@startuml exec-seq
"A"  -> "S": +RUN_CMD
"A"  -> "S": Command Frame
Create P
"S"  -> "P": fork(), exec()

"A"  -> "S": +STDIO
"A"  -> "S": STDIO Frame
"A"  -> "S": STDIO Data

"S"  -> "P": STDIO Data
"P"  -> "S": STDIO Data

"S" --> "A": STDIO
"S" --> "A": STDIO Frame
"S" --> "A": STDIO Data

Note across: Repeat ...

"A"  -> "S": +STDIO
"A"  -> "S": STDIO Frame len=0, fin
"S"  -> "P": STDIN EOF

"P"  -> "S": STDOUT EOF
"S" --> "A": STDIO
"S" --> "A": STDIO Frame len=0, fin

"P"  -> "S": STDERR EOF
"S" --> "A": STDIO
"S" --> "A": STDIO Frame len=0, err, fin

"P"  -> "P": exit()
"S" <-> "P" !!: waitpid()
Note over "P": Process reaped
"S" --> "A": Status
"S" --> "A": Status Frame
@enduml