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
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.
From an interactive shell, the commands below exit with a return value of zero:
isatty isatty stdout isatty 2 echo | isatty 1
And these will exit non-zero:
echo | isatty isatty 9 isatty stdout > file isatty 2 2> file
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