這個shell命令中的“：？”是什么意思？

Question

這是shell腳本：

#!/bin/bash

version="0.01";

fibonacci() {
    n=${1:?If you want the nth fibonacci number, you must supply n as the first parameter.}
    if [ $n -le 1 ]; then 
    echo $n
    else
    l=`fibonacci $((n-1))`
    r=`fibonacci $((n-2))`
    echo $((l + r))
    fi
}

for i in `seq 1 10`
do
  result=$(fibonacci $i)
  echo "i=$i result=$result"
done

我對這一行感到困惑：

n=${1:?If you want the nth fibonacci number, you must supply n as the first parameter.}

我在尋找shell手冊，但對“：？”一無所知 其實是說

謝謝

Answer 1

來自man bash：

${parameter:?word}
          Display Error if Null or Unset.  If parameter is null or unset, the  expansion  of  word
          (or  a  message  to that effect if word is not present) is written to the standard error
          and the shell, if it is not interactive, exits.  Otherwise, the value  of  parameter  is
          substituted.

在這種情況下，要檢查的參數為$ 1（第一個位置參數）