是什么“：？”意思是在这个shell命令中？

Question

这里是shell脚本：

#!/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 

我往困惑这一行：

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

我找壳的手册，但什么也没得到什么，那么“？”实际上是指。

THX

Answer 1

从人的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. 

在这种情况下被检查的参数是$ 1（第一位置参数）