How to identify what file descriptors are associated to STDIN, STDOUT etc

Question

I seem to remember some command or envvar that tells you this. I'd like basically something that executed on terminal, for instance, give you

echo $STDIN 
/dev/tty
echo $STDOUT
/dev/tty
fdescribe 0 or fdescribe /dev/stdin
/dev/tty
fdescribe 1 or fdescribe /dev/stdout
/dev/tty

but that can be executed in a script or subprocess

Answer 1

Well, fd 0 is associated to stdin and fd 1 to stdout by definition.

If you want to know what files are opened on those fds in a (Bourne-like or csh-like¹) shell, you can do:

lsof -ap "$$" -d 0-1

On Linux or Cygwin, see also:

realpath /dev/stdin /dev/stdout

Though the output is less useful if it's sockets or pipes.

On Linux again, adding -E to lsof gives more information for some types of files such as pipes, pseudo-ttys or some sockets as to what's at the other end if any.

Recent versions of util-linux also have a lsfd utility (Linux-specific) that can give you some of the same information as lsof:

lsfd -p "$$" -Q '(fd == 0) or (fd == 1)'

Or

lsfd -p "$$" -Q 'assoc =~ "^[01]$"'

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^{¹ in rc-like shells use $pid instead of $$ and in fish: $fish_pid}