Print and concatenate files. More information: https://manned.org/cat.1posix.
stdout
:cat path/to/file
cat path/to/file1 path/to/file2 ... > path/to/output_file
cat path/to/file1 path/to/file2 ... >> path/to/output_file
cat -u /dev/tty12 > /dev/tty13
stdin
to a file:cat - > path/to/file