This command is part of the TclX package.

read_file ?-nonewline? fileName
read_file fileName numBytes

Returns the contents of the file called fileName. If -nonewline is given, discard the last character if it is a newline. If numBytes is given, read only that number of bytes from the front of the file (unless the file is shorter than that, in which case the whole file is returned).

Compare with open and read.