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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (Using Square Brackets in Bash: Part 2)
+[#]: via: (https://www.linux.com/blog/learn/2019/4/using-square-brackets-bash-part-2)
+[#]: author: (Paul Brown https://www.linux.com/users/bro66)
+
+Using Square Brackets in Bash: Part 2
+======
+
+![square brackets][1]
+
+We continue our tour of square brackets in Bash with a look at how they can act as a command.
+
+[Creative Commons Zero][2]
+
+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.
+
+Square brackets can also be used as a command. Yep, for example, in:
+
+```
+[ "a" = "a" ]
+```
+
+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.
+
+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.
+
+Try it out:
+
+```
+[ "a" = "a" ]
+echo $?
+```
+
+And now try:
+
+```
+[ "a" = "b" ]
+echo $?
+```
+
+In the first case, you will get a 0 (the premise is true), and running the second will give you a 1 (the premise is false). Remember that, in Bash, an exit status from a command that is 0 means it exited normally with no errors, and that makes it `true`. If there were any errors, the exit value would be a non-zero value (`false`). The `[ ... ]` command follows the same rules so that it is consistent with the rest of the other commands.
+
+The `[ ... ]` command comes in handy in `if ... then` constructs and also in loops that require a certain condition to be met (or not) before exiting, like the `while` and `until` loops.
+
+The logical operators for testing stuff are pretty straightforward:
+
+```
+[ STRING1 = STRING2 ] => checks to see if the strings are equal
+[ STRING1 != STRING2 ] => checks to see if the strings are not equal
+[ INTEGER1 -eq INTEGER2 ] => checks to see if INTEGER1 is equal to INTEGER2
+[ INTEGER1 -ge INTEGER2 ] => checks to see if INTEGER1 is greater than or equal to INTEGER2
+[ INTEGER1 -gt INTEGER2 ] => checks to see if INTEGER1 is greater than INTEGER2
+[ INTEGER1 -le INTEGER2 ] => checks to see if INTEGER1 is less than or equal to INTEGER2
+[ INTEGER1 -lt INTEGER2 ] => checks to see if INTEGER1 is less than INTEGER2
+[ INTEGER1 -ne INTEGER2 ] => checks to see if INTEGER1 is not equal to INTEGER2
+etc...
+```
+
+You can also test for some very shell-specific things. The `-f` option, for example, tests whether a file exists or not:
+
+```
+for i in {000..099}; \
+ do \
+  if [ -f file$i ]; \
+  then \
+   echo file$i exists; \
+  else \
+   touch file$i; \
+   echo I made file$i; \
+  fi; \
+done
+```
+
+If you run this in your test directory, line 3 will test to whether a file is in your long list of files. If it does exist, it will just print a message; but if it doesn't exist, it will create it, to make sure the whole set is complete.
+
+You could write the loop more compactly like this:
+
+```
+for i in {000..099};\
+do\
+ if [ ! -f file$i ];\
+ then\
+  touch file$i;\
+  echo I made file$i;\
+ fi;\
+done
+```
+
+The `!` modifier in the condition inverts the premise, thus line 3 would translate to " _if the file`file$i` does not exist_ ".
+
+Try it: delete some random files from the bunch you have in your test directory. Then run the loop shown above and watch how it rebuilds the list.
+
+There are plenty of other tests you can try, including `-d` tests to see if the name belongs to a directory and `-h` tests to see if it is a symbolic link. You can also test whether a files belongs to a certain group of users (`-G`), whether one file is older than another (`-ot`), or even whether a file contains something or is, on the other hand, empty.
+
+Try the following for example. Add some content to some of your files:
+
+```
+echo "Hello World" >> file023
+echo "This is a message" >> file065
+echo "To humanity" >> file010
+```
+
+and then run this:
+
+```
+for i in {000..099};\
+do\
+ if [ ! -s file$i ];\
+ then\
+  rm file$i;\
+  echo I removed file$i;\
+ fi;\
+done
+```
+
+And you'll remove all the files that are empty, leaving only the ones you added content to.
+
+To find out more, check the manual page for the `test` command (a synonym for `[ ... ]`) with `man test`.
+
+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.
+
+To find out more about extended operators [check out this full list of Bash expressions][5].
+
+### Next Time
+
+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!
+
+_Read more:_
+
+  1. [The Meaning of Dot (`.`)][6]
+  2. [Understanding Angle Brackets in Bash (`<...>`)][7]
+  3. [More About Angle Brackets in Bash(`<` and `>`)][8]
+  4. [And, Ampersand, and & in Linux (`&`)][9]
+  5. [Ampersands and File Descriptors in Bash (`&`)][10]
+  6. [Logical & in Bash (`&`)][4]
+  7. [All about {Curly Braces} in Bash (`{}`)][11]
+  8. [Using Square Brackets in Bash: Part 1][3]
+
+
+
+--------------------------------------------------------------------------------
+
+via: https://www.linux.com/blog/learn/2019/4/using-square-brackets-bash-part-2
+
+作者：[Paul Brown][a]
+选题：[lujun9972][b]
+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
+[b]: https://github.com/lujun9972
+[1]: https://www.linux.com/sites/lcom/files/styles/rendered_file/public/square-brackets-3734552_1920.jpg?itok=hv9D6TBy (square brackets)
+[2]: /LICENSES/CATEGORY/CREATIVE-COMMONS-ZERO
+[3]: https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1
+[4]: https://www.linux.com/blog/learn/2019/2/logical-ampersand-bash
+[5]: https://www.gnu.org/software/bash/manual/bashref.html#Bash-Conditional-Expressions
+[6]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
+[7]: https://www.linux.com/blog/learn/2019/1/understanding-angle-brackets-bash
+[8]: https://www.linux.com/blog/learn/2019/1/more-about-angle-brackets-bash
+[9]: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
+[10]: https://www.linux.com/blog/learn/2019/2/ampersands-and-file-descriptors-bash
+[11]: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash