In Bash, how to add "Are you sure [Y/n]" to any command or alias?

Question

These are more compact and versatile forms of Hamish's answer. They handle any mixture of upper and lower case letters:

read -r -p "Are you sure? [y/N] " response
case "$response" in
    [yY][eE][sS]|[yY]) 
        do_something
        ;;
    *)
        do_something_else
        ;;
esac

Or, for Bash >= version 3.2:

read -r -p "Are you sure? [y/N] " response
if [[ "$response" =~ ^([yY][eE][sS]|[yY])$ ]]
then
    do_something
else
    do_something_else
fi

Note: If $response is an empty string, it will give an error. To fix, simply add quotation marks: "$response". – Always use double quotes in variables containing strings (e.g.: prefer to use "$@" instead $@).

Or, Bash 4.x:

read -r -p "Are you sure? [y/N] " response
response=${response,,}    # tolower
if [[ "$response" =~ ^(yes|y)$ ]]
...

Edit:

In response to your edit, here's how you'd create and use a confirm command based on the first version in my answer (it would work similarly with the other two):

confirm() {
    # call with a prompt string or use a default
    read -r -p "${1:-Are you sure? [y/N]} " response
    case "$response" in
        [yY][eE][sS]|[yY]) 
            true
            ;;
        *)
            false
            ;;
    esac
}

To use this function:

confirm && hg push ssh://..

or

confirm "Would you really like to do a push?" && hg push ssh://..

Here is roughly a snippet that you want. Let me find out how to forward the arguments.

read -p "Are you sure you want to continue? <y/N> " prompt
if [[ $prompt == "y" || $prompt == "Y" || $prompt == "yes" || $prompt == "Yes" ]]
then
  # http://stackoverflow.com/questions/1537673/how-do-i-forward-parameters-to-other-command-in-bash-script
else
  exit 0
fi

Watch out for yes | command name here :)

Confirmations are easily bypassed with carriage returns, and I find it useful to continually prompt for valid input.

Here's a function to make this easy. "invalid input" appears in red if Y|N is not received, and the user is prompted again.

prompt_confirm() {
  while true; do
    read -r -n 1 -p "${1:-Continue?} [y/n]: " REPLY
    case $REPLY in
      [yY]) echo ; return 0 ;;
      [nN]) echo ; return 1 ;;
      *) printf " \033[31m %s \n\033[0m" "invalid input"
    esac 
  done  
}

# example usage
prompt_confirm "Overwrite File?" || exit 0

You can change the default prompt by passing an argument

To avoid explicitly checking for these variants of 'yes' you could use the bash regular expression operator '=~' with a regular expression:

read -p "Are you sure you want to continue? <y/N> " prompt
if [[ $prompt =~ [yY](es)* ]]
then
(etc...)

That tests whether the user input starts with 'y' or 'Y' and is followed by zero or more 'es's.