *msg)
+{
+    fprintf(stderr, "%s: %s\n", msg, strerror(errno));
+}
+
+static void diesys(const char *msg)
+{
+    warnsys(msg);
+    exit(1);
+}
+
+static void set_close_on_exec(int fd)
+{
+    int flags;
+
+    if ((flags = fcntl(fd, F_GETFD)) == -1) {
+        diesys("cannot get file descriptor flags");
+    }
+    flags |= FD_CLOEXEC;
+    if (fcntl(fd, F_SETFD, flags) == -1) {
+        diesys("cannot set close-on-exec");
+    }
+}
+
+value_t builtin_spawn(value_t *args, uint32_t nargs)
+{
+    pid_t child;
+    int status, exec_errno;
+    int fds[2];
+    struct pollfd pollfd;
+    ssize_t nbyte;
+    value_t exec_args;
+    char *exec_argv[16];
+    size_t exec_argv_fill = 0;
+
+    argcount("spawn", nargs, 1);
+    exec_args = args[0];
+    if (exec_args == FL_NIL) {
+        lerror(ArgError, "executable argument list is empty");
+    }
+    for (;;) {
+        if (!iscons(exec_args)) {
+            lerror(ArgError, "executable arguments not a proper list");
+        }
+        exec_argv[exec_argv_fill++] =
+        tostring(car_(exec_args), "executable argument");
+        if ((exec_args = cdr_(exec_args)) == FL_NIL) {
+            break;
+        }
+    }
+    exec_argv[exec_argv_fill++] = 0;
+
+    // TODO: Signal mask for the child process.
+    if (pipe(fds) == -1) {
+        diesys("cannot create pipe");
+    }
+    set_close_on_exec(fds[0]);
+    set_close_on_exec(fds[1]);
+    if ((child = fork()) == -1) {
+        diesys("cannot fork");
+    }
+    if (!child) {
+        // We are in the new child process.
+        close(fds[0]);
+        execvp(exec_argv[0], exec_argv);
+        exec_errno = errno;
+        nbyte = write(fds[1], &exec_errno, sizeof(exec_errno));
+        if (nbyte == (ssize_t)-1) {
+            warnsys("completely borked (child)");
+        } else if (nbyte != (ssize_t)sizeof(exec_errno)) {
+            warn("completely borked (child)");
+        }
+        _exit(126);
+    }
+    // We are in the old parent process.
+    close(fds[1]);
+    memset(&pollfd, 0, sizeof(pollfd));
+    pollfd.fd = fds[0];
+    pollfd.events = POLLIN;
+    if (poll(&pollfd, 1, -1) == -1) {
+        diesys("cannot poll");
+    }
+    exec_errno = 0;
+    if (pollfd.revents & POLLIN) {
+        nbyte = read(pollfd.fd, &exec_errno, sizeof(exec_errno));
+        if (nbyte == 0) {
+            // We don't get any data, means the pipe was closed.
+        } else if (nbyte == (ssize_t)-1) {
+            warnsys("completely borked (parent)");
+        } else if (nbyte != (ssize_t)sizeof(exec_errno)) {
+            warn("completely borked (parent)");
+        }
+    }
+    if (waitpid(child, &status, 0) == -1) {
+        warnsys("cannot wait for child process");
+        return FL_NIL;
+    }
+    if (!WIFEXITED(status)) {
+        warn("child process did not exit normally");
+        return FL_NIL;
+    }
+    if (exec_errno != 0) {
+        errno = exec_errno;
+        warnsys("cannot execute command");
+        return FL_NIL;
+    }
+    return fixnum(WEXITSTATUS(status));
+}
diff --git a/scripts/build.sh b/scripts/build.sh
index ca800ee..cab699c 100755
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -21,6 +21,7 @@ o_files="$o_files libraries.o"
 o_files="$o_files lltinit.o"
 o_files="$o_files os_$os.o"
 o_files="$o_files os_unix.o"
+o_files="$o_files os_unix_process.o"
 o_files="$o_files ptrhash.o"
 o_files="$o_files random.o"
 o_files="$o_files socket.o"
@@ -101,6 +102,7 @@ $CC $CFLAGS -c ../c/libraries.c
 $CC $CFLAGS -c ../c/lltinit.c
 $CC $CFLAGS -c ../c/os_"$os".c
 $CC $CFLAGS -c ../c/os_unix.c
+$CC $CFLAGS -c ../c/os_unix_process.c
 $CC $CFLAGS -c ../c/ptrhash.c
 $CC $CFLAGS -c ../c/random.c
 $CC $CFLAGS -c ../c/socket.c