site/docs/3.4/cmds/isatty.html
isatty [FILE DESCRIPTOR]
isatty tests if a file descriptor is a terminal (as opposed to a file). The name is derived from the system call of the same name, which for historical reasons refers to a teletypewriter (TTY).
FILE DESCRIPTOR may be either the number of a file descriptor, or one of the strings stdin, stdout, or stderr. If not specified, zero is assumed.
If the specified file descriptor is a terminal device, the exit status of the command is zero. Otherwise, the exit status is non-zero. No messages are printed to standard error.
The -h or --help option displays help about using this command.
From an interactive shell, the commands below exit with a return value of zero:
isattyisatty stdoutisatty 2echo | isatty 1
And these will exit non-zero:
echo | isattyisatty 9isatty stdout \> fileisatty 2 2\> file