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[#]: collector: (lujun9972)
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[#]: translator: (zero-MK)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Logical & in Bash)
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[#]: via: (https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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Logical & in Bash
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======
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One would think you could dispatch `&` in two articles. Turns out you can't. While [the first article dealt with using `&` at the end of commands to push them into the background][1] and then diverged into explaining process management, the second article saw [`&` being used as a way to refer to file descriptors][2], which led us to seeing how, combined with `<` and `>`, you can route inputs and outputs from and to different places.
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This means we haven't even touched on `&` as an AND operator, so let's do that now.
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### & is a Bitwise Operator
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If you are at all familiar with binary operations, you will have heard of AND and OR. These are bitwise operations that operate on individual bits of a binary number. In Bash, you use `&` as the AND operator and `|` as the OR operator:
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如果您完全熟悉二进制字符的各种操作，您肯定听说过AND和OR。这些是按位操作，对二进制数的各个位进行操作。在Bash中，使用`&`作为AND运算符，使用`|`作为OR运算符：
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**AND**
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```
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0 & 0 = 0
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0 & 1 = 0
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1 & 0 = 0
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1 & 1 = 1
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```
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**OR**
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```
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0 | 0 = 0
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0 | 1 = 1
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1 | 0 = 1
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1 | 1 = 1
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```
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You can test this by ANDing any two numbers and outputting the result with `echo`:
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您可以通过对任何两个数字进行AND运算并使用`echo`输出结果来测试：
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```
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$ echo $(( 2 & 3 )) # 00000010 AND 00000011 = 00000010
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2
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$ echo $(( 120 & 97 )) # 01111000 AND 01100001 = 01100000
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96
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```
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The same goes for OR (`|`):
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OR（`|`）也是如此：
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```
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$ echo $(( 2 | 3 )) # 00000010 OR 00000011 = 00000011
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3
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$ echo $(( 120 | 97 )) # 01111000 OR 01100001 = 01111001
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121
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```
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Three things about this:
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关于这个的三件事：
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1. You use `(( ... ))` to tell Bash that what goes between the double brackets is some sort of arithmetic or logical operation. `(( 2 + 2 ))`, `(( 5 % 2 ))` (`%` being the [modulo][3] operator) and `((( 5 % 2 ) + 1))` (equals 3) will all work.
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2. [Like with variables][4], `$` extracts the value so you can use it.
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3. For once spaces don't matter: `((2+3))` will work the same as `(( 2+3 ))` and `(( 2 + 3 ))`.
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4. Bash only operates with integers. Trying to do something like this `(( 5 / 2 ))` will give you "2", and trying to do something like this `(( 2.5 & 7 ))` will result in an error. Then again, using anything but integers in a bitwise operation (which is what we are talking about now) is generally something you wouldn't do anyway.
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**TIP:** If you want to check what your decimal number would look like in binary, you can use _bc_ , the command-line calculator that comes preinstalled with most Linux distros. For example, using:
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```
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bc <<< "obase=2; 97"
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```
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will convert `97` to binary (the _o_ in `obase` stands for _output_ ), and ...
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```
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bc <<< "ibase=2; 11001011"
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```
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will convert `11001011` to decimal (the _i_ in `ibase` stands for _input_ ).
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### && is a Logical Operator
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Although it uses the same logic principles as its bitwise cousin, Bash's `&&` operator can only render two results: 1 ("true") and 0 ("false"). For Bash, any number not 0 is “true” and anything that equals 0 is “false.” What is also false is anything that is not a number:
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```
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$ echo $(( 4 && 5 )) # Both non-zero numbers, both true = true
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1
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$ echo $(( 0 && 5 )) # One zero number, one is false = false
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0
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$ echo $(( b && 5 )) # One of them is not number, one is false = false
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0
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```
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The OR counterpart for `&&` is `||` and works exactly as you would expect.
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All of this is simple enough... until it comes to a command's exit status.
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### && is a Logical Operator for Command Exit Status
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[As we have seen in previous articles][2], as a command runs, it outputs error messages. But, more importantly for today's discussion, it also outputs a number when it ends. This number is called an _exit code_ , and if it is 0, it means the command did not encounter any problem during its execution. If it is any other number, it means something, somewhere, went wrong, even if the command completed.
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So 0 is good, any other number is bad, and, in the context of exit codes, 0/good means "true" and everything else means “false.” Yes, this is **the exact contrary of what you saw in the logical operations above** , but what are you gonna do? Different contexts, different rules. The usefulness of this will become apparent soon enough.
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Moving on.
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Exit codes are stored _temporarily_ in the [special variable][5] `?` \-- yes, I know: another confusing choice. Be that as it may, [remember that in our article about variables][4], and we said that you read the value in a variable using a the `$` symbol. So, if you want to know if a command has run without a hitch, you have to read `?` as soon as the command finishes and before running anything else.
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Try it with:
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```
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$ find /etc -iname "*.service"
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find: '/etc/audisp/plugins.d': Permission denied
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/etc/systemd/system/dbus-org.freedesktop.nm-dispatcher.service
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/etc/systemd/system/dbus-org.freedesktop.ModemManager1.service
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[etcetera]
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```
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[As you saw in the previous article][2], running `find` over _/etc_ as a regular user will normally throw some errors when it tries to read subdirectories for which you do not have access rights.
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So, if you execute...
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```
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echo $?
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```
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... right after `find`, it will print a `1`, indicating that there were some errors.
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(Notice that if you were to run `echo $?` a second time in a row, you'd get a `0`. This is because `$?` would contain the exit code of `echo $?`, which, supposedly, will have executed correctly. So the first lesson when using `$?` is: **use`$?` straight away** or store it somewhere safe -- like in another variable, or you will lose it).
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One immediate use of `?` is to fold it into a list of chained commands and bork the whole thing if anything fails as Bash runs through it. For example, you may be familiar with the process of building and compiling the source code of an application. You can run them on after another by hand like this:
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```
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$ configure
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.
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.
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.
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$ make
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.
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.
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.
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$ make install
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.
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.
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.
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```
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You can also put all three on one line...
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```
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$ configure; make; make install
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```
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... and hope for the best.
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The disadvantage of this is that if, say, `configure` fails, Bash will still try and run `make` and `sudo make install`, even if there is nothing to make or, indeed, install.
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The smarter way of doing it is like this:
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```
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$ configure && make && make install
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```
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This takes the exit code from each command and uses it as an operand in a chained `&&` operation.
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But, and here's the kicker, Bash knows the whole thing is going to fail if `configure` returns a non-zero result. If that happens, it doesn't have to run `make` to check its exit code, since the result is going to be false no matter what. So, it forgoes `make` and just passes a non-zero result onto the next step of the operation. And, as `configure && make` delivers false, Bash doesn't have to run `make install` either. This means that, in a long chain of commands, you can join them with `&&`, and, as soon as one fails, you can save time as the rest of the commands get canceled immediately.
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You can do something similar with `||`, the OR logical operator, and make Bash continue processing chained commands if only one of a pair completes.
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In view of all this (along with the stuff we covered earlier), you should now have a clearer idea of what the command line we set at the beginning of [this article does][1]:
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```
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mkdir test_dir 2>/dev/null || touch backup/dir/images.txt && find . -iname "*jpg" > backup/dir/images.txt &
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```
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So, assuming you are running the above from a directory for which you have read and write privileges, what it does it do and how does it do it? How does it avoid unseemly and potentially execution-breaking errors? Next week, apart from giving you the solution, we'll be dealing with brackets: curly, curvy and straight. Don't miss it!
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
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作者：[Paul Brown][a]
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选题：[lujun9972][b]
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译者：[zero-MK](https://github.com/zero-MK)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[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/2019/2/and-ampersand-and-linux
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[2]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
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[3]: https://en.wikipedia.org/wiki/Modulo_operation
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[4]: https://www.linux.com/blog/learn/2018/12/bash-variables-environmental-and-otherwise
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[5]: https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html
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229
translated/tech/T20190219 Logical - in Bash.md
Normal file
229
translated/tech/T20190219 Logical - in Bash.md
Normal file
@ -0,0 +1,229 @@
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[#]: collector: "lujun9972"
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[#]: translator: "zero-mk"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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[#]: subject: "Logical & in Bash"
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[#]: via: "https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash"
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[#]: author: "Paul Brown https://www.linux.com/users/bro66"
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Bash中的逻辑和（`&`）
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======
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有人可能会认为两篇文章中的`&`意思差不多，但实际上并不是。虽然 [第一篇文章讨论了如何在命令末尾使用`&`来将命令转到后台运行][1] 之后分解为解释流程管理， 第二篇文章将 [`&` 看作引用文件描述符的方法][2], 这些文章让我们知道了，与 `<` 和 `>` 结合使用后，你可以将输入或输出引导到别的地方。
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但我们还没接触过作为 AND 操作符使用的`&`。所以，让我们来看看。
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### & 是一个按位运算符
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如果您完全熟悉二进制数操作，您肯定听说过 AND 和 OR 。这些是按位操作，对二进制数的各个位进行操作。在 Bash 中，使用`&`作为AND运算符，使用`|`作为 OR 运算符：
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**AND**
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```
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0 & 0 = 0
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0 & 1 = 0
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1 & 0 = 0
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1 & 1 = 1
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```
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**OR**
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```
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0 | 0 = 0
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0 | 1 = 1
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1 | 0 = 1
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1 | 1 = 1
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```
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您可以通过对任何两个数字进行 AND 运算并使用`echo`输出结果：
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```
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$ echo $(( 2 & 3 )) # 00000010 AND 00000011 = 00000010
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2
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$ echo $(( 120 & 97 )) # 01111000 AND 01100001 = 01100000
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96
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```
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OR（`|`）也是如此：
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```
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$ echo $(( 2 | 3 )) # 00000010 OR 00000011 = 00000011
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3
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$ echo $(( 120 | 97 )) # 01111000 OR 01100001 = 01111001
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121
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```
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关于这个不得不说的三件事：
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1. 使用`(( ... ))`告诉 Bash 双括号之间的内容是某种算术或逻辑运算。`(( 2 + 2 ))`, `(( 5 % 2 ))` (`%`是[求模][3]运算符）和`((( 5 % 2 ) + 1))`（等于3）一切都会奏效。
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2. [像变量一样][4], 使用`$`提取值，以便你可以使用它。
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3. 空格并没有影响: `((2+3))` 将等价于 `(( 2+3 ))` 和 `(( 2 + 3 ))`。
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4. Bash只能对整数进行操作. 试试这样做： `(( 5 / 2 ))` ，你会得到"2"；或者这样 `(( 2.5 & 7 ))` ，但会得到一个错误。然后，在按位操作中使用除整数之外的任何东西（这就是我们现在所讨论的）通常是你不应该做的事情。
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**提示:** 如果您想看看十进制数字在二进制下会是什么样子，你可以使用 _bc_ ，这是一个大多数 Linux 发行版都预装了的命令行计算器。比如：
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```
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bc <<< "obase=2; 97"
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```
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这个操作将会把 `97`转换成十二进制（`obase` 中的 _o_ 代表 _output_ ，也即，_输出_）。
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```
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bc <<< "ibase=2; 11001011"
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```
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这个操作将会把 `11001011`转换成十进制（`ibase` 中的 _i_ 代表 _input_ ，也即，_输入_）。
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### &&是一个逻辑运算符
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虽然它使用与其按位表达相同的逻辑原理，但Bash的`&&`运算符只能呈现两个结果：1（“true”）和0（“false”）。对于Bash来说，任何不是0的数字都是“true”，任何等于0的数字都是“false”。什么也是false也不是数字：
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```
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$ echo $(( 4 && 5 )) # 两个非零数字, 两个为true = true
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1
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$ echo $(( 0 && 5 )) # 有一个为零, 一个为false = false
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0
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$ echo $(( b && 5 )) # 其中一个不是数字, 一个为false = false
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0
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```
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与 `&&` 类似， OR 对应着 `||` ，用法正如你想的那样。
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以上这些都很简单... 直到进入命令的退出状态。
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### &&是命令退出状态的逻辑运算符
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[正如我们在之前的文章中看到的][2],当命令运行时，它会输出错误消息。更重要的是，对于今天的讨论，它在结束时也会输出一个数字。此数字称为_exit code_（即_返回码_），如果为0，则表示该命令在执行期间未遇到任何问题。如果是任何其他数字，即使命令完成，也意味着某些地方出错了。
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所以 0 意味着非常棒，任何其他数字都说明有问题发生，并且，在返回码的上下文中，0 意味着“真”，其他任何数字都意味着“假”。对！这 **与您所熟知的逻辑操作完全相反** ，但是你能用这个做什么？ 不同的背景，不同的规则。这种用处很快就会显现出来。
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让我们继续！
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返回码 _临时_ 储存在 [特殊变量][5] `?` 中— 是的，我知道：这又是一个令人迷惑的选择。但不管怎样, [别忘了我们在讨论变量的文章中说过][4], 那时我们说你要用 `$` 符号来读取变量中的值，在这里也一样。所以，如果你想知道一个命令是否顺利运行，你需要在命令结束后，在运行别的命令之前马上用 `$?` 来读取 `?` 的值。
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试试下面的命令：
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```
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$ find /etc -iname "*.service"
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find: '/etc/audisp/plugins.d': Permission denied
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/etc/systemd/system/dbus-org.freedesktop.nm-dispatcher.service
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/etc/systemd/system/dbus-org.freedesktop.ModemManager1.service
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[等等内容]
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```
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[正如你在上一篇文章中看到的一样][2]，普通用户权限在 _/etc_ 下运行 `find` 通常将抛出错误，因为它试图读取你没有权限访问的子目录。
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所以，如果你在执行 `find` 后立马执行...
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```
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echo $?
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```
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...，，它将打印 `1`，表明存在错误。
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注意：当你在一行中运行两遍 `echo $?` ，你将得到一个 `0` 。这是因为 `$?` 将包含 `echo $?` 的返回码，而这条命令按理说一定会执行成功。所以学习如何使用 `$?` 的第一课就是： **单独执行 `$?`** 或者将它保存在别的安全的地方 —— 比如保存在一个变量里，不然你会很快丢失它。）
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一个直接使用 `?` 的用法是将它并入一串链式命令列表，这样 Bash 运行这串命令时若有任何操作失败，后面命令将终止。例如，您可能熟悉构建和编译应用程序源代码的过程。你可以像这样手动一个接一个地运行它们：
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```
|
||||
$ configure
|
||||
|
||||
.
|
||||
|
||||
.
|
||||
|
||||
.
|
||||
|
||||
$ make
|
||||
|
||||
.
|
||||
|
||||
.
|
||||
|
||||
.
|
||||
|
||||
$ make install
|
||||
|
||||
.
|
||||
|
||||
.
|
||||
|
||||
.
|
||||
```
|
||||
|
||||
你也可以把这三行合并成一行...
|
||||
|
||||
```
|
||||
$ configure; make; make install
|
||||
```
|
||||
|
||||
... 但你要希望上天保佑。
|
||||
|
||||
为什么这样说呢？因为你这样做是有缺点的，比方说 `configure` 执行失败了， Bash 将仍会尝试执行 `make` 和 `sudo make install`——就算没东西可 make ，实际上，是没东西会安装。
|
||||
|
||||
聪明一点的做法是：
|
||||
|
||||
```
|
||||
$ configure && make && make install
|
||||
```
|
||||
|
||||
这将从每个命令中获取退出代码，并将其用作链式 `&&` 操作的操作数。
|
||||
但是，没什么好抱怨的，Bash 知道如果 `configure` 返回非零结果，整个过程都会失败。如果发生这种情况，不必运行 `make` 来检查它的退出代码，因为无论如何都会失败的。因此，它放弃运行 `make` ，只是将非零结果传递给下一步操作。并且，由于 `configure && make` 传递了错误，Bash 也不必运行`make install`。这意味着，在一长串命令中，您可以使用 `&&` 连接它们，并且一旦失败，您可以节省时间，因为其他命令会立即被取消运行。
|
||||
|
||||
你可以类似地使用 `||`，OR 逻辑操作符，这样就算只有一部分命令成功执行，Bash 也能运行接下来链接在一起的命令。
|
||||
鉴于所有这些（以及我们之前介绍过的内容），您现在应该更清楚地了解我们在 [本文开头][1] 开头设置的命令行：
|
||||
|
||||
```
|
||||
mkdir test_dir 2>/dev/null || touch backup/dir/images.txt && find . -iname "*jpg" > backup/dir/images.txt &
|
||||
```
|
||||
|
||||
因此，假设您从具有读写权限的目录运行上述内容，它做了什么以及如何做到这一点？它如何避免不合时宜且可能导致执行错误的错误？下周，除了给你这些答案的结果，我们将讨论 brackets: curly, curvy and straight. 不要错过了哟！
|
||||
|
||||
因此，假设您在具有读写权限的目录运行上述内容，它会执行的操作以及如何执行此操作？它如何避免不合时宜且可能导致执行错误的错误？下周，除了给你解决方案，我们将处理包括：卷曲，曲线和直线。不要错过！
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
|
||||
|
||||
作者：[Paul Brown][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[zero-MK](https://github.com/zero-mk)
|
||||
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/2019/2/and-ampersand-and-linux
|
||||
[2]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
|
||||
[3]: https://en.wikipedia.org/wiki/Modulo_operation
|
||||
[4]: https://www.linux.com/blog/learn/2018/12/bash-variables-environmental-and-otherwise
|
||||
[5]: https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html
|
||||
Loading…
Reference in New Issue
Block a user