击变串联 [英] Bash variable concatenation

问题描述

哪种形式是最有效的？

1）

v=''
v+='a'
v+='b'
v+='c'

2）

v2='a'` `'b'` `'c'

假设可读性都是一模一样的给你，这是一个延伸，将1）意味着创建和在Python扔掉一些字符串immutables（等）或作为一个Java的StringBuffer的缓冲能力的周期性扩张？如何在Bash的内部处理字符串连接？

Assuming readability were exactly the same to you, and that's a stretch, would 1) mean creating and throwing away a few string immutables (like in Python) or act as a Java "StringBuffer" with periodical expansion of the buffer capacity? How are string concatenations handled internally in Bash?

如果2）只是为可读你为1），将反引号产卵子shell和那会是更加昂贵，甚至作为一个潜在的无操作比什么是1做）？

If 2) were just as readable to you as 1), would the backticks spawn subshells and would that be more costly, even as a potential 'no-op' than what is done in 1) ?

推荐答案

好了，最简单，最有效的机制将选择0：

Well, the simplest and most efficient mechanism would be option 0:

v="abc"

第一种机制涉及四个任务。

The first mechanism involves four assignments.

第二个机制是奇怪的（而且是绝对无法读取）。它（标称）运行在两个子壳（两个``部分）的空指挥和并置的三个常量输出（空字符串）。如果shell只是执行的back-tick命令而不提的是，他们是空的（这不是不合理的，它不会注意到，这是尝试奇怪的事情 - 我不记得看​​到它在我的$ P $完成pvious30年shell脚本），这绝对是大大慢。

The second mechanism is bizarre (and is definitely not readable). It (nominally) runs an empty command in two sub-shells (the two ` ` parts) and concatenates the outputs (an empty string) with the three constants. If the shell simply executes the back-tick commands without noting that they're empty (and it's not unreasonable that it won't notice; it is a weird thing to try — I don't recall seeing it done in my previous 30 years of shell scripting), this is definitely vastly slower.

因此​​，给定的唯一的选择（1）和（2），使用的选项（1），但在一般情况下，使用选项（0），如上所示。

So, given only options (1) and (2), use option (1), but in general, use option (0) shown above.

为什么你会被改小建立字符串这样呢？什么是缺少从你的榜样，使原来的code明智但小于显示懂事减少code。

Why would you be building up the string piecemeal like that? What's missing from your example that makes the original code sensible but the reduced code shown less sensible.

v=""
x=$(...)
v="$v$x"
y=$(...)
v="$v$y"
z=$(...)
v="$v$z"

这会更有意义，特别是如果你使用的每个 $ x的， $ Y 和 $ Z而言之后，和/或使用的中间值 $ v （也许在命令重新三重点psented $ p $） 。用于将与任何的Bourne shell衍生而来的工作串联符号;替代 + = shell会用更少的贝壳工作，但可能会更有效（与强调略）。

This would make more sense, especially if you use each of $x, $y and $z later, and/or use intermediate values of $v (perhaps in the commands represented by triple dots). The concatenation notation used will work with any Bourne-shell derivative; the alternative += shell will work with fewer shells, but is probably slightly more efficient (with the emphasis on 'slightly').

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