When you combine cat (concatenate) with > (output redirection),
you can create and write to a file from the command line.
$ cat > test.txt
Hit enter after the file name, and your terminal will wait for input. End the
file with CTRL + D.
$ cat > test.txt
one
two
three
$ cat test.txt
one
two
three
>> works, too; appending to the file as expected.