Bash script that checks for package dependencies and installs them if necessary

Question

I'm new to writing bash scripts and was wondering if I could get someone's advice on a part of the script I'm working on.

Intended purpose of code

Check if a package exists using dpkg, and if it doesn't, offer to install it for the user. This snippet is part of a larger script that installs a particular Conky configuration along with all of its dependencies with minimal effort from the user.

Concerns

• I feel as though there is a more elegant way to check if a package is installed using dpkg (code was found on Stack Overflow).
• Is there a better way of handling the (y/n) response?

Here is the code that I am using:

declare -a packages=("conky-all" "lm-sensors");
for i in "${packages[@]}"; do
 if [ $(dpkg-query -W -f='${Status}' $i 2>/dev/null | grep -c "ok installed") -eq 0 ]; then
 echo "$i is not installed, would you like to install it now? (Y/N)";
 read response
 if [ "$response" == "y" ] || [ "$response" == "Y" ]; then
 sudo apt-get install "$i";
 else
 echo "Skipping the installation of $i...";
 echo "Please note that this Conky configuration will not work without the $i package.";
 fi
 else
 echo "The $i package has already been installed.";
 fi
done

Answer 1

Instead of this:

 if [ $(dpkg-query -W -f='${Status}' $i 2>/dev/null | grep -c "ok installed") -eq 0 ]; then

A better way to write the same thing:

 if ! dpkg-query -W -f='${Status}' $i 2>/dev/null | grep -q "ok installed"; then

Instead of this:

 if [ "$response" == "y" ] || [ "$response" == "Y" ]; then

A simpler way to write is:

 if [[ $response == [yY]* ]]; then

This is not example the same. It will match anything that starts with "y" or "Y". If you want to match strictly only those letters, just drop the * from the pattern:

 if [[ $response == [yY] ]]; then

Finally, all the ; at line endings are unnecessary. The purpose of ; is to separate multiple statements on the same line. Line breaks naturally serve as statement separators.

• John P.: Thank you very much. Just for the sake of clarity, why are the double brackets necessary for the "if [[ $response == [yY] ]]" statement?
• janos: [ ... ] cannot match patterns like that. [[ ... ]] is more modern and sophisticated, it can do this.
• 200_success: Just make it a habit to quote all variable expansions. It's not worth it to save a few quote characters.
• janos: @200_success I guess you're right. I dropped that point about quoting