Lately I've been learning Vim more in depth, beyond just Vim's modal editing. With that, I've been learning more about Unix and the shell. As they say, "Unix is an IDE", and Vim is just one of its tools. I'm going to keep it simple and use the terms input & output to refer to stdin & stdout, the more technically correct terms here.
program > file Redirects the output of a program to a file. If the file exists, it will be overwritten (be careful).
program >> file Redirects the output of a program to a file. If the file exists, it will be appended to (safer option).
program < file Redirects a file to be the input of a program. From what I can tell, this is rarely useful on its own, since nearly all programs which accept an input stream, also accept a file argument. Hence, these two are equivalent: cat < file & cat file. More details on that here:
How does input redirection work? - Ask Ubuntu
output | program Redirects the output of a program, to be the input of another program.
Example: echo $PATH | less
This is functionally equivalent to:echo $PATH > temp_file && less < temp_file
Herestrings & Heredocs
program <<< string Redirects a string to be the input of a program, as if it were a file's contents.
Example:python <<< "print(len('Dude no way'))"
11
program << delimiter
multi-line string
delimiter Redirects a multi-line string to be the input of a program, as if it were a file's contents.
Example:python << EOF
heredoc> print('Sooo')
heredoc> print('Powerful')
heredoc> EOF
Sooo
Powerful
EOF (end of file) is just a convention here, the delimiter could be almost any sequence of characters.
Process Substitution
program <(output) Redirects the output of a program to a temp file, to be treated as a file argument.
Example:cat <(echo "It's almost like...\nI'm a file")
Note that cat accepts files as arguments, here we provide output instead.
Another example:vimdiff <(/usr/local/bin/vim --version) <(/usr/bin/vim --version)