Double quotes behave similarly to single quotes, but they perform certain kinds of expansion within them, for shell variables and substitutions. This can be used to include the value of a variable as part of a literal string:

$ echo "This is my login shell: $SHELL"
This is my login shell: /bin/bash

Compare this to the literal output of single quotes:

$ echo 'This is my login shell: $SHELL'
This is my login shell: $SHELL

Other kinds of parameter expansion within double quotes are possible, which we will examine in later chapters.

You can include a literal dollar sign or backslash in a string by escaping it:

$ echo "Not a variable: \$total"
Not a variable: $total
$ echo "Back\\to\\back\\slashes"
Back\to\back\slashes

Exclamation marks are a special case, due to history expansion; you will generally need to quote them with a backslash or single quotes instead of double quotes:

$ echo "Hello $USER"'!!'
Hello bashuser!!

For historical reasons, you will also need to escape backtick characters (`):

$ echo "Backticks: \`\`\`"
Backticks: ```