How does `less` take data from stdin while still be able to read commands from user?

As mentioned by William Pursell, less reads the user’s keystrokes from the terminal. It explicitly opens /dev/tty, the controlling terminal; that gives it a file descriptor, separate from standard input, from which it can read the user’s interactive input. It can simultaneously read data to display from its standard input if necessary. (It could also write directly to the terminal if necessary.)

You can see this happen by running

some_command | strace -o less.trace -e open,read,write less

Move around the input, exit less, and look at the contents of less.trace: you’ll see it open /dev/tty, and read from both file descriptor 0 and whichever one was returned when it opened /dev/tty (likely 3).

This is common practice for programs wishing to ensure they’re reading from and writing to the terminal. One example is SSH, e.g. when it asks for a password or passphrase.

As explained by schily, if /dev/tty can’t be opened, less will read from its standard error (file descriptor 2). less’s use of /dev/tty was introduced in version 177, released on April 2, 1991.

If you try running cat /dev/tty | less, as suggested by Hagen von Eitzen, less will succeed in opening /dev/tty but won’t get any input from it until cat closes it. So you’ll see the screen blank, and nothing else until you press CtrlC to kill cat (or kill it in some other way); then less will show whatever you typed while cat was running, and allow you to control it.

UNIX gives two methods to read users input while stdin has been redirected:

• The original method is to read from stderr. Stderr is open for writing and reading and this is still mentioned in POSIX.

• Later UNIX versions did (around 1979) add a /dev/tty driver interface that allows to open the controlling tty of a process. Since there are processes without a controlling tty, it is possible that an attempt to open /dev/tty fails. Friendly written software therefore has a fallback to the original method and then tries to read from stderr.