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翻译完成 20190226 All about -Curly Braces- in Bash.md
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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (All about {Curly Braces} in Bash)
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[#]: via: (https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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All about {Curly Braces} in Bash
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======
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At this stage of our Bash basics series, it would be hard not to see some crossover between topics. For example, you have already seen a lot of brackets in the examples we have shown over the past several weeks, but the focus has been elsewhere.
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For the next phase of the series, we’ll take a closer look at brackets, curly, curvy, or straight, how to use them, and what they do depending on where you use them. We will also tackle other ways of enclosing things, like when to use quotes, double-quotes, and backquotes.
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This week, we're looking at curly brackets or _braces_ : `{}`.
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### Array Builder
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You have already encountered curly brackets before in [The Meaning of Dot][1]. There, the focus was on the use of the dot/period (`.`), but using braces to build a sequence was equally important.
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As we saw then:
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```
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echo {0..10}
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```
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prints out the numbers from 0 to 10. Using:
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```
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echo {10..0}
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```
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prints out the same numbers, but in reverse order. And,
|
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```
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echo {10..0..2}
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```
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prints every second number, starting with 10 and making its way backwards to 0.
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Then,
|
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|
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```
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echo {z..a..2}
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```
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prints every second letter, starting with _z_ and working its way backwards until _a_.
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And so on and so forth.
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Another thing you can do is combine two or more sequences:
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```
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echo {a..z}{a..z}
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```
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This prints out all the two letter combinations of the alphabet, from _aa_ to _zz_.
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Is this useful? Well, actually it is. You see, arrays in Bash are defined by putting elements between parenthesis `()` and separating each element using a space, like this:
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|
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```
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month=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")
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```
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To access an element within the array, you use its index within brackets `[]`:
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```
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$ echo ${month[3]} # Array indexes start at [0], so [3] points to the fourth item
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Apr
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```
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You can accept all those brackets, parentheses, and braces on faith for a moment. We'll talk about them presently.
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Notice that, all things being equal, you can create an array with something like this:
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```
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letter_combos=({a..z}{a..z})
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```
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and `letter_combos` points to an array that contains all the 2-letter combinations of the entire alphabet.
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You can also do this:
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```
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dec2bin=({0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1})
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```
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This last one is particularly interesting because `dec2bin` now contains all the binary numbers for an 8-bit register, in ascending order, starting with 00000000, 00000001, 00000010, etc., until reaching 11111111. You can use this to build yourself an 8-bit decimal-to-binary converter. Say you want to know what 25 is in binary. You can do this:
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```
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$ echo ${dec2bin[25]}
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00011001
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```
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Yes, there are better ways of converting decimal to binary as we saw in [the article where we discussed & as a logical operator][2], but it is still interesting, right?
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### Parameter expansion
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Getting back to
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```
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echo ${month[3]}
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```
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Here the braces `{}` are not being used as apart of a sequence builder, but as a way of generating _parameter expansion_. Parameter expansion involves what it says on the box: it takes the variable or expression within the braces and expands it to whatever it represents.
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In this case, `month` is the array we defined earlier, that is:
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|
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```
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month=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")
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```
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And, item 3 within the array points to `"Apr"` (remember: the first index in an array in Bash is `[0]`). That means that `echo ${month[3]}`, after the expansion, translates to `echo "Apr"`.
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Interpreting a variable as its value is one way of expanding it, but there are a few more you can leverage. You can use parameter expansion to manipulate what you read from variable, say, by cutting a chunk off the end.
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Suppose you have a variable like:
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```
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a="Too longgg"
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```
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The command:
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|
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```
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echo ${a%gg}
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```
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chops off the last two gs and prints " _Too long_ ".
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Breaking this down,
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* `${...}` tells the shell to expand whatever is inside it
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* `a` is the variable you are working with
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* `%` tells the shell you want to chop something off the end of the expanded variable ("Too longgg")
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* and `gg` is what you want to chop off.
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This can be useful for converting files from one format to another. Allow me to explain with a slight digression:
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[ImageMagick][3] is a set of command line tools that lets you manipulate and modify images. One of its most useful tools ImageMagick comes with is `convert`. In its simplest form `convert` allows you to, given an image in a certain format, make a copy of it in another format.
|
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|
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The following command takes a JPEG image called _image.jpg_ and creates a PNG copy called _image.png_ :
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|
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```
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convert image.jpg image.png
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```
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ImageMagick is often pre-installed on most Linux distros. If you can't find it, look for it in your distro's software manager.
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Okay, end of digression. On to the example:
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With variable expansion, you can do the same as shown above like this:
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```
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i=image.jpg
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convert $i ${i%jpg}png
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```
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What you are doing here is chopping off the extension `jpg` from `i` and then adding `png`, making the command `convert image.jpg image.png`.
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You may be wondering how this is more useful than just writing in the name of the file. Well, when you have a directory containing hundreds of JPEG images, you need to convert to PNG, run the following in it:
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```
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for i in *.jpg; do convert $i ${i%jpg}png; done
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```
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... and, hey presto! All the pictures get converted automatically.
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If you need to chop off a chunk from the beginning of a variable, instead of `%`, use `#`:
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```
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$ a="Hello World!"
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$ echo Goodbye${a#Hello}
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Goodbye World!
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```
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There's quite a bit more to parameter expansion, but a lot of it makes sense only when you are writing scripts. We'll explore more on that topic later in this series.
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### Output Grouping
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Meanwhile, let's finish up with something simple: you can also use `{ ... }` to group the output from several commands into one big blob. The command:
|
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```
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echo "I found all these PNGs:"; find . -iname "*.png"; echo "Within this bunch of files:"; ls > PNGs.txt
|
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```
|
||||
|
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will execute all the commands but will only copy into the _PNGs.txt_ file the output from the last `ls` command in the list. However, doing
|
||||
|
||||
```
|
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{ echo "I found all these PNGs:"; find . -iname "*.png"; echo "Within this bunch of files:"; ls; } > PNGs.txt
|
||||
```
|
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|
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creates the file _PNGs.txt_ with everything, starting with the line " _I found all these PNGs:_ ", then the list of PNG files returned by `find`, then the line "Within this bunch of files:" and finishing up with the complete list of files and directories within the current directory.
|
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|
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Notice that there is space between the braces and the commands enclosed within them. That’s because `{` and `}` are _reserved words_ here, commands built into the shell. They would roughly translate to " _group the outputs of all these commands together_ " in plain English.
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|
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Also notice that the list of commands has to end with a semicolon (`;`) or the whole thing will bork.
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### Next Time
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In our next installment, we'll be looking at more things that enclose other things, but of different shapes. Until then, have fun!
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|
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Read more:
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[And, Ampersand, and & in Linux][4]
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[Ampersands and File Descriptors in Bash][5]
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|
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[Logical & in Bash][2]
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash
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|
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作者：[Paul Brown][a]
|
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选题：[lujun9972][b]
|
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译者：[译者ID](https://github.com/译者ID)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://www.linux.com/users/bro66
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[b]: https://github.com/lujun9972
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[1]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
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[2]: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
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[3]: http://www.imagemagick.org/
|
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[4]: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
|
||||
[5]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
|
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235
translated/tech/20190226 All about -Curly Braces- in Bash.md
Normal file
235
translated/tech/20190226 All about -Curly Braces- in Bash.md
Normal file
@ -0,0 +1,235 @@
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[#]: collector: (lujun9972)
|
||||
[#]: translator: (HankChow)
|
||||
[#]: reviewer: ( )
|
||||
[#]: publisher: ( )
|
||||
[#]: url: ( )
|
||||
[#]: subject: (All about {Curly Braces} in Bash)
|
||||
[#]: via: (https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash)
|
||||
[#]: author: (Paul Brown https://www.linux.com/users/bro66)
|
||||
|
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浅析 Bash 中的 {花括号}
|
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======
|
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|
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|
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在前面的 Bash 基础系列文章中，我们或多或少地使用了一些还没有讲到的符号。在之前文章的很多例子中，我们都使用到了括号，但并没有重点讲解关于括号的内容。
|
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|
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这个系列接下来的文章中，我们会研究括号们的用法：如何使用这些括号？将它们放在不同的位置会有什么不同的效果？除了圆括号、方括号、花括号以外，我们还会接触另外的将一些内容“包裹”起来的符号，例如单引号、双引号和反引号。
|
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|
||||
在这周，我们先来看看花括号 `{}`。
|
||||
|
||||
### 构造序列
|
||||
|
||||
花括号在之前的《[点的含义][1]》这篇文章中已经出现过了，当时我们只对点号 `.` 的用法作了介绍。但在构建一个序列的过程中，同样不可以缺少花括号。
|
||||
|
||||
我们使用
|
||||
|
||||
```
|
||||
echo {0..10}
|
||||
```
|
||||
|
||||
来顺序输出 0 到 10 这 11 个数。使用
|
||||
|
||||
```
|
||||
echo {10..0}
|
||||
```
|
||||
|
||||
可以将这 11 个数倒序输出。更进一步，可以使用
|
||||
|
||||
```
|
||||
echo {10..0..2}
|
||||
```
|
||||
|
||||
来跳过其中的奇数。
|
||||
|
||||
而
|
||||
|
||||
```
|
||||
echo {z..a..2}
|
||||
```
|
||||
|
||||
则从倒序输出字母表，并跳过其中的第奇数个字母。
|
||||
|
||||
以此类推。
|
||||
|
||||
还可以将两个序列进行组合：
|
||||
|
||||
```
|
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echo {a..z}{a..z}
|
||||
```
|
||||
|
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这个命令会将从 aa 到 zz 的所有双字母组合依次输出。
|
||||
|
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这是很有用的。在 Bash 中，定义一个数组的方法是在圆括号 `()` 中放置各个元素并使用空格隔开，就像这样：
|
||||
|
||||
```
|
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month=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")
|
||||
```
|
||||
|
||||
如果需要获取数组中的元素，就要使用方括号 `[]` 并在其中填入元素的索引：
|
||||
|
||||
```
|
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$ echo ${month[3]} # 数组索引从 0 开始，因此 [3] 对应第 4 个元素
|
||||
|
||||
Apr
|
||||
```
|
||||
|
||||
先不要过分关注这里用到的三种括号，我们等下会讲到。
|
||||
|
||||
注意，像上面这样，我们可以定义这样一个数组：
|
||||
|
||||
```
|
||||
letter_combos=({a..z}{a..z})
|
||||
```
|
||||
|
||||
其中 `letter_combos` 变量指向的数组依次包含了从 aa 到 zz 的所有双字母组合。
|
||||
|
||||
因此，还可以这样定义一个数组：
|
||||
|
||||
```
|
||||
dec2bin=({0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1})
|
||||
```
|
||||
|
||||
在这里，`dec2bin` 变量指向的数组按照升序依次包含了所有 8 位的二进制数，也就是 00000000、00000001、00000010，……，11111111。这个数组可以作为一个十进制数到 8 位二进制数的转换器。例如将十进制数 25 转换为二进制数，可以这样执行：
|
||||
|
||||
```
|
||||
$ echo ${dec2bin[25]}
|
||||
|
||||
00011001
|
||||
```
|
||||
|
||||
对于进制转换，确实还有更好的方法，但这不失为一个有趣的方法。
|
||||
|
||||
### <ruby>参数展开<rt>parameter expansion</rt></ruby>
|
||||
|
||||
再看回前面的
|
||||
|
||||
```
|
||||
echo ${month[3]}
|
||||
```
|
||||
|
||||
在这里，花括号的作用就不是构造序列了，而是用于参数展开。顾名思义，参数展开就是将花括号中的变量展开为这个变量实际的内容。
|
||||
|
||||
我们继续使用上面的 `month` 数组来举例：
|
||||
|
||||
```
|
||||
month=("Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec")
|
||||
```
|
||||
|
||||
注意，Bash 中的数组索引从 0 开始，因此 3 代表第 4 个元素 `"Apr"`。因此 `echo ${month[3]}` 在经过参数展开之后，相当于 `echo "Apr"`。
|
||||
|
||||
像上面这样将一个数组展开成它所有的元素，只是参数展开的其中一种用法。另外，还可以通过参数展开的方式读取一个字符串变量，并对其进行处理。
|
||||
|
||||
例如对于以下这个变量：
|
||||
|
||||
```
|
||||
a="Too longgg"
|
||||
```
|
||||
|
||||
如果执行：
|
||||
|
||||
```
|
||||
echo ${a%gg}
|
||||
```
|
||||
|
||||
可以输出“too long”，也就是去掉了最后的两个 g。
|
||||
|
||||
在这里，
|
||||
|
||||
* `${...}` 告诉 shell 展开花括号里的内容
|
||||
* `a` 就是需要操作的变量
|
||||
* `%` 告诉 shell 需要在展开字符串之后从字符串的末尾去掉某些内容
|
||||
* `gg` 是被去掉的内容
|
||||
|
||||
|
||||
|
||||
这个特性在转换文件格式的时候会比较有用，我来举个例子：
|
||||
|
||||
[ImageMagick][3] 是一套可以用于操作图像文件的命令行工具，它有一个 `convert` 命令。这个 `convert` 命令的作用是可以为某个格式的图像文件制作一个另一格式的副本。
|
||||
|
||||
下面这个命令就是使用 `convert` 为 JPEG 格式图像 `image.jpg` 制作一个 PNG 格式的图像副本 `image.png`：
|
||||
|
||||
```
|
||||
convert image.jpg image.png
|
||||
```
|
||||
|
||||
在很多 Linux 发行版中都预装了 ImageMagick，如果没有预装，一般可以在发行版对应的软件管理器中找到。
|
||||
|
||||
继续来看，在对变量进行展开之后，就可以批量执行相类似的操作了：
|
||||
|
||||
```
|
||||
i=image.jpg
|
||||
convert $i ${i%jpg}png
|
||||
```
|
||||
|
||||
这实际上是将变量 `i` 末尾的 `"jpg"` 去掉，然后加上 `"png"`，最终将整个命令拼接成 `convert image.jpg image.png`。
|
||||
|
||||
如果你觉得并不怎么样，可以想象一下有成百上千个图像文件需要进行这个操作，而仅仅运行：
|
||||
|
||||
```
|
||||
for i in *.jpg; do convert $i ${i%jpg}png; done
|
||||
```
|
||||
|
||||
就瞬间完成任务了。
|
||||
|
||||
如果需要去掉字符串开头的部分，就要将上面的 `%` 改成 `#` 了：
|
||||
|
||||
```
|
||||
$ a="Hello World!"
|
||||
$ echo Goodbye${a#Hello}
|
||||
Goodbye World!
|
||||
```
|
||||
|
||||
参数展开还有很多用法，但一般在写脚本的时候才会需要用到。在这个系列以后的文章中就继续提到。
|
||||
|
||||
### 合并输出
|
||||
|
||||
最后介绍一个花括号的用法，这个用法很简单，就是可以将多个命令的输出合并在一起。首先看下面这个命令：
|
||||
|
||||
```
|
||||
echo "I found all these PNGs:"; find . -iname "*.png"; echo "Within this bunch of files:"; ls > PNGs.txt
|
||||
```
|
||||
|
||||
以分号分隔开的几条命令都会执行，但只有最后的 `ls` 命令的结果输出会被重定向到 `PNGs.txt` 文件中。如果将这几条命令用花括号包裹起来，就像这样：
|
||||
|
||||
```
|
||||
{ echo "I found all these PNGs:"; find . -iname "*.png"; echo "Within this bunch of files:"; ls; } > PNGs.txt
|
||||
```
|
||||
|
||||
执行完毕后，可以看到 `PNGs.txt` 文件中会包含两次 `echo` 的内容、`find` 命令查找到的 PNG 文件以及最后的 `ls` 命令结果。
|
||||
|
||||
需要注意的是，花括号与命令之间需要有空格隔开。因为这里的花括号 `{` 和 `}` 是作为 shell 中的保留字，shell 会将这两个符号之间的输出内容组合到一起。
|
||||
|
||||
另外，各个命令之间要用分号 `;` 分隔，否则命令无法正常运行。
|
||||
|
||||
### 下期预告
|
||||
|
||||
在后续的文章中，我会介绍其它“包裹”类符号的用法，敬请关注。
|
||||
|
||||
相关阅读：
|
||||
|
||||
[And, Ampersand, and & in Linux][4]
|
||||
|
||||
[Ampersands and File Descriptors in Bash][5]
|
||||
|
||||
[Logical & in Bash][2]
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash
|
||||
|
||||
作者：[Paul Brown][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[HankChow](https://github.com/HankChow)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://www.linux.com/users/bro66
|
||||
[b]: https://github.com/lujun9972
|
||||
[1]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
|
||||
[2]: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
|
||||
[3]: http://www.imagemagick.org/
|
||||
[4]: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
|
||||
[5]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
|
||||
|
||||
Loading…
Reference in New Issue
Block a user