history length = 1 .! echo ${two:-}history length = 1 Couldn't parse historical command 'echo ( a )': , ) None (word.CompoundWord parts:[(word_part.LiteralPart token:(token id:Lit_Chars val:a span_id:4))]) None> .history length = 1 ! echo -lhistory length = 1 ! echo xxhistory length = 1 .! echo yyhistory length = 2 ! echo "${five@P}"history length = 2 ! echo $threehistory length = 2 .! echo -n $three ${4:-} "${five@P}"history length = 3 ! echo ${two:-}history length = 3 ! echo ${two:-}history length = 3 ! echo 1history length = 3 ! ls /echo/history length = 3 ! echo ${two:-}history length = 3 history length = 3 history length = 3 ! ls /echo/history length = 3 . ---------------------------------------------------------------------- Ran 5 tests in 0.002s OK ! echo /echo/