Bash Basic Tips and Tricks

https://stackoverflow.com/questions/2013547/assigning-default-values-to-shell-variables-with-a-single-command-in-bash/25895084

Very close to what you posted, actually:

FOO=${VARIABLE:-default}  # If variable not set or null, use default.

Or, which will assign default to VARIABLE as well:

FOO=${VARIABLE:=default}  # If variable not set or null, set it to default.

https://stackoverflow.com/questions/7522712/how-to-check-if-command-exists-in-a-shell-script/39983421
In general, that depends on your shell, but if you use bash, zsh, ksh or sh (as provided by dash), the following should work:

if ! type "$foobar_command_name" > /dev/null; then
  # install foobar here
fi
For a real installation script, you'd probably want to be sure that type doesn't return successfully in the case when there is an alias foobar. In bash you could do something like this:

if ! foobar_loc="$(type -p "$foobar_command_name")" || [[ -z $foobar_loc ]]; then
  # install foobar here
fi

This is preferable to which for a few reasons:

1) the default which implementations only support the -a option that shows all options, so you have to find an alternative version to support aliases

2) type will tell you exactly what you are looking at (be it a bash function or an alias or a proper binary).

3) type doesn't require a subprocess

4) type cannot be masked by a binary (for example, on a linux box, if you create a program called which which appears in path before the real which, things hit the fan. type, on the other hand, is a shell built-in [yes, a subordinate inadvertently did this once]

Five ways, 4 for bash and 1 addition for zsh:

• type foobar &> /dev/null
• hash foobar &> /dev/null
• command -v foobar &> /dev/null
• which foobar &> /dev/null
• (( $+commands[foobar] )) (zsh only)

            if [ "$T1" = "$T2" ]; then

                echo expression evaluated as true

            else

                echo expression evaluated as false

            fi