What's a good mnemonic for shell double vs. single quotes?
Single quotes are simple quotes, with a single standard: every character is literal.
Double quotes have a double standard: some characters are literal, others are still interpreted unless there's a backslash before them.
Single quotes work alone: backslash inside single quotes is not special.
Double quotes pair up with backslash: backslash inside double quotes makes the next character non-special.
"Double, double toil and trouble;
Dollars burn, and backquotes bubble."
Single, in contrast, is harmless.
Double-quotes ("
) are weak quotes- you need two of them, so they come in pairs, and variables punch right through them despite their numbers.
Single-quotes ('
) are strong quotes- they come singly, and variables are no match for them.