Bash for循环语法 [英] Bash for loop syntax

问题描述

我正在习惯使用shell脚本，并且遇到了一个我觉得很有趣而且没有解释的行为。在下面的代码中，第一个for循环将正确执行，但第二个不会。

  declare letters =（a b c d e f g）
 for i in {0..7}; 
 echo $ {letters [i]} 
完成
 for {0 .. $ {＃letters [*]}}做
 echo $ {letters [i]} 
完成
  

循环会导致以下错误：

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $语法错误操作数预期错误标记是{0..7}）

令我困扰的是 $ {＃letters [*]} 显然正确地被评估为数字7.但是尽管如此，即使我们刚刚看到与 {0..7} 完美的作品。

这是什么原因？

我运行的是OS X 10.12.2，GNU bash version 3.2 。

解决方案

在扩展参数之前进行扩展扩展，因此不能将变量作为范围的一部分使用。 / p>

将数组扩展为值列表：

  $ {字母[@]};做
 echo$ letter
完成
  

或者，扩大索引把数组放到一个列表中：

$ $ $ $ $ $！！$ {@字母[@]};做
回声$ {letters [i]}
完成

在评论（感谢）中提到，这两种方法也适应稀疏阵列;你不能总是假定一个数组为每个 0 和 $ {＃字母[@ ]} 。

I'm working on getting accustomed to shell scripting and ran across a behavior I found interesting and unexplained. In the following code the first for loop will execute correctly but the second will not.

declare letters=(a b c d e f g)
for i in {0..7}; do
  echo ${letters[i]}
done
for i in {0..${#letters[*]}}; do
  echo ${letters[i]}
done

The second for loop results in the following error:

syntax error: operand expected (error token is "{0..7}")

What confuses me is that ${#letters[*]} is clearly getting evaluated, correctly, to the number 7. But despite this the code fails even though we just saw that the same loop with {0..7} works perfectly fine.

What is the reason for this?

I am running OS X 10.12.2, GNU bash version 3.2.57.

解决方案

Brace expansion occurs before parameter expansion, so you can't use a variable as part of a range.

Expand the array into a list of values:

for letter in "${letters[@]}"; do
    echo "$letter"
done

Or, expand the indices of the array into a list:

for i in ${!letters[@]}; do 
    echo "${letters[i]}"
done

As mentioned in the comments (thanks), these two approaches also accommodate sparse arrays; you can't always assume that an array defines a value for every index between 0 and ${#letters[@]}.

这篇关于Bash for循环语法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！