With Homebrew, how to check if a software/package is installed?

I'm installing a set of softwares/packages/formulas via homebrew with command like

brew install <formula1>
...
brew cask install <formula2>
...

I wonder if it's a good idea to verify if the softwares <formula1>, ..., <formula2>, ... are already installed and only trigger the above commands for the ones NOT already installed. If so, how?

you could do something like this:

brew list <formula1> || brew install <formula1>

This will error on list and continue with install if not installed otherwise it will just list package files. (one could modify this further as a function or alias in .bashrc to make it easier to type)

It should also be noted that you can type brew info <formula> which will tell you whether or not a formula is installed. You can parse the response for "Not installed" and then run the installer if it finds the string.

Building on the earlier answers, but packed into a ready to use function without all the homebrew logging:

brew_install() {
    echo "\nInstalling $1"
    if brew list $1 &>/dev/null; then
        echo "${1} is already installed"
    else
        brew install $1 && echo "$1 is installed"
    fi
}

brew_install "ripgrep"

In similar vein to Hans Fredric, here is a snippet I actually use myself. The funny looking <(cmd) is Bash command substitution.

alias strip-empty="egrep -v '^\s*$'"
NOT_INSTALLED=$(comm -23 <(sort < apps.local) <( brew list --versions | awk '{print $1}' ) | strip-empty)
while read FORMULA; do
    brew install "$FORMULA"
done <<< "$NOT_INSTALLED"

Here, apps.local is just a list of apps to install, one per line. The improvement over just looping over each app and trying something like brew_install basically comes down to speed. Invoking brew list is slow (like up to a second), so I just do the test once by listing out all installed apps. The difference is very noticeable if you have > 5 apps.

If you need something with the same speed, but that works equally well with apps installed using a cask, you need something more elaborate (like this).