Introduction to Command Line

IntroductionA Brief OverviewWe communicate with modern computers by clicking, tapping, sliding, hovering, typing, shaking, and speaking. Through simple gestures, we are able to command the computer. Many computeroperating systems such as Windows and OS X provide a graphical user interface that enables us to open and view les, play games, turn on music, etc., all by simple movements and clicks of the mouse. Most operating systems that run on servers, desktops, and laptops provide another way of commanding your computer that many people have never even heard of. !t is aptly called the command line interface "or #$!%, the command line, or the command prompt.$ike a graphical user interface, the command line is one way thatthe operating system represents the computer&s les, directories,and programs "which are also les% to the user. !n fact, the command line is a te't(based interface through which one can navigate, create, e'ecute, and act on a computer&s les and directories with precision.What We'll Cover in this BookSince the $inu' operating system is the most commonly used operating system for servers, we will cover its command line interface in this book. Other operating systems such as Mac OS Xand )ni' are very similar, so you&ll be able to use everything you *learn in this book on those operating systems. Other operating systems, such as Windows, have command prompts, but they di+er in some signicant ways that make it di,cult to cover theirusage in one book.How to Use the Command Line)sing the command line is as simple as opening up Terminal "available on Mac and $inu'% or iTerm "available on Mac only% on your computer, typing a command and pressing enter. Most commands will return some kind of output. -o ahead and open up Terminal and try typing the following command./ echo 01ello World02ou should see something like this./ echo 01ello World01ello World/There&s nothing spectacular there, but the above e'ercise illustrates how you can command the computer to do something and then see its result.What's in it for me?While it may seem di,cult at rst, being able to use the command line will empower you as a computer user. Once you&ve3learned the basics of using the #$!, you&ll be able to simplify and speed up many tasks that were previously tedious. 2ou can be very precise about how you want your computer to perform certain tasks, or about which pieces of information it should display. 4y reading this book and doing its e'ercises, you&ll gain the following knowledge and skills. )nderstand commands that tutorials tell you to useo 5s you learn programming, you will inevitably have to use the command line to install software, compile or run code, and perform other types of system administration.o $earning the basics of the command line will help you understand why you are running the commands, and give you a general idea of how they work. 6ower over the computero 4eing able to communicate with your computer or server via command line gives you immense and precise power over your computer.o With command line, you can easily monitor your computer and its resources.o 2ou can take advantage of a lot of powerful tools in their raw, powerful form.7o 2ou can learn to diagnose issues with your own softwareand others& software.PreparationsWho should read this bookThis book is intended to help you on your path to becoming a skilled programmer. !t covers the basics of command line usage in a )ni'(like environment, which includes $inu' and Mac OS X. !t does not include Windows 6owerShell or Windows #ommand 6rompt usage. !t is assumed that you know the basics of using a computer and have run a command on the command line before.8nowledge of programming is not re9uired.ettin! the "ost Out of this BookThere are a few things you must do to successfully learn the lessons in this book. :irst, try things out and make what you&ve learned your own. ;on&t want to mess up your own computer or server< =o problem. you can easily set up a separate server or virtual machine to mess around with. See the ne't section for some suggestions. Second, do the e'ercises. They are there to give you a real(world understanding of the command line and will help you remember what you&ve learned. Third, e'plain whatyou&ve learned to a friend or coworker, or to a rubber duck if that&s more comfortable.>#erver #etu$!f you don&t want to mess up your computer&s console while going through the e'amples in this book, you can set up a serverusing one of many online cloud services or set up your own virtual machine. The following are some of the available options.*. Sign up for a service that provides a full, in(browser development environment, such as =itrous or #loud ? . Once you&ve set up one of these accounts, you can ignore everything e'cept the terminal or console. We recommend this option if you&re new to the command line, as it takes the least amount of e+ort to set up.3. )se @ackspace or ;igital Ocean to set up a managed server that provides console access. :or this option, you&ll need a little bit of e'perience with servers.7. Set up a virtual machine on your computer. 2ou can use Airtualbo' ,AMWare , Aagrant , ;ocker , or a combination of these. Setting up a virtual machine is out of the scope of this book, but a 9uick -oogle search should be enough to get you going. 2ou should have a lot of hands(on e'perience with conguring software and in(depth knowledge of computers before taking this option.The Command Line InterfaceBWhat is an interface?5ccording to the online Merriam Webster dictionary, an interface is 0a system that is used for operating a computer. a system thatcontrols the way information is shown to a computer user and the way the user is able to work with the computer.0* 5s this denition e'plains, an interface has two parts. *% a display of information about what the computer is doing and 3% a method for telling the computer what to do. The command line interface is a very basic interface that uses mostly te't for both the display and input components of the interface. !n a nutshell, the command line allows for te't(based communication with a computer. $et&s look at the two parts of the command line interface. the display and the input.CLI %is$la&The command line can be the default interface for a computer, but most personal computers use a program "like Terminal% within the desktop graphical interface to provide the command line interface. $et&s dissect the #$!&s language for a moment. To follow along, log in to your virtual machine or server.4elow is an e'ample of the prompt on an )buntu $inu' server.ubuntuCchopin.D/The above prompt follows this format.EFuserGCFhostnameG.FcurrentHdirectoryG/The FuserG portion "0ubuntu0% represents the current user that is logged in to the command line interface. The FhostnameG portion represents the computer&s name. :ollowing those two pieces is .FcurrentHdirectoryG. The colon is Iust for separation, and the FcurrentHdirectoryG displays the path of the directory that you are in. !f you&ve Iust logged in, it&s probably Iust a tilde "D%, which represents the home directory. The last piece is / "note the trailing space%. This whole piece of te't is called the prompt, or 6S*. !t can be modied to t your needs, but usually displays some very basic information that shows you the conte't of what you&re doing. Whenever you log in to a server, or whenever you open up Terminal on your own computer, you&ll be presented witha prompt like this one. :or the rest of this book, we&ll represent the prompt as a simple /.5t the end of the prompt, you should see a cursor which, in the world of the command line, is Iust a bo' that blinks on for a second and o+ for a second. The cursor shows where you are able to input additional te't. The thing that allows you to input te't is called ST;!= "standard input%.The last piece of the display portion of the #$! is the output of your commands. 5s you can see in the image below, when you type a command, it may have te'tual output.J/ ls Kbin homelibE> opt sbinusrbootinitrd.imglostLfoundprocsrv vardev initrd.img.oldmedia rootsys vmlinuMetc lib mnt run tmp vmlinuM.oldWhen the output has printed to the screen and the command or program e'its, the prompt is displayed again below the output. To review, the components of the #$! display are the prompt, the cursor, the input "te't you have typed in%, and the output of your commands and programs.CLI In$utThe second component of this te'tual interface is the input. 4y typing te't into the command line interface, you are creating input for the #$! to interpret and act on. )sing commands such as echo or ls, you can command the computer to do your bidding. $et&s try a command. Try typingecho 01ello World0 into the console./ echo 01ello World0The echo command is very simple, but it can also be very useful. 5ll it does is send te't to the #$!&s output. 5nother useful command is the pwdcommand. !f you want to know where you arein your computer&s le system, you can type pwd and press enter to e'ecute it. 2ou should get something like the following as output from that command.N/ pwdKhomeKubuntuThe commands above are simple, and don&t actually make any changes to your command line environment, the computer, or itsles (( they only display some output. 4ut other commands such as rm, cp, and source can directly a+ect the les on your computer or can change your command line environment.'he Anatom& of a Command#ommands come in all shapes and siMes, but they have many features in common. When you type a command into a terminal, it will always take the following format.FcommandG Farguments...G5 command can be the path to a le "e.g. KpathKtoKle%, or it can be a command that your terminal is already aware of "e.g. echo%. Thesecond portion of the command usually consists of what are called 0arguments0. 5rguments are strings that are passed to theprogram that you are e'ecuting. Stated more simply, these are pieces of information that you are providing to your command.To better understand how this works, we can compare the command line and its commands to a well(trained army. !n this army, each soldier has a very specic duty, and is trained to do itwith perfection. Some duties, like cleaning the dishes, don&t take any e'tra information for them to be accomplished successfully. ?Other duties re9uire, or at least benet from, e'tra information. !fyou are the general and you tell a soldier to go on patrol, he maybe able to go out and patrol somewhere, but it may or may not be where you need him most. 1e&ll probably Iust go to his usual patrolling area and start there.#ommand line commands are basically the same. !f you type a command by itself, it will sometimes e'ecute Iust ne, using default information to do its Iob. Other times, however, you reallyneed to give the command more information so that it can know e'actly how to perform its duty. Take, for e'ample, the tar command. !f you open the command line prompt, typetar, and press enter, it won&t do much. !n fact, all it does is ask you for more information. 02ou want me to patrol< Where should ! patrol opt sbinusrbootinitrd.imglostLfoundprocsrv vardev initrd.img.oldmedia rootsys vmlinuMetc lib mnt run tmp vmlinuM.oldWithout any arguments, ls Iust prints out a list of the les and directories in the current directory. !t doesn&t tell you much about those les and directories, though. !f you add a couple of arguments, you can get the lscommand to give up more information about each item in the directory./ ls (lahtotal N>8drw'r('r('33 root root >.Q8 May *> *J.7* .drw'r('r('33 root root >.Q8 May *> *J.7* ..drw'r('r(' 3 root root >.Q8 May *3 37.3Q bindrw'r('r(' 7 root root >.Q8 May *3 37.3Q bootdrw'r('r('*7 root root 7.?8 May *> *J.7* dev*Jdrw'r('r('?E root root >.Q8 Yun 33 *J.*B etcdrw'r('r(' 7 root root >.Q8 May *3 37.*? homelrw'rw'rw' * root root 77 May *3 37.3Q initrd.img (Z bootKinitrd.img(7.*7.Q(B3(genericlrw'rw'rw' * root root 77 Mar 3B **.B* initrd.img.old (Z bootKinitrd.img(7.*7.Q(>N(genericdrw'r('r('3* root root >.Q8 May *7 *?.Q7 libdrw'r('r(' 3 root root >.Q8 Mar 3B **.BQ libE>drw'(((((( 3 root root*E8 Mar 3B **.B7 lostLfounddrw'r('r(' 3 root root >.Q8 Mar 3B **.BQ mediadrw'r('r(' 3 root root >.Q8 5pr *Q3Q*> mntdrw'r('r(' 3 root root >.Q8 Mar 3B **.BQ optdr('r('r(' *3J root rootQ May *> *J.7Q procdrw'(((((( 7 root root >.Q8 May *7 *?.QQ rootdrw'r('r('*N root rootJQQ Yul* *>.*3 rundrw'r('r(' 3 root root >.Q8 May *3 37.3Q sbindrw'r('r(' > ubuntu ubuntu >.Q8 Yun 33 *J.QE srvdr('r('r('*7 root rootQ May *> *J.7Q sysdrw'rw'rwt > root root >.Q8 Yul* *>.>Q tmpdrw'r('r('*Q root root >.Q8 Mar 3B **.BQ usrdrw'r('r('*3 root root >.Q8 Mar 3B **.B3 varlrw'rw'rw' * root root 7Q May *3 37.3Q vmlinuM (Z bootKvmlinuM(7.*7.Q(B3(genericlrw'rw'rw' * root root 7Q Mar 3B **.B* vmlinuM.old (Z bootKvmlinuM(7.*7.Q(>N(genericWe&ll go into detail of what everything means in later chapters, but for now, you can see that adding the arguments (l, (a, and (h "grouped into one argument above%, *Nadds a lot more information and formats the items into a more easy(to(read list.?. The command line o+ers a bunch of commands to work withles and directories. This e'ercise will show the basic usage of the following.touch, mkdir, mv, cp, and rm.:irst, let&s make sure we&re in your home directory "recall that D stands for your home directory%./ cd D/ pwdKhomeKubuntu=ow, let&s create a practice directory to mess around with./ P Make a directory called 0practice0/ mkdir practice/ lspractice2ou can see that now there&s a directory named 0practice0 inyour home directory. $et&s change our current directory to the 0practice0 directory that we Iust created./ cd practice*?=ow, in this directory we can create new les, move or rename them, copy them, and remove them. 5fter that, we&llremove the whole 0practice0 directory to clean up./ P #reate an empty le and verify that it got created/ touch e'ample.t't/ lse'ample.t'tThe touch command created the empty le 0e'ample.t't0 in the current directory. 2ou can move or rename a le with the mv command./ P @ename e'ample.t't to e'ample*.t't/ mv e'ample.t't e'ample*.t't/ lse'ample*.t't/ P Make another directory/ mkdir tmp/ P Move e'ample*.t't to the new 0tmp0 directory/ mv e'ample*.t't tmpK/ ls tmpKe'ample*.t't/ lstmp/ P Move it back and rename it/ mv tmpKe'ample*.t't e'ample3.t't/ ls3Qe'ample3.t't tmp2ou can see from the above e'amples that moving or renaming a le is basically the same thing on the command line, and follows this pattern./ mv FsourceG FdestinationG=ow, let&s remove the e'ample le and then the whole practice folder./ rm e'ample3.t't/ lstmpTo remove a folder and all its contents, you need to specify the (r"recursive% option./ cd ../ lspractice/ rm (r practice/ lsThe practice folder "and all its contents% are now gone.Warning. using the rm command is dangerous and permanent. ;o not issue this command until you know for 3*certain you are deleting the right le. )sing the rm (r command is even more dangerous, as it will delete recursively.*Q. One of the most common tasks when using the command line interface is reading the contents of a le. Thise'ercise will go over some common commands for doing so. cat, more, less, head, and tail.To print out all the contents of a le, use cat./ cat le.t't5ll the content of le.t't printed out here.To print out the rst few lines of a le, use head./ head KetcKmime.typesPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPM!MR media types and the e'tensions that represent them.PPThe format of this le is a media type on the left and Mero or morePlename e'tensions on the right.6rograms using this le will mapPles ending with those e'tensions to the associated type.PPThis le is part of the 0mime(support0 package.6lease report a bug usingPthe 0reportbug0 command of the 0reportbug0 package if you would like new33To print out the last few lines of a le, use tail./ tail KetcKmime.typestail KetcKmime.typesvideoK'(ms(wm'wm'videoK'(ms(wv'wv'videoK'(msvideo avivideoK'(sgi(movie movievideoK'(matroskampv mkv'(conferenceK'(cooltalk ice'(epocK'(sis'(app sis''(worldK'(vrmlTo print out the contents of a le, but only ll one screen&s worth at a time, use more./ more KetcKmime.typesP The rst page of KetcKmime.types will show up here, and you can use theP down arrow to go to the ne't line, or the space bar to go to the ne'tP page.)se less when you need to navigate backward and forward ina le./ less KetcKmime.types37The less command allows you to go forward one line with the down arrow, backward one line with the up arrow, and backward and forward a page with the page up and page down keys. 2ou can also use the space bar in the same way you can with the more command.=ote. to e'it more or less, type the 9 key.**. 2ou can get more information about what a command does, how it works, and which Sags you can use by referencing the manual for that command. To read the manual pages "typically called manpages% for a command, use the man command.*3. / man touch*7. TO)#1"*%)ser #ommands TO)#1"*%*>.*B.*E.*J. =5MR*N.touch ( change le timestamps*?.3Q. S2=O6S!S3*.touch FO6T!O=G... :!$R...33.37. ;RS#@!6T!O=3>.)pdatethe access and modication times of each :!$R to the3B.current time.3>3E. ...The manual pages will sometimes have the available Sags and the possible orders they can be used in in the 0S2=O6S!S0 section. !n the case of the touch command, the Sags can be used in any order Iust after the touch command itself, and the le is the last argument. The possible options are described in the 0;RS#@!6T!O=0 section.2ou may have noticed that the man command uses less to display the content of the manpages, so you can use the up(and down(arrow keys to navigate the contents of the manual.To e'it man, you can type the 9 key.Files, Directories, and Executables'he Core of the Command Line!n a desktop environment, like that of OS X, you have windows, menu bars, and the desktop to give conte't to what you are doing. !n the command line, however, the conte't is solely the le system. !n fact, les and directories are what make up the command line. 5lmost everything you do at the prompt will deal 3Bwith les. Rvery time you type a command, you are telling the computer to e'ecute a le.=ot only does the le system provide conte't for you when you&re trying to work on or get information about les and directories, it also provides conte't for the commands you run. :or e'ample, you can use the lscommand to list the les in a directory. When you run the command by itself, it uses your current directory as conte't, and lists the les that are in the directory you are in.Linu)*Uni) +ile #&stem Le!end$et&s look for a moment at some of the symbols that will help us navigate the command line. K ( The root directory or a separator when listing directories . ( The current directory "also .K% or the same level .. ( The directory one level up "also ..K% ..K.. ( Two levels up D ( 2our 0home0 directory, or the directory you are placed in when you log in. [ ( The 0splat0 or 0glob0 operator. This is the wildcard of the command line and represents 0any characters.03EThe above symbols can be combined with directory and le names to represent their locations. The path KhomeKubuntu can be dissected as follows.root directory L 0home0 directory L directory separator L 0ubuntu0 "user%directoryWhile it may not make sense to do so, somewhat 0nonsensical0 combinations can represent valid paths. The following path is thesame asKhomeKubuntu.KhomeKubuntuK..K..K.KhomeKubuntuK$et&s break the above path into its parts. KhomeKubuntuK ( The ubuntu directory within the home directory within the root "K% directory. ..K..K ( )p two directories "which takes us back to the root directory%. .K ( The same directory "which is still K%. homeKubuntuK ( 4ack down into the home directory, then the ubuntu directory.=ote that there&s a dramatic di+erence between a path that starts with a leading slash vs one that doesn&t. :or e'ample.3J KhomeKubuntu This path species a le called 0ubuntu0 that lives under your root directory, then home directory. homeKubuntu This path species a completely di+erent le. This path means there&s a le called 0ubuntu0 that lives in a directory called 0home0, which is itself in the current directory.The leading slash makes all the di+erence. Make sure to pay careful attention to that whenever you&re looking at paths.,avi!atin!=avigating your computer&s le system is pretty easy with the help of the :ile System $egend above, and a few simple commands. cd ( change directory ls ( list les pwd ( display the current working directory$et&s get a feel for navigating the command line interface by opening up Terminal or logging in to your managed server or virtual machine and treading water for a bit. 5fter logging in, type the following commands and note what happens after each step.P #hange directory "with no arguments%3N/ cd/ pwdKhomeKubuntuP #hange to root directory/ cd K/ pwdKP #hange directory "with no arguments again%/ cd/ pwdKhomeKubuntuP -o up one directory/ cd ..K/ pwdKhomeP Show les and directories "with no arguments, it uses current directory%/ lsubuntuP =avigate into 0ubuntu0 directory/ cd ubuntu/ pwdKhomeKubuntu3?P #hange to D "home% directory/ cd D/ pwdKhomeKubuntuP Show les and directories in root directory/ ls KbinbootdevetchomeliblibE>mediamntoptprocrootrootfsrunsbinselinu'srvsystmpusrvarP Show les that match a certain patternP using the splat operator "asterisk%/ ls K[otboot root;id you notice that typing cd by itself takes you to your 0home0 directory< Which command"s% made you 0descend0 into a directory< Which command"s% made you go up a directory< What was the di+erence in the output of ls vs. ls K< What do you think the output of ls .K would be if you rst ran cd K/ ls (d .[. .bashHhistory.bashHprole.cache .gem.npm.viminfo...bashHlogout .bashrc.cong.local.prole.ssh3. $ist the hidden les in the parent directory. Then list the lesin the parent directory&s parent directory.#olution2ou can list a parent directory&s les using the following command./ ls ..ubuntuWhat happens if you run that command from the root directory opt sbinusrbootinitrd.imglostLfoundprocsrv vardev initrd.img.oldmedia rootsys vmlinuMetc lib mnt run tmp vmlinuM.old5s you can see, since the root directory doesn&t have a parent directory, it Iust prints out the les in the root directory itself.7BTo list the les of the parent&s parent directory, Iust add a slash and an e'tra .../ cd/ ls ..K..bin homelibE> opt sbinusrbootinitrd.imglostLfoundprocsrv vardev initrd.img.oldmedia rootsys vmlinuMetc lib mnt run tmp vmlinuM.old7. What&s the di+erence between abcK and Kabclibe'eclocalsbinsharesrctmp)se the (lah set of Sags as arguments to the ls command to get more detailed information about les and directories./ cd K/ ls (lahtotal **38dr('r('r(' 3> root root >.Q8 Mar *E 3Q.BE .dr('r('r(' 3> root root >.Q8 Mar *E 3Q.BE ..(rw(r((r((* root rootQ Mar *E 3Q.BE .autofsckdr('r('r('3 root root >.Q8 Oct*3Q*> bindr('r('r('7 root root >.Q8 Oct*3Q*> bootdrw'r('r(' *E root root 3.N8 Yun7 3*.>N devdrw'r('r(' JE root root >.Q8 Yun7 3*.>N etcdrw'r('r('7 root root >.Q8 Sep 3?3Q*> homedr('r('r('J root root >.Q8 Mar 3B3Q*> libdr('r('r(' *Q root root*38 Oct*3Q*> libE>drw'r('r('3 root root >.Q8 Mar 3B3Q*> localdrw'((((((3 root root*E8 Mar 3B3Q*> lostLfounddrw'r('r('3 root root >.Q8 YanE3Q*3 mediadrw'r('r('3 root root >.Q8 YanE3Q*3 mntdrw'r('r('7 root root >.Q8 Mar 3B3Q*> optdr('r('r(' JB root rootQ Mar *E 3Q.BE procdr('r('(((7 root root >.Q8 Yun7 3*.>N rootdrw'r('r('> root root >.Q8 Sep 3?3Q*> rundr('r('r('3 root root*38 Oct*3Q*> sbindrw'r('r('3 root root >.Q8 YanE3Q*3 selinu'>Qdrw'r('r('3 root root >.Q8 YanE3Q*3 srvdr('r('r(' *7 root rootQ Mar *E 3Q.BE sysdrw'rw'rwt7 root root >.Q8 Yun *Q Q7.7> tmpdrw'r('r(' *7 root root >.Q8 Mar 3B3Q*> usrdrw'r('r(' *? root root >.Q8 Mar 3B3Q*> varN. !n this e'ercise we&re going to play with e'ecutables. We&ll nd the location of so(called 0default e'ecutables0 by using the which command, then e'ecute a le using its absolute path.To gure out a default e'ecutable&s location, you can use the whichcommand./ which touchKbinKtouch=ow we can e'ecute the touch command using it&s full path or it&s name./ cd/ touch test/ lstest/ KbinKtouch test3/ lstest test3>*We can do the same thing now with the rm command.WarningX The 0rm0 command is e'tremely dangerous. There is no easy, or even moderately di,cult, way to restore what you delete with the 0rm0 commandX/ cd/ rm test/ lstest3/ which rmKbinKrm/ KbinKrm test3/ ls/The Environment(nvironment -ariables!n the last chapter, we talked about how your 0current directory0 provides conte't for commands you run. 5nother way of providing conte't is through something called environment variables. !n programming, variables are used to store data and to be able to reference and retrieve that data at a later point using a name. !n the command cd /1OMR, the /1OMR part is a reference to the 1OMR variable, and is replaced by the path to your home directory when the command is run. !n other words, >3running cd /1OMR is the same as running cd KhomeKubuntu, assuming your home directory is KhomeKubuntu.When you log in to the command line, a variety of environment variables are automatically set. 2ou can see e'actly what variables have been set, along with their values, by running env at the command line. Type env, hit enter, and nd the value for 1OMR. !t should say something like KhomeKubuntu, where ubuntu will be replaced by your username. !f you&re doing this ona Mac, the value will probably be something like K)sersKbob. This is the path to your home directory.How to Chan!e &our Command Line (nvironmentWhile there are several environment variables that are set for you automatically, you can also set your own or modify e'isting variables. 2ou can do this on the Sy, so that your changes only a+ect the current command or the current session, or you can make the changes more permanent so that they stick between sessions.=ote. The term 0session0 refers to the state of being logged in to a computer&s command line interface. When you log in, you start a new session, in which your commands will be recorded and other conte'tual information will be maintained. When you close Terminal or type 0e'it0, your session is closed and that conte't and data is lost.#ettin! (nvironment -ariables on the +l&>7There are two ways to set an environment variable on the Sy.*. Set the variable on its own line, then use it anywhere./ SOMRT1!=-\&some value&/ echo /SOMRT1!=-some value3. Set the variable before a command, on the same line./ SOMRT1!=-\&a value& env...SOMRT1!=-\a value...=ote. 2ou cannot "very easily% use a value on the same line that you set it. That&s because variables are evaluated before the setting occurs./ SOMRT1!=-\&something else& echo /SOMRT1!=-P no output;id you notice that when you set a variable you don&t prepend the dollar sign, but when you reference it, you do< 5lso note that there should be no spaces between the variable and the e9ual sign or the e9ual sign and the value. $astly, it&s usually best to use 9uotations around the value that you are assigning to the >>variable, but you don&t have to when the value doesn&t have any special characters.$et&s try changing our current session&s environment. Maybe you&d like to simplify your prompt. To change your prompt, adIustthe 6S* variable to whatever you&d like it to be./ 6S*\0"testprompt%Z 0"testprompt%Z5s you can see, the prompt is now "testprompt%Z, and every time a command nishes, it will show up again. !f you want a more complicated prompt, try the following./ 6S*\0]n]F]eFQO7Jm]GF]hG ]eFQO7Bm]G]d]eFQm]G]n]F]eFQO7*m]G]u]F]eFQO7>m]G in ]F]eF*O77m]G]w]F]eFm]G]F]eFQO7*m]G]n]F]Q77F7Bm]G/]F]Q77FQQm]G 0FchopinG Wed 5pr QNubuntu in D/The new prompt is multi(line and has color(coding. !f you want to revert to your old prompt, Iust close your session and start a newone. Since we made the changes to the environment variable 6S* on the Sy, they won&t be used in future sessions."akin! "ore .ermanent Chan!es>B!t is possible to make more permanent changes to the command line environment. When you start a command line session by opening a new Terminal window, one or more environment les are e'ecuted. These les can be used to modify or create environment variables. They are usually located in your home directory and include the following les. .bashrc,.bashHprole, .bashHlogin, .login, .prole. 4ecause they start with a ., they are considered to be 0hidden0 les, and using the ls command alone won&t show them. Type ls (a D to see them listed along with other les in your home directory. @emember, the ls is the command, and the (a and Dare arguments to the ls command. The (a Sag tells the ls command to include les that start with . in its output, while the D is the directory that lsshould inspect "recall that D means your 0home0 directory%./ ls (a D..cache.sudoHasHadminHsuccessful.. .mys9lHhistory.vbo'Hversion.bashHhistorypostinstall.sh.veeweeHversion.bashHlogout .prole.vim.bashrc.ssh.viminfoThe rules behind which environment le is read for a new sessionare complicated and depend on how the session is created. :or our purposes, using .bashrc should be su,cient. !f your edits aren&t working, try one of the other les.$og in to your console and type the following command.>E/ cat DK.bashrc2ou should see some lines that start with e'port. The e'port command basically means 0make this variable available globally,0 and variable assignment that starts with e'port in your .bashrc will be included in your environment when you log in. =ote that if an environment variable has beene'ported once, you don&t need to continually put e'port before it when you set its value.!f you&re ready to customiMe your prompt a bit more permanently,add your custom prompt to the bottom of your .bashrc le.6S*\0Fyour custom prompt goes hereG 0The e'port isn&t needed because the variable is already available globally. )se the following pieces along with any custom te't to make your prompt./h 1ostname/u )ser name/w #urrent directory/W 4asename of current directory/d #urrent date>J/n =ewline;id you notice that Iust editing and saving your .bashrc le didn&t do anything< The le is only evaluated, or run, when you rst login. !f you want to re(run a particular environment le like .bashrc or .bashHprole, use thesource command./ source DK.bashrc;o revert back to your old prompt, edit the same le and removeyour 6S*setting. Then run source on that le.Fmy custom promptG/ source DK.bashrc/Usin! (nvironment -ariables$et&s look for a moment at the di+erent ways we can use environment variables.01 As .arts of Commands:irst, variables can be used as arguments to commands. Take a look at the following e'ample./ MRSS5-R\&1ello, worldX&/ echo /MRSS5-R1ello, worldX>NThis is a very simple e'ample, but you can see that the /MRSS5-R variable is used as the rst "and only% argument to the echo command. 2ou can actually use variables as commands as well./ MRSS5-R\&1ello, worldX&/ #OMM5=;\0echo0/ /#OMM5=; /MRSS5-R1ello, worldX21 Inter$olated in #trin!sAariables can also be interpolated, or included, in other strings. Take the following e'ample./ MRSS5-R*\0This is message *.0/ MRSS5-R3\0This is message 3.0/ MRSS5-R\0/MRSS5-R* /MRSS5-R30/ echo /MRSS5-RThis is message *. This is message 3.To have a variable get interpolated, you have to use the double 9uotation mark "0%, not the single 9uotes "&%. Try the following e'ample in your command line./ MRSS5-R*\0This is message *.0/ MRSS5-R3\0This is message 3.0/ MRSS5-R\&/MRSS5-R* /MRSS5-R3&>?/ echo /MRSS5-R/MRSS5-R* /MRSS5-R331 Behind the #cenesRnvironment variables can be used by commands "programs% behind the scenes. !n other words, you can set a variable, then run a command without passing the variable as an e'plicit argument to that command, and the command could use that variable. The 6W; variable is automatically used by any commandthat tries to get the user&s current directory. The 1OMR variable is automatically used by cd when you don&t pass any arguments to it. !f you make up a custom variable "like 6!\7.*>%, only programs that know about it will be able to use it without e'plicitly using it as an argument.!f you want to temporarily change a variable before it gets used in a command behind the scenes, you can set the variable immediately preceding the command on the same line.P Set home to root directory and change to home./ 1OMR\K cd/ pwdKP #hange to home directory./ cd/ pwdBQKhomeKubuntu=ote how that the second cd takes you to your original home directory, whereas the rst cd takes you to the root directory because that&s what you set 1OMR to.4.A'H and ()ecutablesOne of the most important environment variables you&ll work with on the command line is 65T1. !n the last chapter, we discussed how commands are really Iust les, but we didn&t talk about how the command line knew which le to e'ecute for commands like cd or echo or other built(in or installed programs. The 65T1 variable provides the additional conte't that the command line needs to gure out which particular le to e'ecute. $et&s look at a 65T1 variable&s value./ echo /65T1KusrKlocalKsbin.KusrKlocalKbin.KusrKsbin.KusrKbin.Ksbin.Kbin.KusrKgames!f you e'amine the output of the echo /65T1 command above, you&ll see that it is a bunch of paths connected by colons. 2ou may have noticed that most of the paths end in Kbin. This is because bin is short for 0binary0, and bin is a standard directory name for e'ecutable les, or programs.$et&s look at what resides in one of the directories listed in the 65T1 variable.B*/ ls KusrKbinF mys9lanalyMe3to7mys9lbinlog3to7(3.Jmys9lbuga3p mys9lcheckaclocal mys9lHclientHtestaclocal(*.**mys9lHconvertHtableHformat...mys9l Mdumpmys9laccess Msoelimmys9ladmin;epending on your computer, di+erent types of les may have di+erent colors. On many computers, for e'ample, e'ecutables will probably be colored green. !f you look at the les located in your home directory, however, they are probably white and blue,which tells us that they are not e'ecutable.One of the items in the KusrKbin directory is man. !f ! type man on thecommand line and hit enter, it will e'ecute that le. 1ow can ! besure that it will e'ecute that le, and not some other le that happens to be named manon my server4 Yul *> *B.3> .drw'r('r('J acmeairpublic 37N4 Yul *> *B.37 ..(rw(r((r((* acmeairvipQ4 Yul *> *B.37 co+eeB?drw'rw'rw'3 acmeairacmeair EN4 Yul *> *B.37 custodianHcloset(rw(r((r((* acmeairvip34 Yul *> *B.3> donut(rw'r((r((* acmeairvip *-4 Yul *> *B.3> internetHaccess5s mentioned above, permissions have three access types. read "r%, write "w%, and e'ecute "'%. 4oth les and directories have access types assigned to them, and each can be assigned a user and a group. The 0other0 level of ownership always includes everyone else implicitly. !t looks like this. 5 le or directory has.o an owner L owner&s rw' access levelso a group L group&s rw' access levelso other&s rw' access levels!n the vipHlounge e'ample above, you can see to the left of each le a string of characters that starts with a dash "which can also be a d to indicate a directory%, then ? more characters consisting of r "0read0%, w "0write and delete0%, ' "0e'ecute0%, and ( "0permission not granted0%. Where a letter e'ists, that access type is granted, and where the dash e'ists, that permission is not granted.5fter the string of d, rs, ws, 's, and dashes in the rst item in the list, you can see acmeair vip. These two strings represent the user and group assigned to the le or directory. !n this case, the nameof the user is 0acmeair0 and the name of the group is 0vip0.EQThe following chart shows what each part of the permissions represents. Rach of the rst *Q characters can be replaced with adash to show that that access level is not granted or, in the case of the d, that the obIect in 9uestion is not a directory. $et&s take alook at a diagram to make this a bit easier to understand.P L(((((((( ;irectory or notP UL((((((( )ser @ead, Write, R'ecuteP UU L((((((( -roup @ead, Write, R'ecuteP UU U L((((( Other @ead, Write, R'ecuteP UU U U L((( The name of the userP UU U U U L((( The name of the groupP UU U U U UP dUrw'Urw'Urw' user group=ow that we understand how the permissions are laid out, we can interpret the output of the ls (lah command we ran previously. !f you look at the rst item displayed, the . directory "also vipHlounge%, we can gather the following. d. !t is a directory. rw'. !ts user has read, write, and e'ecute access. r('. !ts group has read and e'ecute access. r('. Other also has read and e'ecute access. The user named 0acmeair0 has been assigned to it.E* The group called 0vip0 has been assigned to it.!n practice, this means the directory&s default permissions allow for les to be read and e'ecuted by any user, and written to by the user 0acmeair.0!n this hypothetical le system, the vip group would have multipleusers associated with it, such as acmeair, bob, dave, etc. $et&s pretend that the following users e'ist and belong to the groups listed beneath them "we&ll be talking about groups more in a later section%./ groups bobbob vip/ groups acmeairacmeair vip/ groups Ie+Ie+ acmeair!n this scenario, anyone that belongs to the vip group would have group access to anything that is assigned the vip group. The user bob, based on his groups as listed above, would have the ability to enter the vipHloungedirectory and partake of "i.e. read% itsdonut, co+ee, and internet access content. The bob user, however, wouldn&t have access to enter thecustodianHcloset content or use its contents, but Ie+ and acmeair would.5n important thing to note from the above e'ample is that the permissions are not bestowed upon the user or groups. @ather, E3the les and directories are assigned groups and users, with particular permission levels for those. !t&s kind of like a reservation at a restaurant table. There&s nothing about a person that inherently gives them permission to dine at a restaurant at aparticular table and at a particular time. !t&s because the restaurant has labeled, or set apart, that table for that time that the person is able to dine there. So it is with the le system. There&s nothing inherently special about any group or user "e'cept the root user%. 5 user&s privileges are dened by the les and directories themselves.#ettin! .ermissionsThere are a few ways of setting the permissions on a le or directory. Thechmod command will be your friend here. To add write permissions to a le, for e'ample, you can do something like this./ chmod Lw sample.t'tThe Lw means 0add write access.0 !f you want to get more granular in how you set permissions on a le or directory, you can pre' the permission withu, g, o, or a, which stand for 0user "owner%0, 0group0, 0other0, and 0all0, respectively./ ls (lah test.t't(rw'r((r((* bobsta+*-4 Yul *> *B.3> test.t'tE7/ P @emove write access for user/ chmod u(w test.t't/ ls (lah test.t't(r('r((r((* bobsta+*-4 Yul *> *B.3> test.t't/ P 5dd e'ecute access for group/ chmod gL' test.t't/ ls (lah test.t't(r(wr'(r((* bobsta+*-4 Yul *> *B.3> test.t'tWhat if you want to set access level permissions for the user, group, and other all at once< 2ou can do so with 7 numbers, eachfrom Q to J. Why Q(J< There are 37 "which is 3 [ 3 [3, or N% permission levels. When you count from Q to J, there are actuallyeight numbers. =ote that it&s really common in computer languages to count starting from Mero rather than *. !n this octal system, read, write, and e'ecute permissions each add *, 3, and > respectively, resulting in non(ambiguous designations of permissions. !n short, here are the di+erent combinations of permissions based on the octal mask.The following table shows what access level each number represents.,umber .ermissionQ =o permission granted.* #an e'ecute.E>,umber .ermission3 #an write.7 #an write and e'ecute "3 L * \ 7%.> #an read.B #an read and e'ecute "> L* \ B%.E #an read and write "> L 3 \ E%.J #an read and write and e'ecute "> L 3 L * \ J%.!f you combine the permissions from the table aboveTone each for owner, group, and otherTyou can dene the whole set of permissions for a le or directory./ chmod JJJ test.sh/ ls (l test.sh(rw'rw'rw'* bob admin Q4 Yul *B *B.3> test.sh/ chmod QQQ test.sh/ ls (l test.sh((((((((((* bob admin Q4 Yul *B *B.3> test.shEB/ chmod JB> test.sh/ ls (l test.sh(rw'r('r((* bob admin Q4 Yul *B *B.3> test.sh=ote. in order to change the permissions of a le or directory, you much be its owner, be root, or use sudo. See the @oot )ser and Sudo section below .Users and rou$sWe know that a user and a group are assigned to all les and directories in $inu' and )ni' systems, and it is pretty obvious that if your user is assigned to a le, you will have the ability to read, write, or e'ecute the le according to the access types granted. 4ut how do groups work< )sers can belong to multiple groups, and groups can have multiple users. !f a user belongs to a group, it will have the access types granted to the assigned group of any le or directory. So, in our ls (lah e'ample above, the le named 0donut.t't0 can be read "or consumedX% by any user inthe 0vip0 group.To determine if your user is part of a certain group, use the groupscommand./ groupsubuntu adm dialout cdrom Soppy sudo audio dip video plugdev netdevEE!n the above e'ample, the 0ubuntu0 user is part of several groups, the rst being a group of the same name as the user. 0ubuntu0.5oot User and #udo!n )ni' and $inu' systems, there is a special user called 0root.0 The root user is the super userTit can read, write, and delete any le. !f the 0acmeair0 user is like an airline company, then theroot user is basically a god. 4ecause the root user has so much power, it is a common recommendation to not log in as root, and to only run commands with root when necessary. !f you are logged in as a non(root user and know the root user&s password, you can switch to the root user account at any time with the following command./ su (6assword.!f you put in the above command, you will be re9uired to put in the root user&s password. =ote. you may not have root access if you are using a managed server environment.5lthough the root user can read, write, and delete "almost% any le, it cannot e'ecute Iust any le. 5s mentioned in the chapter 0:iles, :olders, and R'ecutables0, a le can only be e'ecuted if it has the e'ecute permission granted. !n the case of the root user, it doesn&t matter who the permission is granted toO as long as it is granted to the user, the group, or other, root can e'ecute it.EJ#udoWhile you will be able to log in as root directly in some scenarios,it is more common to use the sudo command to perform actions that re9uire root privileges. 2ou can think of it as borrowing the god(like powers of the root user for a moment. The sudo command allows you to 0do0 something as a 0super user.0 When you use this command, you will usually be re9uired to input a passwordO but instead of the root user&s password, you&ll be putting in your own password. The following is an e'ample of a command that re9uires root privileges, but that canbe run using sudo "don&t run this unless you want your computer to rebootX%.sudo reboot4ut why use sudo instead of Iust logging in as root< There are several reasons for doing this, including the following. The server administrator wants you to have root access for some commands andKor directories, but not for everything. !n this case the administrator will set up sudo to have restrictions or whitelisted commands. @unning commands while logged in as root can be dangerous. )sing a non(root user makes it obvious when youare running a command that re9uires root privileges because you have to pre' your command withsudo.EN The sudo command provides a detailed audit trail so that system administrators can track what commands individualsused on system les. Sudo uses a ticketing system where you put in your password once, then you don&t have to until you haven&t run any sudo commands for ve minutes or longer. This adds security to your command line session, preventing others from gaining root access if you leave your Terminal open on accident.#ummar&:ile system permissions are complicated, and are probably one ofthe biggest sources of headaches for command line beginners. 4ut having a basic understanding of how permissions work will get you a long way. @emember. 6ermissions are assigned to les and directoriesTnot users and groups. 5ccess levels are determined by the rw' "read, write, e'ecute% permissions for the owner, group, and other. 5 le must have the ' "e'ecute% permission to e'ecute that le directly. To change the permissions of a le or directory, you either must be logged in as that le&s owner or the root user, or you must use the sudocommand.E?With an understanding of the above, you&ll be able to resolve many issues that you will surely run into while programming.()ercises*. Which user and group are assigned to the Ketc folder on your computerServer version. B.E.*7 Source distribution#opyright "c% 3QQQ, 3Q*7, Oracle andKor its a,liates. 5ll rights reserved.Oracle is a registered trademark of Oracle #orporation andKor itsa,liates. Other names may be trademarks of their respectiveowners.Type &helpO& or &]h& for help. Type &]c& to clear the current input statement.mys9lZThe prompt is now simply mys9lZ, and commands such as ls, echo, and cdno longer work. !nstead, only SW$ and other MySW$ commands are available for use. The mys9l program essentially hiIacks the command line session until you e'it out using the W)!T command.JE#onnecting to a MySW$ server with the mys9l command allows you to 9uery databases and otherwise manage the database server. Yust like the regular command line interface, there is input "ST;!=% and output "ST;O)T%. !f you type S1OW ;5T545SRSO, it will print to the screen a list of databases on the server. !f you type SR$R#T * L *O, it will output a table with the result of * L *.To get back to the command line interface, type &W)!T& and hit enter.5ead6(val6.rint6Loo$ 75(.L86rograms that follow the pattern of reading user input, evaluating the input, printing results of the input to the screen, and then allowing for more input, are called 0@R6$s0, or 0@ead(Rval(6rint($oop0 programs or interfaces. Themys9l command provides a simple @R6$. The bash shell, which is another name forthe command line interface that we&ve discussed in this book, is also a @R6$. !nteractive interfaces for scripting languages such as @uby, 6erl, 6ython, and 616 are commonly referred to as @R6$Teven more so than database clients and command line shells. These @R6$s read user input, evaluate it using the scripting language&s interpreter, print the result of the programming statement, then provide a prompt for more input.!n your console, try using the irb @R6$. !f @uby is installed, you should see something like this./ irbJJirb"main%.QQ*.QZYust as with the mys9l program, irb provides a prompt. 5s you can see in the following image, the irb program evaluates input, then performs what the input tells it to do, then outputs the result of what it did./ irbirb"main%.QQ*.QZ a \ *\Z *irb"main%.QQ3.QZ puts 01ello, worldX01ello, worldX\Z nilirb"main%.QQ7.QZ!f you type a multi(line script, it will automatically postpone evaluating the input until the statement is nished./ irbirb"main%.QQ*.QZ a \ *\Z *irb"main%.QQ3.QZ if a \\ 3irb"main%.QQ7.*Z puts 05 is 30irb"main%.QQ>.*Z elseirb"main%.QQB.*[ puts 05 is not 30irb"main%.QQE.*Z end5 is not 3\Z nilJNirb"main%.QQJ.QZType e'it to leave irb.Other scripting languages, such as 1askell, 6ython, and 616, also provide @R6$s. 5s you learn to program, you will nd that @R6$s are very useful for e'perimenting with what you learn andfor performing small computational tasks.(ditorsSome programmers use the command line as their !;R "!ntegrated ;evelopment Rnvironment%. 6art of that !;R is the te't editor, of which there are several available for the commandline. The following interfaces may look familiar.J?The above screenshots are all of te't editors that are often used on the command line. When you see other developers working ininterfaces like the ones above, they are probably not in the mainshell @R6$ "the command line%, and are instead using a te't editor program within the command line.NQ=ote. !f you start typing command line commands while in an editor, those commands may be treated as normal te't, or may be treated as editor commands. Watch out for thatX 2ou must e'it the editor mode, and be back in the command line to issue command line "or 0bash0% commands.The process of e'iting is di+erent for each of the editors mentioned above. 4elow is a list of the commands to e'it each editor.-imR'it and save.`RS#Z L .w9R'it without saving.`RS#Z L .9X,anoR'it and save.`#trlZ L o then `RnterZ, then `#trlZ L 'R'it without saving.`#trlZ L ' then n to discard changes(macsR'it and save.N*`#trlZ L ', `#trlZ L s, then `#trlZ L ', `#trlZ L cR'it without saving.`#trlZ L ', `#trlZ L c, then n, `RnterZ and yes, `RnterZ to discard any changes.Bash9 a #$ecial InterfaceSometimes, you may end up in a situation where you&re working in a 0command line within a command line0. This is because the command line interface we&ve been working in is itself a special interface that can be nested. :or e'ample, while using the command line you may think that you&re in the normal commandline interface, but when you type e'it and hit enter, you remain what appears to be another #$!. This is most likely because you&ve unknowingly entered another bash "or shell% interface. When on the command line, typing bash, Msh, or similar will createa new nested shell interface for you to work in. 2ou can e'it the innermost 0shell0 by typing e'it. When you&re in the outermost shell and you type e'it, you&ll see something like F6rocess completedG without a way to enter more commands. 2ou&ll have to close and restart Terminal at this point and start a new shell. !f you&re working normally, you should very rarely encounter a nested command line situation, but it can happen.#ummar&When you open up Terminal and start using it, you may not always be using the default command line. bash. 6rograms N3like mys9l, vim, and irb provide interfaces of their own right within Terminal. !t&s very important to understand whether you&re in an application(specic @R6$, like mys9l, or if you&re in the bash shell @R6$ "aka, the command line%. Sometimes, the only clue you have is the prompt. 4ut as we&ve seen earlier, we can even modify the prompt by setting the 6S* environment variable, so it can be 9uite tricky.ConclusionOverview of Conce$ts1opefully by this point, the command line is a bit less intimidating. While we&ve covered the basic concepts of the command line in this book, there&s a lot more to learn. 4ut don&t get discouragedX 2ou have all the tools you need to become an adept #$! user. this book, the man command to get more information about commands, and your favorite search engine.$et&s brieSy review the concepts we discussed in this book.: 'he command line is an interface to &our com$uter's ;les and directories1The command line provides a way to communicate with your computer. !ts language takes the form of te't(based input and output.: (ver&thin! &ou do in the command line is related to ;les< directories< and e)ecutables1N7When working in the command line, everything is a le, a directory, or an e'ecutable "and e'ecutables are les%. When you run commands such ascd or ls, you are really Iust e'ecuting les.: (nvironment variables $rovide conte)t for what &ou do in the command line1#onte't for the commands you run is provided by environment variables. 2our 0current directory0 "6W;%, the list of directories that denes the location of e'ecutables "65T1%, and your home directory "1OMR% are e'amples of environment variables that a+ect the commands you run and your ability to run commands. 2ou can change environment variables on the Sy using theA5@!54$R\0value0 pattern, or you can set them more permanently bye'porting the variables in your DK.bashrc le.: Access to ;les< directories< and e)ecutables is determinedb& their read< write< and e)ecute $ermissions1 'here are $ermissions for the ;le's user 7owner8< !rou$< and other 7ever&one else815nother important piece of conte't is the user account you are currently logged in under. Rach directory and each le is assigned to a user and a group, and permissions for reading, writing, and e'ecuting are granted to the user and group that are assigned to that directory or le. 6ermissions are also granted to everyone else "0other0%, which includes any user that is not the user or doesn&t belong to the le&s group.N>: #ome $ro!rams $rovide a com$letel& new conte)t within the command line interface1While the environment variables and your user account provide conte't for commands that you run while in the command line interface, some commands can temporarily take over handling all input and output. These commands allow users to interact more directly with databases, les, and scripts. They change the display and accept commands that aren&t available in the regularcommand line interface, and commands that are available in theregular #$! aren&t available in these other interfaces. Some of these interfaces are called 0@R6$s0 because they @ead input, Rvaluate the input, 6rint the results, and $oop back to read more input.Where to o from HereWith the basic command line skills you learned in this book, you are prepared to begin learning to program. #ontinue to practice what you&ve learned and e'periment with the command line so that you can retain and improve on what you&ve learned. The more you use the command line, the easier it will get, and the more powerful you will become in your ability to make your computer do your bidding.NB