kinda portable way to do isatty() in a shell

gistfile1.sh

% cat t.sh                                                                                                  ~
#!/bin/sh

test -c /dev/stdin && echo "char"
test -p /dev/stdin && echo "named pipe"
test -S /dev/stdin && echo "socket"
test ! -f /dev/stdin && echo "not regular"
test -f /dev/stdin && echo "regular"
% sh ./t.sh                                                                                                 ~
char
not regular
% echo hi | sh ./t.sh                                                                                       ~
named pipe
not regular
% sh ./t.sh < /etc/passwd                                                                                   ~
regular