From the terminal verify if python 3 is installed

Interactive shell

Simply run python3 --version. You should get some output like Python 3.8.1 if Python 3 is installed.

Shell script

You can use the command or type builtins:

command -v python3 >/dev/null 2>&1 && echo Python 3 is installed  # POSIX-compliant
type -P python3 >/dev/null 2>&1 && echo Python 3 is installed     # Bash only

Using which is not recommended as it requires launching an external process and might give you incorrect output in some cases.

execute the following command. which python3 and check the exit status of the command $?. it will be 0 if user has python 3 installed, 1 otherwise.

You can check it with regular expression match, on the "python3 -V" output.

For example:

[[ "$(python3 -V)" =~ "Python 3" ]] && echo "Python 3 is installed"