How can I find out, whether a specific package is already installed in Arch Linux

You should use Pacman, the package manager of Arch Linux.

You want to use the -Q operation to query the installed local package database and the -i option to get information on the package.

This gives you

pacman -Qi <packageName>

You can then use the exit code to determine if the packages existes on the system or not (0 the package exists, 1 it doesn't)

The usage of -i rather than -s ensures you will check for exactly that package and not for the presence of a a package containing the package name in its name.

For example if I search for chromium (the web browser) on a system where only chromium-bsu (the game) is installed,

# This exits with 1 because chromium is not installedpacman -Qi chromium # This exits with 0 because chromium-bsu is installedpacman -Qs chromium

As Random Citizen pointed out, you certainly want to redirect any output to /dev/null if you are writing a script and don't want Pacman to pollute your output:

pacman -Qi <packageName> > /dev/null

You can use the arch package management tool pacman.
As you can see in the Arch-Wiki, the -Qs option can be used to search within the installed packages.

If the package exists, pacman -Qs will exit with the exit-code 0, otherwise with the exit-code 1

You script might look like:

package=firefoxif pacman -Qs $package > /dev/null ; then  echo "The package $package is installed"else  echo "The package $package is not installed"fi

The > /dev/null pipe is used to suppress the printed output.

I usually just do ls /bin | grep $package (replacing $package with the package I'm looking for). It's quick for the computer too.

It depends on the name of the package though, because this will list all of the installed executables that have $package in their name. Nevertheless, if you have executables installed with $package in their name, there's a big chance the package you're looking for is already installed.

Update

Here is a more accurate one:

package="lshw";check="$(sudo pacman -Qs --color always "${package}" | grep "local" | grep "${package} ")";if [ -n "${check}" ] ; then    echo "${package} is installed";elif [ -z "${check}" ] ; then    echo "${package} is NOT installed";fi;

Even better, how about turning it into a function?

There's 2 examples in the code below. You can use _install to install just one package. You can use _installMany to install as many packages as you want. I included both functions because _installMany is kind of complex, and looking at the slightly simpler _install function might help someone understand it.

#!/bin/bash_isInstalled() {    package="$1";    check="$(sudo pacman -Qs --color always "${package}" | grep "local" | grep "${package} ")";    if [ -n "${check}" ] ; then        echo 0; #'0' means 'true' in Bash        return; #true    fi;    echo 1; #'1' means 'false' in Bash    return; #false}# `_install <pkg>`_install() {    package="$1";    # If the package IS installed:    if [[ $(_isInstalled "${package}") == 0 ]]; then        echo "${package} is already installed.";        return;    fi;    # If the package is NOT installed:    if [[ $(_isInstalled "${package}") == 1 ]]; then        sudo pacman -S "${package}";    fi;}# `_installMany <pkg1> <pkg2> ...`# Works the same as `_install` above,#     but you can pass more than one package to this one._installMany() {    # The packages that are not installed will be added to this array.    toInstall=();    for pkg; do        # If the package IS installed, skip it.        if [[ $(_isInstalled "${pkg}") == 0 ]]; then            echo "${pkg} is already installed.";            continue;        fi;        #Otherwise, add it to the list of packages to install.        toInstall+=("${pkg}");    done;    # If no packages were added to the "${toInstall[@]}" array,    #     don't do anything and stop this function.    if [[ "${toInstall[@]}" == "" ]] ; then        echo "All packages are already installed.";        return;    fi;    # Otherwise, install all the packages that have been added to the "${toInstall[@]}" array.    printf "Packages not installed:\n%s\n" "${toInstall[@]}";    sudo pacman -S "${toInstall[@]}";}package="lshw";_install "${package}";packages=("lshw" "inkscape");_installMany "${packages[@]}";#Or,_installMany "lshw" "inkscape"