VAR = $ {N：-m} Bash中使用 [英] VAR=${n:-m} usage in Bash

问题描述

我想写一个遗传算法基于一张贴在这里bash下的http://father-natures.blogspot.mx/2013/04/implementing-genetic-algorithm-in-bash.html.我事先脚本相当不熟练，我没有得到什么VAR = $ {N：-m}表示。我的猜测是，这样的话：

  POOL_SIZE = $ {1：-6}
 

请$ 1 = -6，但是当我检查$ 1中是空的，当我检查$ POOLSIZE我得到6。

 利伯塔德@ engrane4：〜$回声POOL_SIZE
6
 

这对我来说是相当混乱。如果我想的变量是6，我会写：

  POOL_SIZE = 6
 

你能告诉我我在想什么（还有什么是该做的分配）？

感谢您，

解决方案

它规定的情况下，默认的 $ 1 是空的。

在Bash 3.5.3壳牌参数扩展参考手册：

  

$ {参数：-word}

  
  

如果参数没有设置或者为空，word的扩展将被替换。
  否则，参数的值是取代

示例

  $回声$ {A： - 你好}
你好
$ a =测试
$回声$ {一个： - 你好}
测试
 

---

基于您的评论

  

谢谢，@fedorqui。最初的变量是POOL_SIZE = $ {1：-6}，
  REPRO_CHANCE = $ {2：-30}，BEST_FITS = $ {3} -70。现在我想知道如果
  $ {POOL_SIZE：-6} $ {REPRO_CHANCE：-30}和$ {BEST_FITS：-70}将是
  同样的，为什么是记数需要

如果您有

  POOL_SIZE = $ {1：-6}
REPRO_CHANCE = $ {2：-30}
BEST_FITS = $ {3} -70
 

这是因为 POOL_SIZE ， REPRO_CHANCE 和 BEST_FITS 应该包含 $ 1 ， $ 2 和 $ 3 。任何 $ N 表示第n 参数，例如从脚本。所以，如果你有以下脚本：

  $猫
＃！/斌/庆典POOL_SIZE = $ {1：-6}
REPRO_CHANCE = $ {2：-30}
BEST_FITS = $ {3} -70回声POOL_SIZE = $ POOL_SIZE
回声REPRO_CHANCE = $ REPRO_CHANCE
回声BEST_FITS = $ BEST_FITS
 

然后与不同量的参数执行会产生：

  $ ./a
POOL_SIZE = 6
REPRO_CHANCE = 30
BEST_FITS = 70$ ./a 2 2 2
POOL_SIZE = 2
REPRO_CHANCE = 2
BEST_FITS = 2$ ./a 24 2
POOL_SIZE = 24
REPRO_CHANCE = 2
BEST_FITS = 70
 

我希望它明确。

---

还要注意的是 $ {VAR：-value} 和 $ {VAR值} 是不一样的： $之间的区别是什么{VAR：-word}？和$ {VAR字}

I want to write a Genetic Algorithm for bash based in the one posted here: http://father-natures.blogspot.mx/2013/04/implementing-genetic-algorithm-in-bash.html. I am quite inexpert in advance scripting and I don't get what VAR=${n:-m} stands for. My guess was that things like:

POOL_SIZE=${1:-6}

Make $1=-6, however when I check $1 it is empty and when I check $POOLSIZE I get 6.

libertad@engrane4:~$ echo "POOL_SIZE"
6

This is quite confusing for me. If I wanted the variable to be 6 I would write:

POOL_SIZE=6

Could you tell me what am I missing (what else is this assignation doing)?

Thank you,

解决方案

It sets a default in case $1 is empty.

From 3.5.3 Shell Parameter Expansion in the Bash Reference Manual:

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

Example

$ echo ${a:-"hello"}
hello
$ a="test"
$ echo ${a:-"hello"}
test

---

Based on your comment

Thanks, @fedorqui. The original variables were POOL_SIZE=${1:-6}, REPRO_CHANCE=${2:-30}, BEST_FITS=${3:-70}. Now I am wondering if ${POOL_SIZE:-6}, ${REPRO_CHANCE:-30} and ${BEST_FITS:-70} would be the same and why is the numeration needed

If you have

POOL_SIZE=${1:-6}
REPRO_CHANCE=${2:-30}
BEST_FITS=${3:-70}

it is because POOL_SIZE, REPRO_CHANCE and BEST_FITS are supposed to contain the value of $1, $2 and $3. Any $n means the nth parameter, for example from a script. So if you have the following script:

$ cat a
#!/bin/bash

POOL_SIZE=${1:-6}
REPRO_CHANCE=${2:-30}
BEST_FITS=${3:-70}

echo "POOL_SIZE=$POOL_SIZE"
echo "REPRO_CHANCE=$REPRO_CHANCE"
echo "BEST_FITS=$BEST_FITS"

Then its execution with different amount of parameters would yield:

$ ./a
POOL_SIZE=6
REPRO_CHANCE=30
BEST_FITS=70

$ ./a 2 2 2
POOL_SIZE=2
REPRO_CHANCE=2
BEST_FITS=2

$ ./a 24 2
POOL_SIZE=24
REPRO_CHANCE=2
BEST_FITS=70

I hope it makes it clear.

---

Note also that ${var:-value} and ${var-value} are not the same: What is the difference between ${var:-word} and ${var-word}?.

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