implemented '<>' redirects, and the 'seek' builtin

'seek' is a wrapper around the lseek() syscall

added noclobber to core shopts and implemented '>|' redirection syntax

properly implemented fd close syntax

fixed saved fds being leaked into exec'd programs

src/libsh/sys.rs

@@ -2,6 +2,14 @@ use std::sync::LazyLock;
 
 use crate::prelude::*;
 
+/// Minimum fd number for shell-internal file descriptors.
+const MIN_INTERNAL_FD: RawFd = 10;
+
 pub static TTY_FILENO: LazyLock<RawFd> = LazyLock::new(|| {
-  open("/dev/tty", OFlag::O_RDWR, Mode::empty()).expect("Failed to open /dev/tty")
+  let fd = open("/dev/tty", OFlag::O_RDWR, Mode::empty()).expect("Failed to open /dev/tty");
+  // Move the tty fd above the user-accessible range so that
+  // `exec 3>&-` and friends don't collide with shell internals.
+  let high = fcntl(fd, FcntlArg::F_DUPFD_CLOEXEC(MIN_INTERNAL_FD)).expect("Failed to dup /dev/tty high");
+  close(fd).ok();
+  high
 });