UNIX TUTOR. The UNIX operating system comprises three parts: The kernel, the standard utility programs, and the system configuration files.

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1 UNIX TUTOR Sectin 1: What Is UNIX? UNIX is an perating system. The jb f an perating system is t rchestrate the varius parts f the cmputer -- the prcessr, the n-bard memry, the disk drives, keybards, vide mnitrs, etc. -- t perfrm useful tasks. The perating system is the master cntrller f the cmputer, the glue that hlds tgether all the cmpnents f the system, including the administratrs, prgrammers, and users. When yu want the cmputer t d smething fr yu, like start a prgram, cpy a file, r display the cntents f a directry, it is the perating system that must perfrm thse tasks fr yu. Mre than anything else, the perating system gives the cmputer its recgnizable characteristics. It wuld be difficult t distinguish between tw cmpletely different cmputers, if they were running the same perating system. Cnversely, tw identical cmputers, running different perating systems, wuld appear cmpletely different t the user. UNIX was created in the late 1960s, in an effrt t prvide a multiuser, multitasking system fr use by prgrammers. The philsphy behind the design f UNIX was t prvide simple, yet pwerful utilities that culd be pieced tgether in a flexible manner t perfrm a wide variety f tasks. The UNIX perating system cmprises three parts: The kernel, the standard utility prgrams, and the system cnfiguratin files. The kernel The kernel is the cre f the UNIX perating system. Basically, the kernel is a large prgram that is laded int memry when the machine is turned n, and it cntrls the allcatin f hardware resurces frm that pint frward. The kernel knws what hardware resurces are available (like the prcessr(s), the n-bard memry, the disk drives, netwrk interfaces, etc.), and it has the necessary prgrams t talk t all the devices cnnected t it. The standard utility prgrams These prgrams include simple utilities like cp, which cpies files, and cmplex utilities, like the shell that allws yu t issue cmmands t the perating system. The system cnfiguratin files The system cnfiguratin files are read by the kernel, and sme f the standard utilities. The UNIX kernel and the utilities are flexible prgrams, and certain aspects f their

2 behavir can be cntrlled by changing the standard cnfiguratin files. One example f a system cnfiguratin file is the filesystem table "fstab", which tells the kernel where t find all the files n the disk drives. Anther example is the system lg cnfiguratin file "syslg.cnf", which tells the kernel hw t recrd the varius kinds f events and errrs it may encunter. Sectin 2: Accessing a UNIX System There are many ways that yu can access a UNIX system. The main mde f access t a UNIX machine is thrugh a terminal, which usually includes a keybard, and a vide mnitr. Fr each terminal cnnected t the UNIX system, the kernel runs a prcess called a tty that accepts input frm the terminal, and sends utput t the terminal. Tty prcesses are general prgrams, and must be tld the capabilities f the terminal in rder t crrectly read frm, and write t, the terminal. If the tty prcess receives incrrect infrmatin abut the terminal type, unexpected results can ccur. Cnsle Every UNIX system has a main cnsle that is cnnected directly t the machine. The cnsle is a special type f terminal that is recgnized when the system is started. Sme UNIX system peratins must be perfrmed at the cnsle. Typically, the cnsle is nly accessible by the system peratrs, and administratrs. Dumb terminals Sme terminals are referred t as "dumb" terminals because they have nly the minimum amunt f pwer required t send characters as input t the UNIX system, and receive characters as utput frm the UNIX system. Persnal cmputers are ften used t emulate dumb terminals, s that they can be cnnected t a UNIX system. Dumb terminals can be cnnected directly t a UNIX machine, r may be cnnected remtely, thrugh a mdem r a terminal server. Smart terminals Smart terminals, like the X terminal, can interact with the UNIX system at a higher level. Smart terminals have enugh n-bard memry and prcessing pwer t supprt graphical interfaces. The interactin between a smart terminal and a UNIX system can g beynd simple characters t include icns, windws, menus, and muse actins. Sectin 3: Lgging In and Lgging Out T ensure security and rganizatin n a system with many users, UNIX machines emply a system f user accunts. The user accunting features f UNIX prvide a basis fr analysis and cntrl f system resurces, preventing any user frm taking up mre

3 than his r her share, and preventing unauthrized peple frm accessing the system. Every user f a UNIX system must get permissin by sme access cntrl mechanism. Lgging in Lgging in t a UNIX system requires tw pieces f infrmatin: A username, and a passwrd. When yu sit dwn fr a UNIX sessin, yu are given a lgin prmpt that lks like this: lgin: Type yur username at the lgin prmpt, and press the return key. The system will then ask yu fr yur passwrd. When yu type yur passwrd, the screen will nt display what yu type. Yur username Yur username is assigned by the persn wh creates yur accunt. At ISU, the standard username is the first fur letters f yur last name cncatenated with the first fur letters f yur first name. Yur username must be unique n the system where yur accunt exists since it is the means by which yu are identified n the system. Yur passwrd When yur accunt is created, a passwrd is assigned. The first thing yu shuld d is change yur passwrd, using the passwd utility. T change yur passwrd, type the cmmand passwd after yu have lgged in. The system will ask fr yur ld passwrd, t prevent smene else frm sneaking up, and changing yur passwrd. Then it will ask fr yur new passwrd. Yu will be asked t cnfirm yur new passwrd, t make sure that yu didn't mistype. It is very imprtant that yu chse a gd passwrd, s that smene else cannt guess it. Here are sme rules fr selecting a gd passwrd: D nt use any part f yur name, yur spuse's name, yur child's name, yur pet's name, r anybdy's name. D nt use any backward spellings f any name, either. D nt use an easily-guessable number, like yur phne number, yur scial security number, yur address, license plate number, etc. D nt use any wrd that can be fund in an English r freign-language dictinary. D nt use all the same letter, r a simple sequence f keys n the keybard, like qwerty. D use a mix f upper-case and lwer-case letters, numbers, and cntrl characters. D use at least six characters.

4 If yu have accunts n multiple machines, use a different passwrd n each machine. D nt chse a passwrd that is s difficult t remember that yu must write it dwn. Lgging Out When yu're ready t quit, type the cmmand exit Befre yu leave yur terminal, make sure that yu see the lgin prmpt, indicating that yu have successfully lgged ut. If yu have left any unresved prcesses, the UNIX system will require yu t reslve them befre it will let yu lg ut. Sme shells will recgnize ther cmmands t lg yu ut, like "lgut" r even "bye". It is always a gd idea t clear the display befre yu lg ut, s that the next user desn't get a screenful f infrmatin abut yu, yur wrk, r yur user accunt. Yu can type the cmmand clear right befre yu lg ut, r yu can press the return key until all the infrmatin is scrlled ff the screen. Sectin 4: The UNIX Shell The shell is perhaps the mst imprtant prgram n the UNIX system, frm the enduser's standpint. The shell is yur interface with the UNIX system, the middleman between yu and the kernel. CONCEPT: The shell is a type f prgram called an interpreter. An interpreter perates in a simple lp: It accepts a cmmand, interprets the cmmand, executes the cmmand, and then waits fr anther cmmand. The shell displays a "prmpt," t ntify yu that it is ready t accept yur cmmand. The shell recgnizes a limited set f cmmands, and yu must give cmmands t the shell in a way that it understands: Each shell cmmand cnsists f a cmmand name, fllwed by cmmand ptins (if any are desired) and cmmand arguments (if any are desired). The cmmand name, ptins, and arguments, are separated by blank space.

5 CONCEPT: The shell is a prgram that the UNIX kernel runs fr yu. A prgram is referred t as a prcess while the kernel is running it. The kernel can run the same shell prgram (r any ther prgram) simultaneusly fr many users n a UNIX system, and each running cpy f the prgram is a separate prcess. Many basic shell cmmands are actually subrutines built in t the shell prgram. The cmmands that are nt built in t the shell require the kernel t start anther prcess t run them. CONCEPT: When yu execute a nn built-in shell cmmand, the shell asks the kernel t create a new subprcess (called a "child" prcess) t perfrm the cmmand. The child prcess exists just lng enugh t execute the cmmand. The shell waits until the child prcess finishes befre it will accept the next cmmand. EXERCISE: Explain why the exit (lgut) prcedure must be built in t the shell. EXPLANATION: If the lgut prcedure were nt built in t the shell, the kernel wuld start a new child prcess t run it. The new prcess wuld lgut, and then return yu t the riginal shell. Yu wuld thus find yurself back where yu started, withut having lgged ut. Unlike DOS, the UNIX shell is case-sensitive, meaning that an uppercase letter is nt equivalent t the same lwer case letter (i.e., "A" is nt equal t "a"). Mst all unix cmmands are lwer case. Entering shell cmmands The basic frm f a UNIX cmmand is: cmmandname [-ptins] [arguments] The cmmand name is the name f the prgram yu want the shell t execute. The cmmand ptins, usually indicated by a dash, allw yu t alter the behavir f the cmmand. The arguments are the names f files, directries, r prgrams that the cmmand needs t access. The square brackets ([ and ]) signify ptinal parts f the cmmand that may be mitted. EXAMPLE: Type the cmmand ls -l /tmp t get a lng listing f the cntents f the /tmp directry. In this example, "ls" is the cmmand name, "-l" is an ptin that tells ls t create a lng, detailed utput, and "/tmp" is an argument naming the directry that ls is t list. Abrting a shell cmmand Mst UNIX systems will allw yu t abrt the current cmmand by typing Cntrl-C. T issue a Cntrl-C abrt, hld the cntrl key dwn, and press the "c" key. Special characters in UNIX

6 UNIX recgnizes certain special characters as cmmand directives. If yu use ne f the UNIX special characters in a cmmand, make sure yu understand what it des. The special characters are: / < >! $ % ^ & * { } ~ and ; When creating files and directries n UNIX, is is safest t nly use the characters A-Z, a-z, 0-9, and the perid, dash, and underscre characters. The meaning f the ther characters, and ways t use them, will be intrduced as the tutrial prgresses. Getting help n UNIX T access the n-line manuals, use the man cmmand, fllwed by the name f the cmmand yu need help with. EXAMPLE: Type man ls t see the manual page fr the "ls" cmmand. EXAMPLE: T get help n using the manual, type man man t the UNIX shell. Sectin 5: Wrking with Files and Directries Here is an index t the tpics in this sectin: The UNIX filesystem structure File and directry permissins Changing directries Listing the cntents f a directry Viewing the cntents f a file Cpying files and directries Mving and renaming files Remving files Creating a directry Remving a directry The UNIX filesystem structure All the stred infrmatin n a UNIX cmputer is kept in a filesystem. Any time yu interact with the UNIX shell, the shell cnsiders yu t be lcated smewhere within a filesystem. Althugh it may seem strange t be "lcated" smewhere in a cmputer's

7 filesystem, the cncept is nt s different frm real life. After all, yu can't just be, yu have t be smewhere. The place in the filesystem tree where yu are lcated is called the current wrking directry. CONCEPT: The UNIX filesystem is heirarchical (resembling a tree structure). The tree is anchred at a place called the rt, designated by a slash "/". Every item in the UNIX filesystem tree is either a file, r a directry. A directry is like a file flder. A directry can cntain files, and ther directries. A directry cntained within anther is called the child f the ther. A directry in the filesystem tree may have many children, but it can nly have ne parent. A file can hld infrmatin, but cannt cntain ther files, r directries. CONCEPT: T decribe a specific lcatin in the filesystem heirarchy, yu must specify a "path." The path t a lcatin can be defined as an abslute path frm the rt anchr pint, r as a relative path, starting frm the current lcatin. When specifying a path, yu simply trace a rute thrugh the filesystem tree, listing the sequence f directries yu pass thrugh as yu g frm ne pint t anther. Each directry listed in the sequence is separated by a slash. UNIX prvides the shrthand ntatin f "." t refer t the current lcatin, and ".." t refer t the parent directry. EXERCISE: Specify the abslute path t the directry named "jn" at the bttm f the tree diagram. EXPLANATION: Since the abslute path must always begin at the rt (/) directry, the path wuld be: /users/admin/jn EXERCISE: Specify the relative path frm the directry named "student" t the directry named "jn" in the tree diagram.

8 EXPLANATION: Starting frm the student directry, we wuld first have t mve up the filesystem tree (using the ".." ntatin) t the directry called "users" befre we culd descend t the directry called "jn". The path wuld be:../admin/jn File and directry permissins CONCEPT: UNIX supprts access cntrl. Every file and directry has assciated with it wnership, and access permissins. Furthermre, ne is able t specify thse t whm the permissins apply. Permissins are defined as read, write, and execute. The read, write, and execute permissins are referred t as r, w, and x, respectively. Thse t whm the permissins apply are the user wh wns the file, thse wh are in the same grup as the wner, and all thers. The user, grup, and ther permissins are referred t as u, g, and, respectively. A shrt nte n grups: UNIX allws users t be placed in grups, s that the cntrl f access is made simpler fr administratrs. The meaning f file and directry permissins Read permissin Fr a file, having read permissin allws yu t view the cntents f the file. Fr a directry, having read permissin allws yu t list the directry's cntents. Write permissin Fr a file, write permissin allws yu t mdify the cntents f the file. Fr a directry, write permissin allws yu t alter the cntents f the directry, i.e., t add r delete files. Execute permissin Fr a file, execute permissin allws yu t run the file, if it is an executable prgram, r script. Nte that file execute permissin is irrelevant fr nnexecutable files. Fr a directry, execute permissin allws yu t cd t the directry, and make it yur current wrking directry. Viewing permissins T see the permissins n a file, use the ls cmmand, with the -l ptin. EXAMPLE: Execute the cmmand ls -l /etc/passwd t view the infrmatin n the system passwrd database. The utput shuld lk similar t this: -rw-r--r-- 1 rt sys Apr 17 12:05 /etc/passwd The first 10 characters describe the access permissins. The first dash indicates the type f file (d fr directry, s fr special file, - fr a regular file). The next three characters ("rw-") describe the permissins f the wner f the file: read and write, but n execute.

9 The next three characters ("r--") describe the permissins fr thse in the same grup as the wner: read, n write, n execute. The next three characters describe the permissins fr all thers: read, n write, n execute. Setting permissins UNIX allws yu t set the permissins n files that yu wn. The cmmand t change the file permissin mde is chmd. Chmd requires yu t specify the new permissins yu want, and specify the file r directry yu want the changes applied t. T set file permissins, yu may use t the "rwx" ntatin t specify the type f permissins, and the "ug" ntatin t specify thse the permissins apply t. T define the kind f change yu want t make t the permissins, use the plus sign (+) t add a permissin, the minus sign (-) t remve a permissin, and the equal sign (=) t set a permissin directly. EXAMPLE: Type the cmmand chmd g=rw- ~/.shrc t change the file permissins n the file.shrc, in yur hme directry. Specifically, yu are specifying grup read access and write access, with n execute access. EXERCISE: Change the permissins n the.shrc file in yur hme directry s that grup and thers have read permissin nly. EXPLANATION: Typing the cmmand chmd g=r-- ~/.shrc wuld accmplish the task. Changing Directries In UNIX, yur lcatin in the filesystem heirarchy is knwn as yur "current wrking directry." When yu lg in, yu are autmatically placed in yur "hme directry." T see where yu are, type the cmmand pwd which stands fr "print wrking directry." T change yur lcatin in the filesystem heirarchy, use the cd (change directry) cmmand, fllwed by an argument defining where yu want t g. The argument can be either an abslute path t the destinatin, r a relative path. EXAMPLE: Type the cmmand cd /tmp t g t the /tmp directry. Yu can type pwd t cnfirm that yu're actually there. If yu type the cd cmmand withut an argument, the shell will place yu in yur hme directry. EXERCISE: Type the cmmand

10 pwd and nte the result. Then type cd.. t the shell. Type pwd again t see where yu ended up. EXPLANATION: The "cd.." cmmand shuld have mved yu up ne level in the directry tree, because ".." is UNIX shrthand fr the parent directry. The result f the secnd "pwd" cmmand shuld be the same as the first, with the last directry in the path mitted. Listing the cntents f a directry The ls cmmand allws yu t see the cntents f a directry, and t view basic infrmatin (like size, wnership, and access permissins) abut files and directries. The ls cmmand has numerus ptins, s see the manual page n ls (type man ls) fr a cmplete listing. The ls cmmand als accepts ne r mre arguments. The arguments can be directries, r files. EXAMPLE: Type the cmmand ls -lr /etc/i* t the UNIX shell. In the example, the "l" and "r" ptins f the ls cmmand are invked tgether. Sme cmmands permit yu t grup ptins in that way, and sme cmmands require the ptins t be named separately, e.g., ls -l -r. The l ptin calls fr a lng utput, and the r ptin causes ls t perate recursively, mving dwn directry trees. The last part f the example, "/etc/i*", directs the ls cmmand t list files and directries in the /etc directry, that begin with the letter i. The wildcard character, "*", matches any character(s). EXERCISE: Type the cmmand ls -m /etc/i*g t the shell. Hw did the shell respnd, and why? EXPLANATION: The shell respnded by printing all the entries in the /etc directry that start with the letter i, and end with the letter g. The -m ptin causes the utput t be streamed int a single line. See the manual page fr ls t get a cmplete descriptin f the ls cmmand's ptins. EXERCISE: Find the permissins n yur hme directry. EXPLANATION: There are many ways t accmplish this. Yu culd type cd t get t yur hme directry, and then type ls -la

11 The -a ptin instructs the ls cmmand t list all files, including thse that start with the perid character. The directry permissins are listed next t the "." symbl. Remember that "." is UNIX shrthand fr the current wrking directry. Viewing the cntents f a file CONCEPT: Text files are intended fr direct viewing, and ther files are intended fr cmputer interpretatin. The UNIX file cmmand allws yu t determine whether an unknwn file is in text frmat, suitable fr direct viewing. EXERCISE: Type the cmmand file /bin/sh t see what kind f file the shell is. EXPLANATION: The shell is a shared executable, indicating that the file cntains binary instructins t be executed by the cmputer. The cat cmmand The cat cmmand cncatenates files and sends them t the screen. Yu can specify ne r mre files as arguments. Cat makes n attempt t frmat the text in any way, and lng utput may scrll ff the screen befre yu can read it. EXAMPLE: Send the cntents f yur.prfile file t the screen by typing cat ~/.prfile t the shell. The tilde character (~) is UNIX shrthand fr yur hme directry. The mre cmmand The mre cmmand displays a text file, ne screenful at a time. Yu can scrll frward a line at a time by pressing the return key, r a screenful at a time by pressing the spacebar. Yu can quit at any time by pressing the q key. EXAMPLE: Type mre /etc/rc t the shell. Scrll dwn by pressing return, and by pressing the spacebar. Stp the mre cmmand frm displaying the rest f the file by typing q. The head and tail cmmands The head cmmand allws yu t see the tp part f a file. Yu may specify the number f lines yu want, r default t ten lines. EXAMPLE: Type head -15 /etc/rc t see the first fifteen lines f the /etc/rc file. The tail cmmand wrks like head, except that it shws the last lines f f file. EXAMPLE: Type tail /etc/rc

12 t see the last ten lines f the file /etc/rc. Because we did nt specify the number f lines as an ptin, the tail cmmand defaulted t ten lines. Cpying files and directries The UNIX cmmand t cpy a file r directry is cp. The basic cp cmmand syntax is cp surce destinatin. EXAMPLE: The cmmand cp ~/.prfile ~/pcpy makes a cpy f yur.prfile file, and stres it in a file called "pcpy" in yur hme directry. EXERCISE: Describe the permissins necessary t successfully execute the cmmand in the previus example. EXPLANATION: T cpy the.prfile file, ne must have read permissin n the file. T create the new file called pcpy, ne must have write permissin in the directry where the file will be created. Mving and renaming files The UNIX mv cmmand mves files and directries. Yu can mve a file t a different lcatin in the filesystem, r change the name by mving the file within the current lcatin. EXAMPLE: The cmmand mv ~/pcpy ~/qcpy takes the pcpy file yu created in the cp exercise, and renames it "qcpy". Remving files The rm cmmand is used fr remving files and directries. The syntax f the rm cmmand is rm filename. Yu may include many filenames n the cmmand line. EXAMPLE: Remve the the shrccpy file that yu placed in yur hme directry in the sectin n mving files by typing rm ~/.shrccpy Creating a directry The UNIX mkdir cmmand is used t make directries. The basic syntax is mkdir directryname. If yu d nt specify the place where yu want the directry created (by giving a path as part f the directry name), the shell assumes that yu want the new directry placed within the current wrking directry. EXAMPLE: Create a directry called f within yur hme directry by typing mkdir ~/f EXERCISE: Create a directry called bar, within the directry called f, within yur hme directry.

13 EXPLANATION: Once the f directry is created, yu culd just type mkdir ~/f/bar Alternately, yu culd type cd ~/f; mkdir bar In the secnd slutin, tw UNIX cmmands are given, separated by a semicln. The first part f the cmmand makes f the current wrking directry. The secnd part f the cmmand creates the bar directry in the current wrking directry. Remving a directry The UNIX rmdir cmmand remves a directry frm the filesystem tree. The rmdir cmmand des nt wrk unless the directry t be remved is cmpletely empty. The rm cmmand, used with the -r ptin can als be used t remve directries. The rm -r cmmand will first remve the cntents f the directry, and then remve the directry itself. EXERCISE: Describe hw t remve the "f" directry yu created, using bth rmdir, and rm with the -r ptin. EXPLANATION: Yu culd enter the cmmands rmdir ~/f/bar; rmdir ~/f t accmplish the task with the rmdir cmmand. Nte that yu have t rmdir the bar subdirectry bfre yu can rmdir the f directry. Alternately, yu culd remve the f directry with the cmmand rm -r ~/f Sectin 6: Redirecting Input and Output CONCEPT: Every prgram yu run frm the shell pens three files: Standard input, standard utput, and standard errr. The files prvide the primary means f cmmunicatins between the prgrams, and exist fr as lng as the prcess runs. The standard input file prvides a way t send data t a prcess. As a default, the standard input is read frm the terminal keybard. The standard utput prvides a means fr the prgram t utput data. As a default, the standard utput ges t the terminal display screen. The standard errr is where the prgram reprts any errrs encuntered during executin. By default, the standard errr ges t the terminal display. CONCEPT: A prgram can be tld where t lk fr input and where t send utput, using input/utput redirectin. UNIX uses the "less than" and "greater than" special characters (< and >) t signify input and utput redirectin, respectively. Redirecting input Using the "less-than" sign with a file name like this: < file1

14 in a shell cmmand instructs the shell t read input frm a file called "file1" instead f frm the keybard. EXAMPLE:Use standard input redirectin t send the cntents f the file /etc/passwd t the mre cmmand: mre < /etc/passwd Many UNIX cmmands that will accept a file name as a cmmand line argument, will als accept input frm standard input if n file is given n the cmmand line. EXAMPLE: T see the first ten lines f the /etc/passwd file, the cmmand: head /etc/passwd will wrk just the same as the cmmand: head < /etc/passwd Redirecting utput Using the "greater-than" sign with a file name like this: > file2 causes the shell t place the utput frm the cmmand in a file called "file2" instead f n the screen. If the file "file2" already exists, the ld versin will be verwritten. EXAMPLE: Type the cmmand ls /tmp > ~/ls.ut t redirect the utput f the ls cmmand int a file called "ls.ut" in yur hme directry. Remember that the tilde (~) is UNIX shrthand fr yur hme directry. In this cmmand, the ls cmmand will list the cntents f the /tmp directry. Use tw "greater-than" signs t append t an existing file. Fr example: >> file2 causes the shell t append the utput frm a cmmand t the end f a file called "file2". If the file "file2" des nt already exist, it will be created. EXAMPLE: In this example, I list the cntents f the /tmp directry, and put it in a file called myls. Then, I list the cntents f the /etc directry, and append it t the file myls: ls /tmp > myls ls /etc >> myls Redirecting errr Redirecting standard errr is a bit trickier, depending n the kind f shell yu're using (there's mre than ne flavr f shell prgram!). In the POSIX shell and ksh, redirect the standard errr with the symbl "2>". EXAMPLE: Srt the /etc/passwd file, place the results in a file called f, and trap any errrs in a file called err with the cmmand: srt < /etc/passwd > f 2> err Sectin 7: Pipelines and Filters CONCEPT: UNIX allws yu t cnnect prcesses, by letting the standard utput f ne prcess feed int the standard input f anther prcess. That mechanism is called a pipe.

15 Cnnecting simple prcesses in a pipeline allws yu t perfrm cmplex tasks withut writing cmplex prgrams. EXAMPLE: Using the mre cmmand, and a pipe, send the cntents f yur.prfile and.shrc files t the screen by typing cat.prfile.shrc mre t the shell. EXERCISE: Hw culd yu use head and tail in a pipeline t display lines 25 thrugh 75 f a file? ANSWER: The cmmand cat file head -75 tail -50 wuld wrk. The cat cmmand feeds the file int the pipeline. The head cmmand gets the first 75 lines f the file, and passes them dwn the pipeline t tail. The tail cmmand then filters ut all but the last 50 lines f the input it received frm head. It is imprtant t nte that in the abve example, tail never sees the riginal file, but nly sees the part f the file that was passed t it by the head cmmand. It is easy fr beginners t cnfuse the usage f the input/utput redirectin symbls < and >, with the usage f the pipe. Remember that input/utput redirectin cnnects prcesses with files, while the pipe cnnects prcesses with ther prcesses. Grep The grep utility is ne f the mst useful filters in UNIX. Grep searches line-by-line fr a specified pattern, and utputs any line that matches the pattern. The basic syntax fr the grep cmmand is grep [-ptins] pattern [file]. If the file argument is mitted, grep will read frm standard input. It is always best t enclse the pattern within single qutes, t prevent the shell frm misinterpreting the cmmand. The grep utility recgnizes a variety f patterns, and the pattern specificatin syntax was taken frm the vi editr. Here are sme f the characters yu can use t build grep expressins: The carat (^) matches the beginning f a line. The dllar sign ($) matches the end f a line. The perid (.) matches any single character. The asterisk (*) matches zer r mre ccurrences f the previus character. The expressin [a-b] matches any characters that are lexically between a and b. EXAMPLE: Type the cmmand grep 'jn' /etc/passwd t search the /etc/passwd file fr any lines cntaining the string "jn". EXAMPLE: Type the cmmand

16 grep '^jn' /etc/passwd t see the lines in /etc/passwd that begin with the character string "jn". EXERCISE:List all the files in the /tmp directry wned by the user rt. EXPLANATION: The cmmand ls -l /tmp grep 'rt' wuld shw all prcesses with the wrd "rt" smewhere in the line. That desn't necessarily mean that all the prcess wuld be wned by rt, but using the grep filter can cut the dwn the number f prcesses yu will have t lk at. Sectin 8: Prcess Cntrl and Multitasking CONCEPT: The UNIX kernel can keep track f many prcesses at nce, dividing its time between the jbs submitted t it. Each prcess submitted t the kernel is given a unique prcess ID. Single-tasking perating systems, like DOS, r the Macintsh System, can nly perfrm ne jb at a time. A user f a single-tasking system can switch t different windws, running different applicatins, but nly the applicatin that is currently being used is active. Any ther task that has been started is suspended until the user switches back t it. A suspended jb receives n perating system resurces, and stays just as it was when it was suspended. When a suspended jb is reactivated, it begins right where it left ff, as if nthing had happened. The UNIX perating system will simultaneusly perfrm multiple tasks fr a single user. Activating an applicatin des nt have t cause ther applicatins t be suspended. Actually, it nly appears that UNIX is perfrming the jbs simultaneusly. In reality, it is running nly ne jb at a time, but quickly switching between all f its nging tasks. The UNIX kernel will execute sme instructins frm jb A, and then set jb A aside, and execute instructins frm jb B. The cncept f switching between queued jbs is called prcess scheduling. Viewing prcesses UNIX prvides a utility called ps (prcess status) fr viewing the status f all the unfinished jbs that have been submitted t the kernel. The ps cmmand has a number f ptins t cntrl which prcesses are displayed, and hw the utput is frmatted. EXAMPLE: Type the cmmand ps t see the status f the "interesting" jbs that belng t yu. The utput f the ps cmmand, withut any ptins specified, will include the prcess ID, the terminal frm which the prcess was started, the amunt f time the prcess has been running, and the name f the cmmand that started the prcess. EXAMPLE: Type the cmmand ps -ef

17 t see a cmplete listing f all the prcesses currently scheduled. The -e ptin causes ps t include all prcesses (including nes that d nt belng t yu), and the -f ptin causes ps t give a lng listing. The lng listing includes the prcess wner, the prcess ID, the ID f the parent prcess, prcessr utilizatin, the time f submissin, the prcess's terminal, the ttal time fr the prcess, and the cmmand that started the prcess. EXERCISE: Use the ps cmmand, and the grep cmmand, in a pipeline t find all the prcesses wned by yu. EXPLANATION: The cmmand ps -ef grep yurusername where "yurusername" is replaced by yur user name, will cause the utput f the ps -ef cmmand t be filtered fr thse entries that cntain yur username. Killing prcesses Occasinally, yu will find a need t terminate a prcess. The UNIX shell prvides a utility called kill t terminate prcesses. Yu may nly terminate prcesses that yu wn (i.e., prcesses that yu started). The syntax fr the kill cmmand is kill [-ptins] prcess-id. T kill a prcess, yu must first find its prcess ID number using the ps cmmand. Sme prcesses refuse t die easily, and yu can use the "-9" ptin t frce terminatin f the jb. EXAMPLE: T frce terminatin f a jb whse prcess ID is 111, enter the cmmand kill Sectin 9: Interactin and Jb Cntrl When yu lg in t a UNIX system, the kernel starts a shell fr yu, and cnnects the shell t yur terminal. When yu execute a cmmand frm the shell, the shell creates a child prcess t execute the cmmand, and cnnects the child prcess t yur terminal. By cnnecting the child prcess t yur terminal, the shell allws yu t send input t the child prcess, and receive utput frm it. When the child prcess finishes, the shell regains access t the terminal, redisplays the shell prmpt, and waits fr yur next cmmand. Any task that requires yu t actively participate (like wrd prcessing) must be in the fregrund t run. Such jbs, termed "interactive," must peridically update the display, and accept input frm yu, and s require access t the terminal interface. Other jbs d nt require yu t participate nce they are started. Fr example, a jb that srts the cntents f ne file and places the results in anther file, wuld nt have t accept user cmmands, r write infrmatin t the screen while it runs. Sme UNIX shells allw such nninteractive jbs t be discnnected frm the terminal, freeing the terminal fr interactive use.

18 Nte that it is even pssible t lg ut, and leave a backgrund prcess running. Unfrtunately, there is n way t recnnect a backgrund jb t the terminal after yu've lgged ut. Jbs that are discnnected frm the terminal fr input and utput are called "backgrund" jbs. Yu can have a large number f backgrund jbs running at the same time, but yu can nly have ne fregrund jb. That is because yu nly have ne screen, and ne keybard at yur terminal. Backgrund and fregrund jbs The prcess that is cnnected t the terminal is called the fregrund jb. A jb is said t be in the fregrund because it can cmmunicate with the user via the screen, and the keybard. A UNIX prcess can be discnnected frm the terminal, and allwed t run in the backgrund. Because backgrund jbs are nt cnnected t a terminal, they cannt cmmunicate with the user. If the backgrund jb requires interactin with the user, it will stp, and wait t be recnnected t the terminal. Jbs that d nt require interactin frm the user as they run(like srting a large file) can be placed in the backgrund, allwing the user t access the terminal, and cntinue t wrk, instead f waiting fr a lng jb t finish. Starting a jb in the backgrund T place a jb in the backgrund, simply add the ampersand character (&) t the end f yur shell cmmand. EXAMPLE: T srt a file called "f", and place the results in a file called "bar", yu culd issue the cmmand srt < f > bar & Examining yur jbs The UNIX cmmand jbs allws yu t see a list f all the jbs yu have invked frm the current shell. The shell will list the jb ID f each jb, alng with the status (running, stpped, r therwise), and the cmmand that started the jb. The shell cnsiders the mst recently-created (r mst recently-manipulated) jb t be the current jb, marked with a plus sign. Other jbs are referred t as previus jbs, and are marked with a minus sign. The cmmands related t jb cntrl will apply t the current jb, unless directed t d therwise. Yu may refer t jbs by jb ID by using the percent sign. Thus, jb 1 is referred t as %1, jb 2 is %2, and s frth. Suspending the fregrund jb Yu can (usually) tell UNIX t suspend the jb that is currently cnnected t yur terminal by typing Cntrl-Z (hld the cntrl key dwn, and type the letter z). The shell will infrm yu that the prcess has been suspended, and it will assign the suspended jb a jb ID.

19 There is a big difference between a suspended jb, and a jb running in the backgrund. A suspended jb is stpped, and n prcessing will take place fr that jb until yu run it, either in the fregrund, r in the backgrund. Placing a fregrund jb int the backgrund If yu started a jb in the fregrund, and wuld like t place it in the backgrund, the first thing yu must d is suspend the jb with a Cntrl-Z, freeing yur terminal. Then, yu can issue the UNIX cmmand bg t place the suspended jb in the backgrund. The bg cmmand can accept a jb ID as an argument. If n jb ID is given, bg assumes yu are referring t the current (suspended) jb. Bringing a backgrund jb t the fregrund Yu can recnnect a backgrund jb t yur terminal with the UNIX cmmand fg The fg cmmand will accept a jb ID as an argument. Make sure t include the percent sign: fg %2 will bring jb 2 int the fregrund. If n jb ID is given, fg will assume yu are referring t the current (suspended) jb. Starting a suspended jb If yu have a suspended jb that yu'd like t resume running, first yu must decide whether yu want it running in the fregrund, r the backgrund. Find the jb ID f the suspended jb with the jbs cmmand, and then use bg (t run the jb in the backgrund), r fg (t run the jb in the fregrund). Sectin 10: Editing Text with EMACS One f the mst basic peratins yu will need t perfrm n a UNIX system is text editing. Whether yu are preparing a dcument, writing a prgram, r sending t a clleague, yu will need a utility t allw yu t enter and edit text. There are many editrs available fr UNIX systems, but this discussin will fcus n the emacs text editing prgram because f its pwer, flexibility, extensibility, custmizability, and prevalence. N matter where yu get a UNIX accunt, yu are likely t have emacs at yur dispsal. Here is an index t the tpics in this sectin: General features Wrking with buffers Starting, quitting, and getting help The emacs display Wrking with files

20 Cursr mtin Inserting and deleting text Cutting and pasting text regins General features f the emacs editr Emacs is a visual editr. That means that yu have a representatin f yur entire dcument n yur screen, and yu can mve arund freely, editing any part f the dcument yu wish. Older editrs, referred t as line editrs, required all changes t the file t be made n a line-by-line basis. Each cmmand t a line editr specified a line number, and the changes t be applied t that line. Line editrs are truly hrrible things, and yu shuld feel lucky if yu have never seen ne. Emacs uses cntrl and escape characters t distinguish editr cmmands frm text t be inserted in the buffer. In this dcument, the ntatin "Cntrl-X" means t hld dwn the cntrl key, and type the letter x. Yu dn't need t capitalize the x, r any ther cntrl character, by hlding dwn the shift key. "ESCAPE-X" means t press the escape key dwn, release it, and then type x. Wrking with buffers When yu edit a file in emacs, yu're nt really editing the file itself, as it sits ut n a disk smewhere. Instead, emacs makes a cpy f the file, and stres the cpy in a part f RAM memry called a buffer. All the changes yu make t the file are applied t the buffer. When yu save the file, emacs writes the cntents f the buffer t the disk. Because the buffer exists in RAM memry, it disappears if the pwer is turned ff, r if the system crashes. Thus, yu shuld use the save cmmand ften, flushing yur current buffer t disk. Once the file is n disk, a pwer utage r system crash shuldn't harm it. Basic peratins in emacs Here are sme f the fundamental things yu'll need t d when yu edit a dcument in emacs. Starting emacs T start emacs, just type the cmmand emacs t the UNIX shell. If yu want emacs t start with a file already laded int a buffer, type emacs filename where "filename" is the name f the file yu want t edit. Quitting emacs T exit emacs and return t the UNIX shell, type Cntrl-X-Cntrl-C. If yu have made changes t the buffer since the last time yu saved it t disk, emacs will ask yu if yu want t save. Type y fr yes, r n fr n. Getting help Emacs has an n-line help system that can be invked by typing Cntrl-H. If yu type the questin mark (?), emacs will present a list f help tpics yu can chse.

21 The emacs display The display in emacs is divided int three basic areas. The tp area is called the text windw. The text windw takes up mst f the screen, and is where the dcument being edited appears. At the bttm f the text windw, there is a single mde line. The mde line gives infrmatin abut the dcument, and abut the emacs sessin. The bttm line f the emacs display is called the minibuffer. The minibuffer hlds space fr cmmands that yu give t emacs, and displays status infrmatin. Abrting a cmmand Yu can abrt an emacs cntrl r escape sequence by typing the cmmand Cntrl-G. Wrking with files T read a disk file int an emacs buffer, type the cmmand Cntrl-X-Cntrl-F. Emacs will ask yu fr the name f the file. As yu type the name f the file, it will be displayed in the minibuffer. When yu have entered the file name, press the return key, and emacs will lad the file int a buffer, and display it in the text windw. The cmmand t save the cntents f the buffer t a disk file is Cntrl-X-Cntrl-S. The save cmmand verwrites the ld versin f the file. Yu may als write the cntents f the buffer t a different file with the cmmand Cntrl-X-Cntrl-W. Emacs will prmpt yu fr the name f the file yu want t create. T create a new file, use Cntrl-X-Cntrl-F, just as if the file already existed. When emacs asks yu fr the file name, type in the name yu want yur new file t have, and emacs will create the file, and display an empty buffer fr yu t type in. Emacs will perfrm file name cmpletin fr yu. Type part f the name f the file yu want, and press the spacebar r tab key t get emacs t cmplete a file name. If the partial name yu've given matches mre than ne file, emacs will display a list f all ptential matches. Yu can cntinue typing in mre f the file's name, and pressing either file cmpletin key, until yu zer in n the file yu want. Cursr mtin On well-cnfigured systems, yu will find that the keybard arrw keys will functin crrectly in emacs, mving yu frward r backward ne character at a time, and up r dwn ne line at a time. If the arrw keys d nt wrk, here's hw t accmplish the same functins: Cntrl-F mves the cursr frward t the next character. Cntrl-B mves the cursr back t the previus character. Cntrl-N mves the cursr t the next line. Cntrl-P mves the cursr t the previus line. In additin t basic cursr mtin, emacs prvides sme ther handy cursr mtin functins:

22 Cntrl-A mves the cursr t the start f the current line. Cntrl-E mves the cursr t the end f the current line. ESCAPE-F mves the cursr frward t the next wrd. ESCAPE-B mves the cursr back t the previus wrd. ESCAPE-< mves the cursr t the start f the buffer. ESCAPE-> mves the cursr t the end f the buffer. Inserting and deleting text T insert text int a buffer, place the cursr where yu want t start inserting text, and start typing away. If yu want t insert the cntents f anther file int the current buffer, place the cursr at the desired insertin pint, and type Cntrl-X-I. Emacs will ask yu fr the name f the file yu wish t insert. Yu may als insert text by cutting it frm ne place, and pasting it at the insertin pint. See the next sectin fr infrmatin n cutting and pasting. Deleting text is easy. As yu'd expect, the delete key deletes backward ne character. Here are sme ther ways t delete text: Cntrl-D deletes frward ne letter. Cntrl-K deletes frm the pint t the end f the line. ESCAPE-D deletes frward ne wrd. ESCAPE-delete deletes backward ne wrd. Cutting and pasting text regins Emacs allws yu t select a regin f text, and perfrm cut and paste peratins n the regin. It uses a temprary strage area called the "kill buffer" t allw yu t stre and retrieve blcks f text. There is nly ne kill buffer in emacs, which means that yu can cut text frm ne dcument, and paste it int anther. T define a regin f text, place the cursr at ne end f the regin and press Cntrlspacebar. That sets the mark. Then, mve the cursr t the ther end f the regin. The text between the mark and the cursr defines the regin. T cut a regin f text, and place it in the kill buffer, use the cmmand Cntrl-W (think f Wipe). The paste cmmand is Cntrl-Y. It Yanks the blck f text frm the kill buffer, and places it where the cursr rests. The Cntrl-Y cmmand nly retrieves the mst recently-cut blck f text.

23 Yu can paste in earlier cuts by pressing ESCAPE-Y. The ESCAPE-Y cmmand, used repeatedly, will take yu back thrugh several previus text blcks that were cut. The ESCAPE-Y cmmand des nt wrk unless yu type Cntrl-Y first. Yu may cpy a regin f text int the kill buffer withut cutting it. Define the text blck by setting the mark at ne end, and mving the cursr t the ther end. Then type ESCAPE-W. Unding changes It is pssible t und the changes yu have made t a file by entering the cmmand Cntrl-_. (That's Cntrl-underscre. On sme keybards, yu'll have t hld dwn bth the cntrl and shift keys t enter the underscre character.) Many wrd prcessing prgrams can nly und the mst recent cmmand, but emacs remembers a lng histry f cmmands, allwing yu t und many changes by repeatedly entering the Cntrl-_ cde. Sectin 11: The Executin Envirnment CONCEPT: The exact behavir f cmmands issued in the shell depends upn the executin envirnment prvided by the shell. The UNIX shell maintains a set f envirnment variables that are used t prvide infrmatin, like the current wrking directry, and the type f terminal being used, t the prgrams yu run. The envirnment variables are passed t all prgrams that are nt built in t the shell, and may be cnsulted, r mdified, by the prgram. By cnventin, envirnment variables are given in upper case letters. T view all the envirnment variables, use the cmmand printenv Yu can als view a particular envirnment variable using the ech cmmand: ech $TERM The abve cmmand echs the value f the TERM envirnment variable t the standard utput. The creatin f the executin envirnment When yu lg in, a sequence f events establishes the executin envirnment. The exact sequence f events depends n the particular flavr f UNIX, and als depends upn the default shell fr yur accunt. The fllwing is a descriptin f the lgin prcess fr the HP-UX perating system. Other perating systems may differ. The getty prcess The getty prcess prvides the lgin: prmpt that yu see n the terminal screen. The getty prcess reads yur username, and invkes the lgin prgram. The lgin prgram The lgin prgram receives the username frm getty, and prmpts yu fr yur passwrd. Lgin then cnsults the system database /etc/passwd t verify yur passwrd. (Nte that lgin will request yur passwrd even if there is n entry in /etc/passwd fr the username yu've given. That prevents smene frm finding

24 valid usernames by trial and errr.) Lgin turns ff terminal ech s that yur passwrd is nt displayed n the screen. Having verified yur passwrd, lgin then uses infrmatin in /etc/passwd t invke yur default shell. If n default shell is specified in the /etc/passwd entry, lgin starts the Burne shell (/bin/sh). Shell startup: System lgin scripts When the shell prgram starts, it reads cnfiguratin files called lgin scripts t cnfigure the executin envirnment. On HP-UX, the file /etc/prfile prvides initializatin parameters fr ksh and sh, while the file /etc/csh.lgin is used fr csh. After the system lgin scripts are read, the shell lks fr user-specified lgin scripts. Shell startup: User lgin scripts After the system lgin scripts are read, the shell reads user lgin scripts. User lgin scripts are kept in ne's hme directry, and are the means by which ne can custmize the shell envirnment. Sh and ksh lk fr a file called.prfile. Ksh als reads a file defined in the envirnment variable ENV. Csh reads a file called.cshrc, and (if it is the lgin shell), the file.lgin. Imprtant envirnment variables Here are descriptins f sme f the mst imprtant envirnment variables, and examples f hw sme variables can affect the executin f cmmands. TERM The TERM envirnment variable defines the type f terminal that yu are using. Mst UNIX systems have a database f terminal types, and the capabilities f each terminal type. PATH The PATH variable cntains the names f directries t be searched fr prgrams that crrespnd t cmmand names. When yu issue a cmmand t the shell, the shell searches sequentially thrugh each directry in the PATH list until it finds an executable prgram with the cmmand name yu typed. USER The USER variable cntains yur username. Any time yu access a file r directry, the access permissins are checked against the value f USER. HOME The HOME variable cntains the name f yur hme directry. When yu issue the cd cmmand with n directry argument, yu will be placed in the directry defined in the HOME envirnment variable. The HOME variable is als where the shell will lk fr the user lgin scripts. MAIL The MAIL variable cntains the name f the directry where yur incming mail is stred. When yu start a mail prgram, the prgram will lk in the directry stred in the MAIL envirnment variable fr yur incming mail messages. EDITOR The EDITOR variable is used by prgrams that must invke a text editr t prvide the ability t edit r cmpse dcuments. One example is the elm prgram, which is used t read and send electrnic mail. If yu elect t cmpse a

25 new mail message while in elm, the elm prgram will check the cntents f the EDITOR variable t determine which editr t invke. HOST The HOST envirnment variable cntains the name f the hst machine that is running yur shell prgram. When yu cnnect t a remte hst thrugh telnet r ftp, the name f yur hst is relayed t the remte machine, s the administratrs f the remte machine can keep track f wh is cnnecting, and frm where. Setting envirnment and shell variables The exact mechanism fr setting the envirnment and shell variables depends upn the type f shell yu're using. sh, r ksh T set an envirnment variable in sh r ksh, use the syntax VAR=value;exprt VAR, where VAR is the name f the envirnment variable and value is the value yu wish t assign. D nt put spaces n either side f the equals sign. The exprt cmmand instructs the shell t prpagate the value f the variable t all prgrams that are run by the shell. If an envirnment variable is reset, but nt exprted, the change will nly apply t the shell itself. T set the EDITOR variable t the value emacs in ksh r sh, use the cmmand: EDITOR=emacs;exprt EDITOR It is als pssible t unset envirnment variables, with the unset cmmand. Unsetting an envirnment variable remves the definitin f the variable. csh T set an envirnment variable in csh, use the setenv cmmand. The cmmand has the syntax: setenv VARIABLE value. T set the EDITOR variable t the value emacs in csh, use the cmmand: setenv EDITOR emacs Fr mre infrmatin abut the shell envirnment, cnsult the manual page fr the shell yu're using. Sectin 12: Custmizing the UNIX Shell The UNIX shell is a actually a user prgram that the kernel runs fr yu when yu lg in. There is usually mre than ne shell available n mst UNIX systems. The mst cmmn shells available n UNIX systems are the Burne Shell (sh), the C Shell (csh) and the Krn shell (ksh). Here is a summary f features available n these three shells, adapted frm the Hewlett Packard "Beginner's Guide t HPUX." Feature Functin sh csh ksh Jb cntrl Allws prcesses t be run in the backgrund N Yes Yes Histry substitutin Allws previus cmmands t be saved, edited, and reused N Yes Yes File name cmpletin Allws autmatic cmpletin f partially typed file name N Yes Yes Cmmand line editing Allws the use f an editr t N N Yes