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[Translated] “10 Amazing and Mysterious Uses of…"
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translating by wwy-hust
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10 Amazing and Mysterious Uses of (!) Symbol or Operator in Linux Commands
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================================================================================
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The `'!'` symbol or operator in Linux can be used as Logical Negation operator as well as to fetch commands from history with tweaks or to run previously run command with modification. All the commands below have been checked explicitly in bash Shell. Though I have not checked but a major of these won’t run in other shell. Here we go into the amazing and mysterious uses of `'!'` symbol or operator in Linux commands.
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### 1. Run a command from history by command number. ###
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You might not be aware of the fact that you can run a command from your history command (already/earlier executed commands). To get started first find the command number by running ‘history‘ command.
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$ history
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Now run a command from history just by the number at which it appears, in the output of history. Say run a command that appears at number 1551 in the output of ‘history‘ command.
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$ !1551
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And, it runs the command ([top command][1] in the above case), that was listed at number 1551. This way to retrieving already executed command is very helpful specially in case of those commands which are long. You just need to call it using **![Number at which it appears in the output of history command]**.
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### 2. Run previously executed command as 2nd last command, 7th last command,etc. ###
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You may run those commands which you have run previously by their running sequence being the last run command will be represented as -1, second last as -2, seventh last as -7,….
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First run history command to get a list of last executed command. It is necessary to run history command, so that you can be sure that there is no command like `rm command > file` and others just to make sure you do not run any dangerous command accidentally. And then check Sixth last command, Eight last command and Tenth last command.
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$ history
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$ !-6
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$ !-8
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$ !-10
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Run Last Executed Commands By Numbers
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### 3. Pass arguments of last command that we run to the new command without retyping ###
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I need to list the content of directory ‘/home/$USER/Binary/firefox‘ so I fired.
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$ ls /home/$USER/Binary/firefox
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Then I realized that I should have fired ‘ls -l‘ to see which file is executable there? So should I type the whole command again! No I don’t need. I just need to carry the last argument to this new command as:
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$ ls -l !$
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Here `!$` will carry arguments passed in last command to this new command.
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Pass Arguments of Last Executed Command to New
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### 4. How to handle two or more arguments using (!) ###
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Let’s say I created a text file 1.txt on the Desktop.
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$ touch /home/avi/Desktop/1.txt
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and then copy it to ‘/home/avi/Downloads‘ using complete path on either side with cp command.
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$ cp /home/avi/Desktop/1.txt /home/avi/downloads
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Now we have passed two arguments with cp command. First is ‘/home/avi/Desktop/1.txt‘ and second is ‘/home/avi/Downloads‘, lets handle them differently, just execute `echo [arguments]` to print both arguments differently.
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$ echo “1st Argument is : !^”
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$ echo “2nd Argument is : !cp:2”
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Note 1st argument can be printed as `“!^”` and rest of the arguments can be printed by executing `“![Name_of_Command]:[Number_of_argument]”`.
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In the above example the first command was ‘cp‘ and 2nd argument was needed to print. Hence `“!cp:2”`, if any command say xyz is run with 5 arguments and you need to get 4th argument, you may use `“!xyz:4”`, and use it as you like. All the arguments can be accessed by `“!*”`.
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Handle Two or More Arguments
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### 5. Execute last command on the basis of keywords ###
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We can execute the last executed command on the basis of keywords. We can understand it as follows:
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$ ls /home > /dev/null [Command 1]
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$ ls -l /home/avi/Desktop > /dev/null [Command 2]
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$ ls -la /home/avi/Downloads > /dev/null [Command 3]
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$ ls -lA /usr/bin > /dev/null [Command 4]
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Here we have used same command (ls) but with different switches and for different folders. Moreover we have sent to output of each command to ‘/dev/null‘ as we are not going to deal with the output of the command also the console remains clean.
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Now Execute last run command on the basis of keywords.
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$ ! ls [Command 1]
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$ ! ls -l [Command 2]
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$ ! ls -la [Command 3]
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$ ! ls -lA [Command 4]
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Check the output and you will be astonished that you are running already executed commands just by `ls` keywords.
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Run Commands Based on Keywords
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### 6. The power of !! Operator ###
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You can run/alter your last run command using `(!!)`. It will call the last run command with alter/tweak in the current command. Lets show you the scenario
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Last day I run a one-liner script to get my private IP so I run,
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$ ip addr show | grep inet | grep -v 'inet6'| grep -v '127.0.0.1' | awk '{print $2}' | cut -f1 -d/
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Then suddenly I figured out that I need to redirect the output of the above script to a file ip.txt, so what should I do? Should I retype the whole command again and redirect the output to a file? Well an easy solution is to use `UP` navigation key and add `'> ip.txt'` to redirect the output to a file as.
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$ ip addr show | grep inet | grep -v 'inet6'| grep -v '127.0.0.1' | awk '{print $2}' | cut -f1 -d/ > ip.txt
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Thanks to the life Savior `UP` navigation key here. Now consider the below condition, the next time I run below one-liner script.
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$ ifconfig | grep "inet addr:" | awk '{print $2}' | grep -v '127.0.0.1' | cut -f2 -d:
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As soon as I run script, the bash prompt returned an error with the message `“bash: ifconfig: command not found”`, It was not difficult for me to guess I run this command as user where it should be run as root.
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So what’s the solution? It is difficult to login to root and then type the whole command again! Also (UP Navigation Key) in last example didn’t came to rescue here. So? We need to call `“!!”` without quotes, which will call the last command for that user.
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$ su -c “!!” root
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Here su is switch user which is root, `-c` is to run the specific command as the user and the most important part `!!` will be replaced by command and last run command will be substituted here. Yeah! You need to provide root password.
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The Power of !! Key
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I make use of `!!` mostly in following scenarios,
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1. When I run apt-get command as normal user, I usually get an error saying you don’t have permission to execute.
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$ apt-get upgrade && apt-get dist-upgrade
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Opps error…don’t worry execute below command to get it successful..
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$ su -c !!
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Same way I do for,
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$ service apache2 start
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or
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$ /etc/init.d/apache2 start
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or
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$ systemctl start apache2
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OOPS User not authorized to carry such task, so I run..
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$ su -c 'service apache2 start'
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or
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$ su -c '/etc/init.d/apache2 start'
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or
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$ su -c 'systemctl start apache2'
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### 7. Run a command that affects all the file except ![FILE_NAME] ###
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The `!` (Logical NOT) can be used to run the command on all the files/extension except that is behind `'!'`.
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A. Remove all the files from a directory except the one the name of which is 2.txt.
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$ rm !(2.txt)
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B. Remove all the file type from the folder except the one the extension of which is ‘pdf‘.
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$ $ rm !(*.pdf)
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### 8. Check if a directory (say /home/avi/Tecmint)exist or not? Printf if the said directory exist or not. ###
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Here we will use `'! -d'` to validate if the directory exist or not followed by Logical AND Operator `(&&)` to print that directory does not exist and Logical OR Operator `(||)` to print the directory is present.
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Logic is, when the output of `[ ! -d /home/avi/Tecmint ]` is 0, it will execute what lies beyond Logical AND else it will go to Logical OR `(||)` and execute what lies beyond Logical OR.
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$ [ ! -d /home/avi/Tecmint ] && printf '\nno such /home/avi/Tecmint directory exist\n' || printf '\n/home/avi/Tecmint directory exist\n'
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### 9. Check if a directory exist or not? If not exit the command. ###
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Similar to the above condition, but here if the desired directory doesn’t exist it will exit the command.
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$ [ ! -d /home/avi/Tecmint ] && exit
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### 10. Create a directory (say test) in your home directory if it does not exist. ###
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A general implementation in Scripting Language where if the desired directory does not exist, it will create one.
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[ ! -d /home/avi/Tecmint ] && mkdir /home/avi/Tecmint
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That’s all for now. If you know or come across any other use of `'!'` which is worth knowing, you may like to provide us with your suggestion in the feedback. Keep connected!
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--------------------------------------------------------------------------------
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via: http://www.tecmint.com/mysterious-uses-of-symbol-or-operator-in-linux-commands/
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作者：[Avishek Kumar][a]
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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) 原创翻译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:http://www.tecmint.com/author/avishek/
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[1]:http://www.tecmint.com/12-top-command-examples-in-linux/
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@ -0,0 +1,193 @@
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10个（!）符号在Linux命令行下惊人和奇妙的用法
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================================================================================
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`'!'`符号在Linux中不但可以用作否定符号，还可以用来从历史命令记录中取出命令或不加修改的执行之前运行的命令。下面的所有命令都已经在Bash Shell中经过确切地检查。尽管我没有试过，但大多都不能在别的Shell中运行。这里我们介绍下Linux命令行中符号`'!'`那惊人和奇妙的用法。
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### 1. 使用数字从历史命令列表中找一条命令来执行 ###
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您也许没有意识到您可以从历史命令列表（之前已经执行的命令集）中找出一条来运行。首先，通过"history"命令查找之前命令的序号。
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$ history
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现在，只需要使用历史命令输出中显示在该命令前面的数字便可以运行这个命令。例如，运行一个在`history`输出中编号是1551的命令。
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$ !1551
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这样，编号为1551的命令（上面的例子是[top命令][1]）便运行了。这种通过ID号来执行之前的命令的方式很有用，尤其是在这些命令都很长的情况下。您只需要使用**![history命令输出的序号]**便可以调用它。
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### 2. 运行之前的倒数第二个、第七个命令等 ###
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您可以以另一种方式来运行之前执行的命令，通过使用-1代表最后的命令，-2代表倒数第二个命令，-7代表倒数第七个命令等。
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首先使用history命令来获得执行过的命令的列表。history命令的执行很有必要，因为您可以通过它来确保没有`rm command > file`或其他会导致危险的命令。接下来执行倒数第六个、第八个、第十个命令。
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$ history
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$ !-6
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$ !-8
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$ !-10
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通过负数序号运行之前执行的命令
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### 3. 向最后的命令传递参数，以方便的运行新的命令 ###
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我需要显示`/home/$USER/Binary/firefox`文件夹的内容，因此我执行：
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$ ls /home/$USER/Binary/firefox
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接下来，我意识到我应该执行'ls -l'来查看哪个文件是可执行文件。因此我应该重新输入整个命令么？不，我不需要。我仅需要在新的命令中带上最后的参数，类似：
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$ ls -l !$
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这里`!$`将把最后执行的命令的参数传递到这个新的命令中。
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将上一个命令的参数传递给新命令
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### 4. 如何使用!来处理两个或更多的参数 ###
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比如说我在桌面创建了一个文本文件file1.txt。
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$ touch /home/avi/Desktop/1.txt
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然后在cp命令中使用绝对路径将它拷贝到`/home/avi/Downloads`。
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$ cp /home/avi/Desktop/1.txt /home/avi/downloads
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现在，我们给cp命令传递了两个参数。第一个是`/home/avi/Desktop/1.txt`，第二个是`/home/avi/Downloads`。让我们分别处理他们，使用`echo [arguments]`来打印两个不同的参数。
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$ echo "1st Argument is : !^"
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$ echo "2nd Argument is : !cp:2"
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注意第一个参数可以使用`"!^"`进行打印，其余的命令可以通过`"![命令名]:[参数编号]"`打印。
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在上面的例子中，第一个命令是`cp`，第二个参数也需要被打印。因此是`"!cp:2"`，如果任何命令比如xyz运行时有5个参数，而您需要获得第四个参数，您可以使用`"!xyz:4"`。所有的参数都可以通过`"!*"`来获得。
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处理两个或更多的参数
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### 5. 以关键字为基础执行上个的命令 ###
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我们可以以关键字为基础执行上次执行的命令。可以从下面的例子中理解：
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$ ls /home > /dev/null [命令1]
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$ ls -l /home/avi/Desktop > /dev/null [命令2]
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$ ls -la /home/avi/Downloads > /dev/null [命令3]
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$ ls -lA /usr/bin > /dev/null [命令4]
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上面我们使用了同样的命令（ls），但有不同的开关和不同的操作文件夹。而且，我们还将输出传递到`/dev/null`，我们并未处理输出，因而终端依旧很干净。
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现在以关键字为基础执行上个的命令。
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$ ! ls [命令1]
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$ ! ls -l [命令2]
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$ ! ls -la [命令3]
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$ ! ls -lA [命令4]
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检查输出，您将惊奇发现您仅仅使用关键字`ls`便执行了您已经执行过的命令。
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以关键字为基础执行命令
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### 6. !!操作符的威力 ###
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您可以使用`(!!)`运行/修改您上个运行的命令。它将附带一些修改/调整并调用上个命令。让我给您展示一些实际情境。
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昨天我运行了一行脚本来获得我的私密IP，因此我执行了：
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$ ip addr show | grep inet | grep -v 'inet6'| grep -v '127.0.0.1' | awk '{print $2}' | cut -f1 -d/
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接着，我突然发现我需要将上面脚本的输出重定向到一个ip.txt的文件，因此，我改怎么办呢？我改重新输入整个命令并重定向到一个文件么？一个简单的解决方案是使用向上导航键并添加`'> ip.txt'`来将输出重定向到文件。
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$ ip addr show | grep inet | grep -v 'inet6'| grep -v '127.0.0.1' | awk '{print $2}' | cut -f1 -d/ > ip.txt
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在这里感谢救世主"向上导航键"。现在，考虑下面的情况，这次我运行了下面这一行脚本。
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$ ifconfig | grep "inet addr:" | awk '{print $2}' | grep -v '127.0.0.1' | cut -f2 -d:
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一旦我运行了这个脚本，Bash提示符便返回了错误消息`"bash: ifconfig: command not found"`。原因并不难猜，我运行了本应以root权限的运行的命令。
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所以，怎么解决呢？我们很难以root用户登录并且再次键入整个命令！而且向上导航键也不管用了。因此，我们需要调用`"!!"`（去掉引号），它将为那个用户调用上个命令。
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$ su -c "!!" root
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这里su是用来切换到root用户的，`-c`用来以某用户运行特定的命令，最重要的部分是`!!`，它将被替换为上次运行的命令。对的！您需要提供root密码。
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!!操作符的威力
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我通常在下面的情景中使用`!!`。
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1.当我用普通用户来运行apt-get，我通常收到提示说我没有权限来执行。
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$ apt-get upgrade && apt-get dist-upgrade
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好吧，有错误。但别担心，使用下面的命令来成功的执行...
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$ su -c !!
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同样的适用于：
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$ service apache2 start
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或
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$ /etc/init.d/apache2 start
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或
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$ systemctl start apache2
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||||
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普通用户不被授权执行那些任务，所以我运行：
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||||
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$ su -c 'service apache2 start'
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或
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$ su -c '/etc/init.d/apache2 start'
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或
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$ su -c 'systemctl start apache2'
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### 7.运行一个影响所有除了![FILE_NAME]的文件命令 ###
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`!`（逻辑非）能用来对除了`'!'`后的文件的所有的文件/扩展执行命令。
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A.从文件夹移除所有文件，2.txt除外。
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$ rm !(2.txt)
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||||
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B.从文件夹移除所有的文件类型，pdf类型除外。
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||||
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$ rm !(*.pdf)
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||||
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||||
### 8.检查某个文件夹（比如/home/avi/Tecmint）是否存在？并打印 ###
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||||
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||||
这里，我们使用`'! -d'`来验证文件夹是否存在，当文件夹不存在时，将使用其后跟随AND操作符`(&&)`进行打印，当文件夹存在时，将使用OR操作符`(||)`进行打印。
|
||||
|
||||
逻辑上，当`[ ! -d /home/avi/Tecmint ]`的输出为0时，它将执行AND逻辑符后面的内容，否则，它将执行OR逻辑符`(||)`后面的内容。
|
||||
|
||||
$ [ ! -d /home/avi/Tecmint ] && printf '\nno such /home/avi/Tecmint directory exist\n' || printf '\n/home/avi/Tecmint directory exist\n'
|
||||
|
||||
### 9.检查某文件夹是否存在？如果不存在则退出该命令 ###
|
||||
|
||||
类似于上面的情况，但这里当期望的文件夹不存在时，该命令会退出。
|
||||
|
||||
$ [ ! -d /home/avi/Tecmint ] && exit
|
||||
|
||||
### 10.如果您的home文件夹内不存在一个文件夹（比方说test），则创建它 ###
|
||||
|
||||
这是脚本语言中的一个常用的实现，当期望的文件夹不存在时，创建一个。
|
||||
|
||||
[ ! -d /home/avi/Tecmint ] && mkdir /home/avi/Tecmint
|
||||
|
||||
这便是全部了。如果您知道或偶尔遇到其他值得了解的`'!'`使用方法，请您在反馈的地方给我们提建议。保持联系！
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: http://www.tecmint.com/mysterious-uses-of-symbol-or-operator-in-linux-commands/
|
||||
|
||||
作者：[Avishek Kumar][a]
|
||||
译者：[wwy-hust](https://github.com/wwy-hust)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:http://www.tecmint.com/author/avishek/
|
||||
[1]:http://www.tecmint.com/12-top-command-examples-in-linux/
|
||||
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Reference in New Issue
Block a user