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Hank
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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: (Using Square Brackets in Bash: Part 2)
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[#]: via: (https://www.linux.com/blog/learn/2019/4/using-square-brackets-bash-part-2)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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Using Square Brackets in Bash: Part 2
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======
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![square brackets][1]
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We continue our tour of square brackets in Bash with a look at how they can act as a command.
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[Creative Commons Zero][2]
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Welcome back to our mini-series on square brackets. In the [previous article][3], we looked at various ways square brackets are used at the command line, including globbing. If you've not read that article, you might want to start there.
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Square brackets can also be used as a command. Yep, for example, in:
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```
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[ "a" = "a" ]
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```
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which is, by the way, a valid command that you can execute, `[ ... ]` is a command. Notice that there are spaces between the opening bracket `[` and the parameters `"a" = "a"`, and then between the parameters and the closing bracket `]`. That is precisely because the brackets here act as a command, and you are separating the command from its parameters.
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You would read the above line as " _test whether the string "a" is the same as string "a"_ ". If the premise is true, the `[ ... ]` command finishes with an exit status of 0. If not, the exit status is 1. [We talked about exit statuses in a previous article][4], and there you saw that you could access the value by checking the `$?` variable.
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Try it out:
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```
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[ "a" = "a" ]
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echo $?
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```
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And now try:
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```
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[ "a" = "b" ]
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echo $?
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```
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In the first case, you will get a 0 (the premise is true), and running the second will give you a 1 (the premise is false). Remember that, in Bash, an exit status from a command that is 0 means it exited normally with no errors, and that makes it `true`. If there were any errors, the exit value would be a non-zero value (`false`). The `[ ... ]` command follows the same rules so that it is consistent with the rest of the other commands.
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The `[ ... ]` command comes in handy in `if ... then` constructs and also in loops that require a certain condition to be met (or not) before exiting, like the `while` and `until` loops.
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The logical operators for testing stuff are pretty straightforward:
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```
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[ STRING1 = STRING2 ] => checks to see if the strings are equal
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[ STRING1 != STRING2 ] => checks to see if the strings are not equal
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[ INTEGER1 -eq INTEGER2 ] => checks to see if INTEGER1 is equal to INTEGER2
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[ INTEGER1 -ge INTEGER2 ] => checks to see if INTEGER1 is greater than or equal to INTEGER2
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[ INTEGER1 -gt INTEGER2 ] => checks to see if INTEGER1 is greater than INTEGER2
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[ INTEGER1 -le INTEGER2 ] => checks to see if INTEGER1 is less than or equal to INTEGER2
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[ INTEGER1 -lt INTEGER2 ] => checks to see if INTEGER1 is less than INTEGER2
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[ INTEGER1 -ne INTEGER2 ] => checks to see if INTEGER1 is not equal to INTEGER2
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etc...
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```
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You can also test for some very shell-specific things. The `-f` option, for example, tests whether a file exists or not:
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```
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for i in {000..099}; \
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do \
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if [ -f file$i ]; \
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then \
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echo file$i exists; \
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else \
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touch file$i; \
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echo I made file$i; \
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fi; \
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done
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```
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If you run this in your test directory, line 3 will test to whether a file is in your long list of files. If it does exist, it will just print a message; but if it doesn't exist, it will create it, to make sure the whole set is complete.
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You could write the loop more compactly like this:
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```
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for i in {000..099};\
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do\
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if [ ! -f file$i ];\
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then\
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touch file$i;\
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echo I made file$i;\
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fi;\
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done
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```
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The `!` modifier in the condition inverts the premise, thus line 3 would translate to " _if the file`file$i` does not exist_ ".
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Try it: delete some random files from the bunch you have in your test directory. Then run the loop shown above and watch how it rebuilds the list.
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There are plenty of other tests you can try, including `-d` tests to see if the name belongs to a directory and `-h` tests to see if it is a symbolic link. You can also test whether a files belongs to a certain group of users (`-G`), whether one file is older than another (`-ot`), or even whether a file contains something or is, on the other hand, empty.
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Try the following for example. Add some content to some of your files:
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```
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echo "Hello World" >> file023
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echo "This is a message" >> file065
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echo "To humanity" >> file010
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```
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and then run this:
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```
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for i in {000..099};\
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do\
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if [ ! -s file$i ];\
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then\
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rm file$i;\
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echo I removed file$i;\
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fi;\
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done
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```
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And you'll remove all the files that are empty, leaving only the ones you added content to.
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To find out more, check the manual page for the `test` command (a synonym for `[ ... ]`) with `man test`.
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You may also see double brackets (`[[ ... ]]`) sometimes used in a similar way to single brackets. The reason for this is because double brackets give you a wider range of comparison operators. You can use `==`, for example, to compare a string to a pattern instead of just another string; or < and `>` to test whether a string would come before or after another in a dictionary.
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To find out more about extended operators [check out this full list of Bash expressions][5].
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### Next Time
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In an upcoming article, we'll continue our tour and take a look at the role of parentheses `()` in Linux command lines. See you then!
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_Read more:_
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1. [The Meaning of Dot (`.`)][6]
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2. [Understanding Angle Brackets in Bash (`<...>`)][7]
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3. [More About Angle Brackets in Bash(`<` and `>`)][8]
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4. [And, Ampersand, and & in Linux (`&`)][9]
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5. [Ampersands and File Descriptors in Bash (`&`)][10]
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6. [Logical & in Bash (`&`)][4]
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7. [All about {Curly Braces} in Bash (`{}`)][11]
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8. [Using Square Brackets in Bash: Part 1][3]
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/2019/4/using-square-brackets-bash-part-2
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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)
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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/sites/lcom/files/styles/rendered_file/public/square-brackets-3734552_1920.jpg?itok=hv9D6TBy (square brackets)
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[2]: /LICENSES/CATEGORY/CREATIVE-COMMONS-ZERO
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[3]: https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1
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[4]: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
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[5]: https://www.gnu.org/software/bash/manual/bashref.html#Bash-Conditional-Expressions
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[6]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
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[7]: https://www.linux.com/blog/learn/2019/1/understanding-angle-brackets-bash
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[8]: https://www.linux.com/blog/learn/2019/1/more-about-angle-brackets-bash
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[9]: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
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[10]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
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[11]: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash
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@ -0,0 +1,158 @@
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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: (Using Square Brackets in Bash: Part 2)
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[#]: via: (https://www.linux.com/blog/learn/2019/4/using-square-brackets-bash-part-2)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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在 Bash 中使用[方括号]（二）
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======
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![square brackets][1]
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> 我们继续来看方括号的用法，它们甚至还可以在 Bash 当中作为一个命令使用。
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[Creative Commons Zero][2]
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欢迎回到我们的方括号专题。在[前一篇文章][3]当中，我们介绍了方括号在命令行中可以用于通配操作，如果你已经读过前一篇文章，就可以从这里继续了。
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方括号还可以以一个命令的形式使用，就像这样：
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```
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[ "a" = "a" ]
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```
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上面这种 `[ ... ]` 的形式就可以看成是一个可执行的命令。要注意，方括号内部的内容 `"a" = "a"` 和方括号 `[`、`]` 之间是有空格隔开的。因为这里的方括号被视作一个命令，因此要用空格将命令和它的参数隔开。
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上面这个命令的含义是“判断字符串 `"a"` 和字符串 `"a"` 是否相同”，如果判断结果为真，那么 `[ ... ]` 就会以<ruby>状态码<rt>status code</rt></ruby> 0 退出，否则以状态码 1 退出。在之前的文章中，我们也有介绍过状态码的概念，可以通过 `$?` 变量获取到最近一个命令的状态码。
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分别执行
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```
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[ "a" = "a" ]
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echo $?
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```
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以及
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```
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[ "a" = "b" ]
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echo $?
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```
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这两段命令中，前者会输出 0（判断结果为真），后者则会输出 1（判断结果为假）。在 Bash 当中，如果一个命令的状态码是 0，表示这个命令正常执行完成并退出，而且其中没有出现错误，对应布尔值 `true`；如果在命令执行过程中出现错误，就会返回一个非零的状态码，对应布尔值 `false`。而 `[ ... ]`也同样遵循这样的规则。
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因此，`[ ... ]` 很适合在 `if ... then`、`while` 或 `until` 这种在代码块结束前需要判断是否达到某个条件结构中使用。
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对应使用的逻辑判断运算符也相当直观：
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```
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[ STRING1 = STRING2 ] => checks to see if the strings are equal
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[ STRING1 != STRING2 ] => checks to see if the strings are not equal
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[ INTEGER1 -eq INTEGER2 ] => checks to see if INTEGER1 is equal to INTEGER2
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[ INTEGER1 -ge INTEGER2 ] => checks to see if INTEGER1 is greater than or equal to INTEGER2
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[ INTEGER1 -gt INTEGER2 ] => checks to see if INTEGER1 is greater than INTEGER2
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[ INTEGER1 -le INTEGER2 ] => checks to see if INTEGER1 is less than or equal to INTEGER2
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[ INTEGER1 -lt INTEGER2 ] => checks to see if INTEGER1 is less than INTEGER2
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[ INTEGER1 -ne INTEGER2 ] => checks to see if INTEGER1 is not equal to INTEGER2
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etc...
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```
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方括号的这种用法也可以很有 shell 风格，例如通过带上 `-f` 参数可以判断某个文件是否存在：
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```
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for i in {000..099}; \
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do \
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if [ -f file$i ]; \
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then \
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echo file$i exists; \
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else \
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touch file$i; \
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echo I made file$i; \
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fi; \
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done
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```
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如果你在上一篇文章使用到的测试目录中运行以上这串命令，其中的第 3 行会判断那几十个文件当中的某个文件是否存在。如果文件存在，会输出一条提示信息；如果文件不存在，就会把对应的文件创建出来。最终，这个目录中会完整存在从 `file000` 到 `file099` 这一百个文件。
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上面这段命令还可以写得更加简洁：
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```
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for i in {000..099};\
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do\
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if [ ! -f file$i ];\
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then\
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touch file$i;\
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echo I made file$i;\
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fi;\
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done
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```
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其中 `!` 运算符表示将判断结果取反，因此第 3 行的含义就是“如果文件 `file$i` 不存在”。
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可以尝试一下将测试目录中那几十个文件随意删除几个，然后运行上面的命令，你就可以看到它是如何把被删除的文件重新创建出来的。
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除了 `-f` 之外，还有很多有用的参数。`-d` 参数可以判断某个目录是否存在，`-h` 参数可以判断某个文件是不是一个符号链接。可以用 `-G` 参数判断某个文件是否属于某个用户组，用 `-ot` 参数判断某个文件的最后更新时间是否早于另一个文件，甚至还可以判断某个文件是否为空文件。
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运行下面的几条命令，可以向几个文件中写入一些内容：
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```
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echo "Hello World" >> file023
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echo "This is a message" >> file065
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echo "To humanity" >> file010
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```
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然后运行：
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```
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for i in {000..099};\
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do\
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if [ ! -s file$i ];\
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then\
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rm file$i;\
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echo I removed file$i;\
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fi;\
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done
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```
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你就会发现所有空文件都被删除了，只剩下少数几个非空的文件。
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如果你还想了解更多别的参数，可以执行 `man test` 来查看 `test` 命令的 man 手册（`test` 是 `[ ... ]` 的命令别名）。
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有时候你还会看到 `[[ ... ]]` 这种双方括号的形式，使用起来和单方括号差别不大。但双方括号支持的比较运算符更加丰富：例如可以使用 `==` 来判断某个字符串是否符合某个<ruby>模式<rt>pattern</rt></ruby>，也可以使用 `<`、`>` 来判断两个字符串的出现顺序。
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可以在 [Bash 表达式文档][5]中了解到双方括号支持的更多运算符。
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### 下一集
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在下一篇文章中，我们会开始介绍圆括号 `()` 在 Linux 命令行中的用法，敬请关注！
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/2019/4/using-square-brackets-bash-part-2
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作者：[Paul Brown][a]
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选题：[lujun9972][b]
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译者：[HankChow](https://github.com/HankChow)
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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/sites/lcom/files/styles/rendered_file/public/square-brackets-3734552_1920.jpg?itok=hv9D6TBy "square brackets"
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[2]: /LICENSES/CATEGORY/CREATIVE-COMMONS-ZERO
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[3]: https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1
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[4]: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
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[5]: https://www.gnu.org/software/bash/manual/bashref.html#Bash-Conditional-Expressions
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[6]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
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[7]: https://www.linux.com/blog/learn/2019/1/understanding-angle-brackets-bash
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[8]: https://www.linux.com/blog/learn/2019/1/more-about-angle-brackets-bash
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[9]: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
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[10]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
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[11]: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash
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