PlantUML

https://en.wikipedia.org/wiki/PlantUML
http://www.plantuml.com/plantuml
wget http://vorboss.dl.sourceforge.net/project/plantuml/plantuml.jar
java -Djava.awt.headless=true -jar ~/Downloads/plantuml.jar sequence1.txt
requires graphviz (dot)
http://plantuml.com/class-diagram
java -Djava.awt.headless=true -jar ~/Downloads/plantuml.jar class1.txt

Graphviz for windows https://graphviz.gitlab.io/_pages/Download/windows/graphviz-2.38.msi

Class diagram

@startuml
class Car

interface ICar{
    addWheel(Wheel w)
}

note "This is a floating note" as CarNote
CarNote .. Car

Driver - Car : drives >
Car *- Wheel : have 4 >
Car -- Person : < owns

/' 
<|--  extends 
-- association
..> depends 
<|.. implements / realizes
'/

Car ..|> ICar 
Car : engine : String
Car : startEngine() 

@enduml

Sequence diagram example

@startuml
autonumber
actor Xyz1
boundary Xyz2
control Xyz3
entity Xyz4
database Xyz5

/' comment 
-> synch call
--> response
->> asynch call
'/
Xyz1 -> Xyz2 : To boundary
Xyz1 -> Xyz3 : To control
Xyz1 -> Xyz4 : To entity
Xyz1 -> Xyz5 : To database
Xyz5 --> Xyz1 : To Foo1

@enduml

@startuml
Alice -> Bob: Authentication Request

alt successful case

        Bob -> Alice: Authentication Accepted
        
else some kind of failure

        Bob -> Alice: Authentication Failure
        group My own label
                Alice -> Log : Log attack start
            loop 1000 times
                Alice -> Bob: DNS Attack
               break ccc
               else ddd
               end
            end
                Alice -> Log : Log attack end
        end
        
else Another type of failure
   Bob -> Alice: Please repeat
end

opt aaaa
  Alice -> Bob : llll
else  bbb
  Alice -> Log : kkk
end
@enduml

Use case diagram example

@startuml
'left to right direction
'skinparam packageStyle rect

actor user
actor admin

rectangle System {
  user -- (search stuff)
  (search stuff) .> (show search fields) : include
  (help) .> (search stuff) : extends
  (search stuff) -- admin
}

@enduml

Activity example

@startuml

(*) --> "First Activity"
"First Activity" --> [label] "Second Activity"

"Second Activity" --> "Third Activity"

if "Activities are complete?" then
    --> [true] (*)
else
    --> [false] "Second Activity" 
endif

@enduml