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sources/talk/20140729 Don't Fear The Command Line.md
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Don't Fear The Command Line
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================================================================================
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> Embrace your computer's most useful tool.
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You've probably seen it in movies, even if you haven't ever called it up on your own computer: a blank screen with a simple text prompt and a cursor, just waiting for you to enter the appropriate arcane commands to do your bidding.
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This is the command line. It's a text-based interface that predates the far more familiar windows, icons and tiles of today's major computer operating systems, from Windows to Mac OS X to Linux.
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The command line is an extremely powerful tool for accessing basic functions of your computer. For most people, it's also a confusing, complicated and seemingly irrelevant distraction. It doesn't have to be.
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### Computers Under Your Command ###
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Typing text instructions and hitting Return to get a computer to do something sounds like a major step back compared to the swipe-and-tap touch-sensitive interfaces of mobile devices. Even a toddler can use an iPad, right? Yet the command line can save you time and aggravation, if you know when to use it.
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If you're serious about learning to code—or just understanding computer technology—you absolutely need to get to know your command line.
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Diving into the command line will teach you a lot about how your computer works and organizes information. You might find that some tasks you perform every day with a mouse are actually faster when you type a command or two instead.
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Most important, you’ll be better prepared to learn [languages like Python][1] and [programs like Git][2] that require some command-line setup. By getting familiar with the command line, you’ll break down barriers that may have kept you from learning to program in the past.
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So here's a quick, basic guide to getting around on the command line. It's focused on Mac OS X's Unix-based environment, simply because that's what I'm familiar with. Linux users probably know the command line well already, although newcomers might also find these tips useful. If you're running a Chromebook, Google has some helpful instructions for getting to its [version of the command line][3], which is similar to Mac and Linux systems. Windows users, unfortunately, are stuck with a command language derived from MS-DOS that just barely overlaps with Unix, so this guide isn't going to be much use to you; you might check out [this dosprompt.info tutorial][4].
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### How To Find Your Way Around ###
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The very first thing you’ll need to do is figure out how to access the command line, which is typically done through a program called a "shell." On any Mac running OS X, you'll need to start the Terminal application. You can do this through the Finder (it's in the Utilities folder under Applications), or just click the magnifying glass in the upper right hand corner of your screen and type “terminal,” then select it from the drop-down.
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You’re in, but all you see is a blank box with a space to type prompts. This is the command line! Let’s get to know this window a little bit better.
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Type pwd, which stands for Print Working Directory. In computer parlance, “printing” something has nothing to do with paper. It really just means spitting it out on the screen. The command should result in the computer returning the directory you are currently working in.
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And indeed, /Users/laurenorsini is my home directory. Advanced tip: You can use the tilde symbol (~) as a shortcut for your home directory—it means the same thing as /Users/yourusername. So you can reference your Downloads subdirectory, for instance, as ~/Downloads. (If you look closely at the command prompt above, you'll see a tilde there. That indicates that I'm in my home directory.)
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We don’t want to muddy up our main directory with all our command-line experimenting, so let’s make a new directory with the mkdir command. This is the same as creating a new folder on your desktop operating system. Let's call it "experiments":
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Now we have a new directory. Using the graphic interface, we can visually verify that we actually created a new one. Sure enough, if I open the Finder and go into my home directory—here marked with a little house icon—I now see a folder named “experiments.” I made that on the command line! (The reverse works, too: You can create a folder on your desktop, and see it in the command line. They're just two different ways of looking at the same system.)
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Now I need to change directories and enter the ~/experiments directory with the **cd** (change directory) command.
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I have my command prompt on the default setting, so it automatically shows where my working directory is. But if yours looks different, here's how to make sure “experiments” is truly your working directory: type **pwd** again. It should tell you that your working directory is “experiments.”
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### Getting Filed Away ###
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I create and edit files on the command line every day that I code. It's faster than using the graphical user interface because I can test out my programs on the command line as soon as I finish editing them. And if I also happen to be pushing things to [GitHub][5] at the same time, well, it's even more convenient.
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Now you have a new directory (also called a repository or folder) on your computer to mess around with. Let's start by creating a new file that contains only the words, "Hello World." There are a lot of ways to do this; here I'm using the echo command.
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Now you have a new directory (also called a repository or folder) on your computer to mess around with. Let's start by creating a new file that contains only the words, "Hello World." There are a [lot of ways][6] to do this; here I'm using the **echo** command.
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Oh no! I spelled "newfile" incorrectly. That happens. Let's fix it in two steps. First, I'll create a new file with the correct spelling...
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And then, I'll use the **mv** (move) command to replace my old, misspelled file with my new file. This always takes the form "**mv oldfile newfile**."
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A note about **mv**: like many commands, it's a deceptively powerful one. When we're "moving" newfil.txt into newfile.txt, what we're actually doing is completely overwriting the first file and replacing it with the second. So the text I wrote into newfil.txt is gone forever, replaced by what I wrote into newfile.txt.
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To prove that I only have one file in my directory, I can use **ls** , the list command, to get a list of all the files in this directory.
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See? Just the one. And if I look inside the folder using my computer's graphical user interface, I can see the file there, too.
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But it's just a blank text file. Let's put something inside it using a text editor. On the command line, I tend to use the nano editor since it's simple and it works on just about every type of computer.
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This should immediately bring up a new editing screen right inside your command line window. The basic commands are all laid out for you.
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Write what you want, and then exit with CTRL + X. If it asks you to save and you'd like to, type "Y."
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As you've probably guessed by now, it's possible to also see these changes by using the operating system and navigating to newfile.txt with your mouse. Here you can open and edit the file you've created in any text editor of your choice.
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If you want to delete the file forever, do that with the **rm** (remove) command:
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Keep in mind that the **rm** command is very powerful! A [common trick][7] on hacker forums is to convince a command-line newbie to type **rm -rf** / so she ends up deleting her whole computer. The "/" means the very top-level directory of your computer—and everything underneath it. NEVER type that command!
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### Further Reading ###
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This is just the beginning of the endless possibilities of the command line. You can use this tool to control every aspect of your computer, which is what makes it as dangerous as it is powerful. Make sure to always read up on new command-line prompts before you use them, and never blindly input a prompt that a stranger suggests to you online.
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I've outlined the commands I use every day so I can code, but there are a lot more reasons to master the command line than that. If you're looking for a more thorough overview, you might want to try:
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[The Command Line Crash Course][8]. A free, extended course that covers the basics of command line usage.
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[A Command Line Primer For Beginners][9]. Lifehacker’s collection of helpful commands for first time users.
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[Introduction to the Mac OS X Command Line][10]. Online-education site Treehouse covers the very basics in extreme detail.
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Now that you've finished reading, you're better prepared for any code tutorial I've written in the past, since it's impossible to do any of them without typing in some commands. If you're ready to go, I suggest you check out ReadWrite's [Git tutorial][11], which utilizes the command line to introduce you to collaborative coding. Happy computing!
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*Lead photo by [Jason Scott][12]; all other screenshots by Lauren Orsini for ReadWrite*
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--------------------------------------------------------------------------------
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via: http://readwrite.com/2014/07/18/command-line-tutorial-intro
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作者：[Lauren Orsini][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](http://linux.cn/) 荣誉推出
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[a]:http://readwrite.com/author/lauren-orsini
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[1]:http://readwrite.com/2014/07/08/what-makes-python-easy-to-learn
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[2]:http://readwrite.com/2013/09/30/understanding-github-a-journey-for-beginners-part-1
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[3]:http://www.chromium.org/chromium-os/poking-around-your-chrome-os-device
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[4]:http://dosprompt.info/
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[5]:http://www.github.com/
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[6]:http://www.cyberciti.biz/faq/unix-create-file-from-terminal-window-shell-prompt/
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[7]:http://www.urbandictionary.com/define.php?term=rm+-rf+%2F
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[8]:http://cli.learncodethehardway.org/book/
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[9]:http://lifehacker.com/5633909/who-needs-a-mouse-learn-to-use-the-command-line-for-almost-anything
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[10]:http://blog.teamtreehouse.com/introduction-to-the-mac-os-x-command-line
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[11]:http://readwrite.com/2013/09/30/understanding-github-a-journey-for-beginners-part-1
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[12]:http://en.wikipedia.org/wiki/Computer_terminal#mediaviewer/File:DEC_VT100_terminal.jpg
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How to access Linux command cheat sheets from the command line
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================================================================================
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The power of Linux command line is its flexibility and versatility. Each Linux command comes with its share of command line options and parameters. Mix and match them, and even chain different commands with pipes and redirects. You get yourself literally hundreds of use cases even with a few basic commands, and it's hard even for seasoned system admins to get used to them all. That's when command line cheat sheets come to our rescue.
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[][1]
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I know man pages are still our best friend, but we want to be efficient and strategic by having quick reference cards at our disposal. Your ultimate cheet sheets could be hanging on your office wall with pride, or secretly stored in your hard drive as PDF files, or even be the background image on your desktop.
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Alternatively, use yet(!) another command to access your favorite command line cheat sheets. That is, use [cheat][2]. Which is a command line tool allowing you to access, create or update cheat sheets from the command line. The concept is really simple, yet cheat turns out to be quite useful. This tutorial is about how to use cheat command on Linux. You don't need a cheat sheet for using cheat command. It's that simple.
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### Installing Cheat on Linux ###
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First install Git if you haven't:
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$ sudo apt-get install git (Debian-based system)
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$ sudo yum install git (RedHat-based system)
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Also install [Python package installer pip[3].
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Finally, install cheat using the following commands.
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$ sudo pip install docopt pygments
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$ git clone https://github.com/chrisallenlane/cheat.git
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$ cd cheat
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$ sudo python setup.py install
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### Configuring Cheat ###
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There is not much to configure for cheat command.
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One thing to recommend is to enable command-line autocompletion. That way, when you look up a cheat sheet, you can use [TAB] key to auto-complete the name of the command you want to check. Here is how to enable autocompletion for bash.
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$ wget https://github.com/chrisallenlane/cheat/raw/master/cheat/autocompletion/cheat.bash
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$ sudo cp cheat.bash /etc/bash_completion.d/
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They provide autocompletion scripts for other shells such as zsh and fish as well.
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Another thing is to define an EDITOR environment variable. This variable should point to a text editor that you want to use when creating or updating a cheat sheet. For example, if you want to use Vim editor, put the following in ~/.bashrc.
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export EDITOR=/usr/bin/vim
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Log out and log back in to activate autocompletion and updated .bashrc.
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### Basic Usage of Cheat ###
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One cool thing about the cheat command is that it comes with pre-built cheat sheets for more than 90 popular Linux commands. To get a list of available cheat sheets:
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$ cheat -l
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To access a cheat sheet of a specific command, simply run cheat with the name of the command:
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$ cheat <command-name>
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You can search all the cheat sheets that contain a specific keyword by using "-s" option:
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$ cheat -s <keyword>
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In many cases, cheat sheets that are useful to some folks may not that helpful to others. To personalize pre-built cheat sheets, cheat command allows you to create a new cheat sheet or update existing ones. To do so, cheat command can keep local copies of cheat sheets in ~/.cheat directory.
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To take advantage of cheat's editing feature, first make sure that the EDITOR environment variable is set to the full path of your default text editor. Then copy (non-editable) built-in cheat sheets to ~/.cheat directory. You can find where the built-in cheat sheets are by running the following command. Once you know where they are, simply copy them over to ~/.cheat directory.
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$ cheat -d
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----------
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/usr/lib/python2.6/site-packages/cheat/cheatsheets
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----------
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$ cp /usr/lib/python2.6/site-packages/cheat/cheatsheets/* ~/.cheat
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Now you can create or update a cheat sheet by using "-e" option:
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$ cheat -e openssl
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As you can imagine, the cheat's editing feature is very useful to tailor a local cheat sheet repository to meet your needs. If you believe in sharing knowledge, you are more than welcome to contribute your cheat sheets to the cheat command's [official Git repository][4], so everyone can benefit from them.
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/2014/07/access-linux-command-cheat-sheets-command-line.html
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作者：[Dan Nanni][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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|
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/nanni
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[1]:http://xkcd.com/1168/
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[2]:https://github.com/chrisallenlane/cheat
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[3]:http://ask.xmodulo.com/install-pip-linux.html
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[4]:https://github.com/chrisallenlane/cheat
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131
sources/tech/20140729 How to use awk command in Linux.md
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How to use awk command in Linux
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================================================================================
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Text processing is at the heart of Unix. From pipes to the /proc subsystem, the "everything is a file" philosophy pervades the operating system and all of the tools built for it. Because of this, getting comfortable with text-processing is one of the most important skills for an aspiring Linux system administrator, or even any power user, and awk is one of the most powerful text-processing tools available outside general-purpose programming languages.
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The simplest awk task is selecting fields from stdin; if you never learn any more about awk than this, you'll still have at your disposal an extremely useful tool.
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By default, awk separates input lines by whitespace. If you'd like to select the first field from input, you just need to tell awk to print out $1:
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$ echo 'one two three four' | awk '{print $1}'
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> one
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(Yes, the curly-brace syntax is a little weird, but I promise that's about as weird as it gets in this lesson.)
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Can you guess how you'd select the second, third, or fourth fields? That's right, with $2, $3, and $4, respectively.
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$ echo 'one two three four' | awk '{print $3}'
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(Yes, the curly-brace syntax is a little weird, but I promise that's about as weird as it gets in this lesson.)
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Can you guess how you'd select the second, third, or fourth fields? That's right, with $2, $3, and $4, respectively.
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$ echo 'one two three four' | awk '{print $3}'
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> three
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Often when text munging, you need to create a specific format of data, and that covers more than just a single word. The good news is that awk makes it easy to print multiple fields, or even include static strings:
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$ echo 'one two three four' | awk '{print $3,$1}'
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> three one
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----------
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$ echo 'one two three four' | awk '{print "foo:",$3,"| bar:",$1}'
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> foo: three | bar: one
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Ok, but what if your input isn't separated by whitespace? Just pass awk the '-F' flag with your separator:
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$ echo 'one mississippi,two mississippi,three mississippi,four mississippi' | awk -F , '{print $4}'
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> four mississippi
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Occasionally, you may find yourself working with data with a varied number of fields, and you just know you want the *last* one. awk prepopulates the $NF variable with the *number of fields*, so you can use it to grab the last element:
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$ echo 'one two three four' | awk '{print $NF}'
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> four
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You can also do simple math on $NF, in case you need the next-to-last field:
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$ echo 'one two three four' | awk '{print $(NF-1)}'
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> three
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Or even the middle field:
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$ echo 'one two three four' | awk '{print $((NF/2)+1)}'
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> three
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$ echo 'one two three four five' | awk '{print $((NF/2)+1)}'
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> three
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While this is all very useful, you can get away with forcing sed, cut, and grep into a form to get these results, as well (albeit with a lot more work).
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So, I'll leave you with one last introductory feature of awk, maintaining state across lines.
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$ echo -e 'one 1\ntwo 2' | awk '{print $2}'
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> 1
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>
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> 2
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$ echo -e 'one 1\ntwo 2' | awk '{sum+=$2} END {print sum}'
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> 3
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(The END indicates that we should only perform the following block **after** we finish processing every line.)
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The case where I've used this is to sum up bytes from web server request logs. Imagine we have an access log that looks like this:
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$ cat requests.log
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> Jul 23 18:57:12 httpd[31950]: "GET /foo/bar HTTP/1.1" 200 344
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>
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> Jul 23 18:57:13 httpd[31950]: "GET / HTTP/1.1" 200 9300
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>
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> Jul 23 19:01:27 httpd[31950]: "GET / HTTP/1.1" 200 9300
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>
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> Jul 23 19:01:55 httpd[31950]: "GET /foo/baz HTTP/1.1" 200 6401
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>
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> Jul 23 19:02:31 httpd[31950]: "GET /foo/baz?page=2 HTTP/1.1" 200 6312
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We know the last field is the number of bytes of the response. We've already learned how to extract them using print and $NF:
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$ < requests.log awk '{print $NF}'
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> 344
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>
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> 9300
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>
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> 9300
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>
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> 6401
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>
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> 6312
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And so we can sum into a variable to gather the total number of bytes our webserver has served to clients during the timespan of our log:
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$ < requests.log awk '{totalBytes+=$NF} END {print totalBytes}'
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> 31657
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If you're looking for more to do with awk, you can find used copies of [the original awk book][1] for under 15 USD on Amazon. You may also enjoy Eric Pement's [collection of awk one-liners][2].
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|
||||
--------------------------------------------------------------------------------
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||||
|
||||
via: http://xmodulo.com/2014/07/use-awk-command-linux.html
|
||||
|
||||
作者：[James Pearson][a]
|
||||
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) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:http://xmodulo.com/author/james
|
||||
[1]:http://www.amazon.com/gp/product/020107981X/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=020107981X&linkCode=as2&tag=xmodulo-20&linkId=6NW62B2WBRBXRFJB
|
||||
[2]:http://www.pement.org/awk/awk1line.txt
|
||||
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