feat: add ability to check if a plugin has been installed

docs/advanced-usage/plugin-management.md

@@ -6,6 +6,7 @@
 plugin:disable <name>                    # Disable an installed plugin (third-party only)
 plugin:enable <name>                     # Enable a previously disabled plugin
 plugin:install [--core|git-url [--committish tag|branch|commit|--name custom-plugin-name]]           # Optionally download git-url (with custom tag/committish) & run install trigger for active plugins (or only core ones)
+plugin:installed <name>                  # Checks if a plugin is installed
 plugin:install-dependencies [--core]     # Run install-dependencies trigger for active plugins (or only core ones)
 plugin:list                              # Print active plugins
 plugin:trigger <args...>.                # Trigger an arbitrary plugin hook
@@ -63,6 +64,12 @@ plugn: dev
   trace                0.20.0 enabled    dokku core trace plugin
 ```
 
+You can check if a plugin has been installed via the `pluginn:installed` command:
+
+```shell
+dokku plugin:install postgres
+```
+
 Installing a plugin is easy as well using the `plugin:install` command. This command will also trigger the `install` pluginhook on all existing plugins.
 
 ```shell

plugins/plugin/internal-functions

@@ -12,6 +12,15 @@ cmd-plugin-list() {
   plugn list
 }
 
+cmd-plugin-installed() {
+  declare desc="checks if a plugin is installed"
+  declare cmd="plugin:installed"
+  [[ "$1" == "$cmd" ]] && shift 1
+  declare PLUGIN_NAME="$1"
+
+  plugn list | awk '{print $1}' | grep -q "^${PLUGIN_NAME}$"
+}
+
 plugin_prime_bash_completion() {
   declare desc="primes the bash-completion cache"
 

plugins/plugin/subcommands/installed

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/plugin/internal-functions"
+
+cmd-plugin-installed "$@"

tests/unit/40_plugin.bats

@@ -39,11 +39,21 @@ teardown() {
 }
 
 @test "(plugin) plugin:install, plugin:disable, plugin:update plugin:uninstall" {
+  run /bin/bash -c "dokku plugin:installed $TEST_PLUGIN_NAME"
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure  
+
   run /bin/bash -c "dokku plugin:install $TEST_PLUGIN_GIT_REPO --name $TEST_PLUGIN_NAME"
   echo "output: $output"
   echo "status: $status"
   assert_success
 
+  run /bin/bash -c "dokku plugin:installed $TEST_PLUGIN_NAME"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success  
+
   run /bin/bash -c "dokku plugin:update $TEST_PLUGIN_NAME"
   echo "output: $output"
   echo "status: $status"