S E C T I O N O V E R V I E W

Transcription

1 AN INTRODUCTION TO SHELLS S E C T I O N O V E R V I E W Continuing from last section, we are going to learn about the following concepts: understanding quotes and escapes; considering the importance of the shell; introduces shell s syntax; obtaining user input; passing data into scripts; performing simple arithmetic; fundamentals of shell programming. 7.1 QUOTES AND BACKQUOTES In UNIX shells, there are several characters that have special meaning. These include the dollar sign ($), the greater that (>), less than (<), the ampersand (&) and so on. Each of these serves some kind of functionality. For example, the dollar sign ($) allows us to access the contents of a variable while the greater (>) and less (<) than characters reflect output and input redirection. The set of all characters that have a special meaning for UNIX shells, are known as meta-characters. These are: *? [ ] \ $ ; & ( ) ^ < > new-line space tab FIGURE 6.1: Meta-characters Although, these characters are interpreted by shells with this certain functionality, we may wish to use them as regular ones. For instance, we may wish to display a string on our terminal window that corresponds to the PAGE 1

2 currency of the USA or we would like to display a sentence that describes inequality between two numbers. This can be shorted out by the use of some special characters that indicate the system our intentions. More precisely, these characters are the single quote ( ), double quote ( ) and the backslash (\) symbol. In other words, these characters override the preserved functionality of all meta-characters. Let us consider an example: 1 tiger% echo Hello ; world Hello world: Command not found 2 tiger% SAMPLE OUTPUT 7.1: USING ECHO WITH ; CHARACTER In sample output 7.1, the shell echoed Hello on the terminal display. However, it did not echoed world. The reason of getting this result corresponds to that the shell tried to execute command world. Recalling back from previous sections, the meta-character ; allows us to execute UNIX commands sequentially. Now, let us attempt to override the functionality of semicolon (;) symbol and indicate the shell that we need to echo the string Hello ; world on our terminal window. This is shown in sample output tiger% echo Hello \; world Hello ; world 5 tiger% echo Hello world Hello world 6 tiger% echo Hello world Hello world 7tiger% SAMPLE OUTPUT 7.2: USING ECHO WITH \ ; CHARACTER In this case, we used the backslash symbol followed by the semicolon symbol. This instructed the shell to consider (;) as a semicolon and displayed it on our terminal window. Therefore, world was not considered as a UNIX command by the shell. Of course, as an alternative solution we can use single PAGE 2

3 or double quotes. The result after the execution, as shown, is exactly identical. However, each of these symbols is used differently. Typically, if we wish to use any of the meta-characters shown in figure 7.1 overriding any functionality ascribed to them, we need to use either quotes or the backslash symbol. Quotes are often used for strings while backslash is used with single characters. 11 tiger% set VAR = 'Hello <$NAME>' 12 tiger% echo $VAR Hello <$NAME> 13 tiger% SAMPLE OUTPUT 7.3: VARIABLES INCLUDED IN OTHER VARIABLES In sample output 7.3, we attempted to define a variable that contains a reference of another variable in it. However, the execution of the output did not have the expected outcome. The reason is well known as it has been elaborated previously. Now the problem is to find out how we can achieve that having a reference of a variable in the context of another variable. This can be carried out by the use of brackets that have a special meaning for UNIX shells. 1 tiger% set NAME = Tom 2 tiger% echo $NAME Tom 3 tiger% set HELLO = ('Hello ' $NAME) 4 tiger% echo $HELLO Hello Tom 5 tiger% set HELLO = ("Hello " $NAME) 6 tiger% echo $HELLO Hello Tom 7 tiger% set NAME = Ioannis 8 tiger% echo $HELLO Hello Tom 9 tiger% set HELLO = ('Hello ' $NAME) 10 tiger% echo $HELLO Hello Ioannis 11 tiger% SAMPLE OUTPUT 7.4: USING ECHO WITH \ ; CHARACTER Initially, we defined a variable called NAME and assigned the value Tom into it. Then, we set up a new variable called HELLO. However, the PAGE 3

4 most important point here is that HELLO variable contains a reference of NAME variable. In order to complete its definition, we had to use brackets. Moreover, the string hello was separated from variable $NAME by the use of double quotes. Next, we echoed HELLO variable and we obtained the expected result. Another important point here is that when we decided to change the value of the variable NAME, we initially thought that echoing HELLO would refer to the updated value of variable name. This did not happen initially. However, when we redefined variable HELLO, the expected result was obvious. To this extent, we can conclude that when a variable contains a reference of another variable, the assignment of a value to the former variable happens statically and cannot change dynamically i.e. when we redefine the included variable in the context of another variable. Moreover, when we use quotes we need to remember to close them as shells match an initial single quote with the nearest possible ending single quote. Therefore, although we are allowed to use single quotes into a quoting string, there are ignored by the shell. 14 tiger% set TEST = 'Hello '' how are you' 15 tiger% echo $TEST Hello how are you 16 tiger% set V = 'Hello Unmatched ' 17 tiger% SAMPLE OUTPUT 7.5: WORKING WITH SINGLE QUOTES In sample output 7.5 we can see that csh ignored two single quotes placed in the quoting string. Moreover, when we attempted to define variable V, we did not close the quoting string before pressing the <RETURN> key. This resulted in the generation of an error. A suggestion here is that we need to use as less as possible single quotes in a statement. In Bourne shells, there are some differences. For example, forgetting to close a single quote will result in the appearance of the continuation prompt. This actually informs us to close an opening single quote. PAGE 4

5 $ MYVAR='Hello > world' $ echo $MYVAR Hello world $ SAMPLE OUTPUT 7.6: WORKING WITH SINGLE QUOTES IN BOURNE SHELLS Another way of making reference to variables inside other variables is through the use of double quotes. In addition, double quotes can include single quotes in it. 3 tiger% set MYNAME = "Ioannis" 4 tiger% set HELLO = "Hello $MYNAME" 5 tiger% echo $HELLO Hello Ioannis 6 tiger% echo "Hello 'nice' world" Hello 'nice' world 7 tiger% SAMPLE OUTPUT 7.7: WORKING WITH SINGLE QUOTES IN BOURNE SHELLS In the example provided (7.7), we defined a variable MYNAME with value Ioannis, then we defined an additional variable called HELLO using double quotes. In this variable, during the assignment process, we used the former variable as a part of former variable. The echo command resulted in the expected outcome which corresponds to the string Hello Ioannis. Moreover, we also demonstrated the use of single quotes into double quotes. In fact, these were interpreted as regular symbols as the interpreter ignored any special meaning of them. The use of back quotes is quite different. More precisely, there are several times we need to assign into variables values that are obtained by the execution of a command or a pipeline. This is achieved by the use of back quotes. Assuming that we need to define a variable called YEAR and assign into it the value obtained by the execution of a pipeline. The following combination of commands with the use of back quotes will result in the expected outcome. PAGE 5

6 27 tiger% set YEAR = `date cut -f6 -d' '` 28 tiger% echo $YEAR tiger% SAMPLE OUTPUT 7.7: WORKING WITH SINGLE QUOTES IN BOURNE SHELLS In order to define our variable, we used the well known command set. However, in order to dictate the execution of the pipeline, we placed it into back quotes. After pressing the <RETURN KEY>, csh executed the pipeline command; the output obtained was assigned to the variable YEAR. Moreover, in order to avoid the use of pipelines and obtain date s output that corresponds to the year value, we can also use the option + %Y. Any examples are omitted as this can be carried out easily. After understanding some basic notions about the use of backslash, single quotes, double quotes and back quotes, we are ready to move to the next section that introduces the notion of shells and some basic shell programming. 7.2 INTRODUCTION TO SHELLS There are several notions that have been presented so far. For instance, we considered UNIX commands, environment variables, processes, pipelines and so on. However, as the most important element is the shell, in this section we are going to introduce the components that consist of shells which make it to be a programming language itself. The functionality of the shell can be considered as multi-dimensional. Commands, we have learnt so far can be executed because they can be found in directories included in our PATH variable. These commands allow us to runs programs such as the VI editor, to construct shell scripts, to store files and so on. Moreover, through the use of the shell we can have access to development environments and programming languages that allow us to develop various programs and applications. In addition, through the shell we can interact with the UNIX kernel, check the success or failure of commands PAGE 6

7 (i.e. standard output, standard error), check the state of the file system, obtained and set up values assigned to environment variables. However, the shell is also a programming language itself which is specifically designed for use with the UNIX operating system. Through the use of commands, it allows us to gather information from the kernel and write shell scripts that can process this information and perform various tasks. In fact, this also allows us to develop our commands. Note that although the shell can be considered as a very expressive programming language, it is not recommended for the development of various applications with scope out of the UNIX environment SHELL SYNTAX Each programming language consists of a set of primitive components such as its alphabet and its syntax. The alphabet is a set of symbols that are used by this language while its syntax corresponds to formation rules that define how these symbols can be bound together. Alternatively, these rules determine what is allowed to write and what it is forbidden. Typically, programs written using these constructs are known as scripts. In short, these are files containing lines of commands that can be interpreted by the shell interpreter. Scripts can be simple consisting of few lines or can be more complex with hundreds of lines in them. Simple scripts may be easy understandable but those that consists of several or even hundred of lines may not be easily understood. Therefore, we need to follow some simple rules that can facilitate us and other people who may need to work on these scripts. At the top of the script, we provide information about: the name of the script and a short description about its use; the author of the script; when it was written; any comments related to it i.e. version number etc. A typical example of a script is described in the following sample. PAGE 7

8 # ************************************************** # Timestamp # This script redirects output into a file called timestamp. # It obtains a list of all users who are logged in a machine # There are two option available. The first given information about # users, terminals and dates. Option two gives a list of users Internet # connection. # Developed by Ioannis Svigkos # # Version 1.0 # Comments: It makes use of LOGNAME environment # variable. # ************************************************** SAMPLE OUTPUT 7.8: A TYPICAL SCRIPT STRUCTURE (1) Moreover, we need to provide information about each part of the script. This is achieved by the use of comments expressed by the hash (#) symbol. In fact, everything that is written after # will be ignored by the shell interpreter. Consider for example, the following script: echo "Please select one of the following options:" # echoes a message echo "Press one (1) for login identifiers, terminal and date info" # echoes echo "Press two (2) for Internet connections only" # echos 2nd choice set choice = $< # prompts for user input set choice = "f$choice" # changes choice to include letter f echo "Time stamping starts... you selected option $choice" # echoes (who cut -$choice -d' ') > stamp # performs operations echo "Time stamp successfully finished." # echoes operation ends echo " " # blank line echo "The results are stored in file stamp and are..." # echoes more stamp # displays contents of file stamp on standard output echo SAMPLE OUTPUT 7.9: A TYPICAL SCRIPT STRUCTURE (2) This script includes several echo commands that display some information to our terminal windows after its execution. Although, it is a very simple example, we can see that this script prompts the user to select one of two choices presented. Then it stores this information into a file called stamp. The most important points here are that when this script is executed, users are PAGE 8

9 prompted to enter data. This is achieved by the use of set choice = $< that prompts for user input. Moreover, each line has a comment that explains its use. Further description of this script is omitted as there is a detailed description in the comments provided for each line FORMATTING OUTPUT In order to printout a string on our terminal window, so far we have considered the echo command. However, echo exhibits some limitations. For instance, it can print only one line on screen. A possible solution to this problem may be the use of multiple echo commands. However, this is not very efficient. An alternative is the use of printf command which allows the formatting of an output. This is carried out by giving the string as an argument while in the context of this string we can include some special formatting characters. These characters can be found on table 7.1. CHARACTER DESCRIPTION \a Bell \b Backspace \f Skips one page if this is possible \n New line \r Carriage return \t Tab \v Vertical tab \\ \ %d Integer decimal %o Integer octal %x Integer Hexadecimal %s String %c Character TABLE 7.1: PRINTF FORMATTING CHARACTERS The following sample output describes some common use of the printf command. PAGE 9

10 tiger% printf "Hello. How are you?" Hello. How are you?3 tiger% printf "Hello!\nHow are you?\n" Hello! How are you? 5 tiger% printf "\a" 6 tiger% printf "Hello\tHow are you?" Hello How are you?7 tiger% printf "Hello\t How are you? \n" Hello How are you? SAMPLE OUTPUT 7.10: WORKING WITH PRINTF When, we entered the command printf Hello. How are you?, we avoided putting any control tags in it. The consequence was that the string was printed but our command line was placed at the same line. This is due to the fact that we did not enter the control tag \n that indicates new line should be included there. This was resolved after the inclusion of the corresponding control tag. Next, we used prinf \a and indeed we heard a beep. \t printed the corresponding string and separated Hello from How are you? with a tab between them. Finally, we demonstrated that various control tags can be inserted in a sentence. 8 tiger% printf "Hello %s \n" "Ioannis" Hello Ioannis 9 tiger% printf "Welcome back %s \n. Your account balance is %d OD" "Tom" 50 Welcome back Tom. Your account balance is 50 OD 10 tiger% SAMPLE OUTPUT 7.11: WORKING WITH PRINTF In sample output 7.11 we describe some conversion characters that instruct how a part of an argument given (string) should be displayed. Note that the number of conversion characters, included in a string, should be equal to the number of arguments given excluding the first argument which is the script itself. For example, in printf Hello %s\n Ioannis the conversion character %s informs the shell that a string given as an argument will follow. Indeed the string is Ioannis. "Welcome back %s \n. Your account balance is %d OD" "Tom" 50 command contains two conversion characters and one control tag. The conversion characters are %s that represents a string PAGE 10

11 and %d that corresponds to an integer number. The arguments given for these conversion characters are: Tom and 50, respectively. The \n just instructed the shell to print Your account balance is %d OD to a new line. printf can also be used with environment variables. Note that conversion characters can be used to represent environment variables, as well. 6 tiger% printf "Hello $LOGNAME\n" Hello svigkoi 7 tiger% printf "Hello %s\n" $LOGNAME Hello svigkoi 8 tiger% SAMPLE OUTPUT 7.11: WORKING WITH PRINTF In the above sample output, we initially executed a printf command which has as argument a string and an environment variable both included in quotes. In the second example provided, we used a conversion character while we gave the environment variable as an additional argument. Both examples output exactly the same results OBTAINING USER INPUT When writing scripts, we may need to obtain some user input. This is accomplished by the use of the set command. However, the way we use it in order to achieve our goal is quite different than the usual one. More precisely we need to use something like that set myvar = $<. 25 tiger% csh input Please enter your first name.ioannis Please enter your surname.svigkos Hello Ioannis Svigkos! How are you? SAMPLE OUTPUT 7.11: WORKING WITH PRINTF This instructs the shell to prompt for user input. In sample output 7.9 and 7.11, there are some indicative examples. Note that as syntax in Bourne shells differs, prompting for user input is carried out by the use of the read command. PAGE 11

12 7.2.4 PASSING INPUT AS A SCRIPT ARGUMENT The previous section demonstrated how to enter values into our scripts. Moreover, so far, we have also learnt how to use environment variables in our scripts. Both of these ways allow us to use data in our scripts. However, we may need to pass data as arguments during the initialisation of our script. In fact, a script can be considered as a command that can be executed and it may need to have some arguments. The method of defining arguments is through the use of $<number>. For example, $1 represents our first argument while $2 is the second argument and so on. These specialised variables are known as positional parameters and can be used as regular variables. However, note that they cannot be reset by using the command unset or by using set =. 2 tiger% cat > arguments $1 $2 $3 echo "This is the first argument $1" echo "This is the second argument $2" echo "This is the third argument $3" 3 tiger% csh arguments One Two Three One: Command not found This is the first argument One This is the second argument Two This is the third argument Three 4 tiger% SAMPLE OUTPUT 7.12: PASSING DATA AS SCRIPT ARGUMENTS As shown, we defined three positional parameters $1, $2, $3 in our script called argument that echoes each of them. We executed the script by giving the values One Two Three, respectively. The output is given in sample Note that we can use as many positional parameters we wish but there are some that have a special meaning. These are $0 that represents the name of the script and $#argv that corresponds to the number of positional parameters given. Whilst $argv[*] represents a word list of any parameters given excluding the name of the script. PAGE 12

13 16 tiger% more arguments $1 $2 $3 echo "This is the first argument $1" echo "This is the second argument $2" echo "This is the third argument $3" echo "*************************************" echo "SCRIPT'S NAME: $0" echo "POSSITIONAL PARAMETERS: $#argv" echo WORD LIST PARAMETERS: $argv[*] 17 tiger% csh arguments : Command not found This is the first argument 1 This is the second argument 2 This is the third argument 3 ************************************* SCRIPT'S NAME: arguments POSSITIONAL PARAMETERS: 3 WORD LIST PARAMETERS: tiger% SAMPLE OUTPUT 7.13: PASSING DATA AS SCRIPT ARGUMENTS This sample output demonstrated the use of some specific positional parameters. The upper half of this sample describes the script we developed while the lower half part corresponds to the output given after the execution of the script EXIT STATUS Anytime we perform a UNIX command, the command is executed by the shell and any results are typically displayed on our terminal window. These results may include something obtained through the standard output or they may be error messages directed to the standard error. However, the problem here is that machines cannot interpret any sentences given as output and understand the meaning of an error message. Therefore, the success of failure of each UNIX command should be represented by an integer In fact, every time we execute a command, its termination returns a number known as exit status. Typically, the success of each command is denoted by the integer zero (0) while any failures return number (1). In order to find out the exit status of the last command executed we need to access the PAGE 13

14 contents of an environment variable known as status. This is applicable to C shells. In Bourne shells, we simply need to type in our command prompt $?. 12 tiger% date Mon May 30 02:06:06 BST tiger% echo $status 0 14 tiger% acommand acommand: Command not found. 15 tiger% echo $status 1 16 tiger% ls -l file -rwx svigkoi staff 6 May 30 02:20 file 17 tiger% chmod 500 file 18 tiger% ls l file -r-x svigkoi staff 6 May 30 02:20 file 19 tiger% cat >> file file: Permission denied 20 tiger% echo $status 1 21 tiger% SAMPLE OUTPUT 7.10: EXIT STATUS We initially typed date which was executed successfully by displaying the date and time on our terminal window. Next, we echoed the contents of variable status. This displayed number 0 and it was expected as the execution of date was successful. Next, we typed command acommand which could not be found in the environment path. Therefore, an error message was returned. In an attempt to see the contents of variable status, we saw now its value is set to number 1. In addition, we also attempted to write information into a writeprotected file. Again, we received an error message while the status variable was set to number 1 which indicated there was an error LIST COMMANDS In order to understand the use of exit status, we can consider the use of list commands. These are commands that consist of simple UNIX commands or can be composed of by pipelines separated by or &&. The use of the former symbol refers to an or-list while the use of the latter one refers to an and-list. PAGE 14

15 OR-list: A typical example is pipelinea pipelineb This means that if pipelinea starts its execution and returns a non-zero exit status then pipelineb will start its execution. If pipelinea returns zero, pipelineb will never be executed. Moreover, the return status of this list will be 0, if pipelinea will be executed successfully or it will have a return status that stems from the execution of pipelineb which can be zero or non-zero. If non-zero is the outcome then this means that Or-list exited with an error. AND-list: An example is pipelinea && pipelineb When pipelinea is run if after its execution returns a zero exit status, pipelineb is run. The exit status of an and-list corresponds to pipelinea if zero. However, if pipelinea or pipelineb returns a non-zero exit status then the exit status of the pipeline corresponds to the exit status of the pipeline which returns the non-zero exit status. In general, an and-list succeeds if and only if both pipelines will be executed successfully. 2 tiger% who cut -f1 -d' ' echo "Command failed" w w getovv beusdul w svigkoi w w w w dowling xanthisl w tiger% echo $status 0 4 tiger% who cat -f1 -d' ' echo "Command failed" cat: illegal option -- f usage: cat [ -usvtebn ] [- file]... Command failed 5 tiger% echo $status 6 tiger% 1 7 tiger% SAMPLE OUTPUT 7.11: OR-LIST PAGE 15

16 After entering the or-list command who cut -f1 -d' ' echo "Command failed" and pressing the <Return> key, it was executed successfully. In fact, in our terminal display we could see a list of all users logged in tiger.westminster.ac.uk. As, the pipeline who cut f1 d returned exit status 0, the echo command was not executed. The exit status of or-list was 0, as well. We attempted to execute who cat -f1 -d' ' echo "Command failed" but because of a syntax error, the UNIX shell could not execute the pipeline on the left hand side. Therefore, the command echo Command Failed was executed informing us about this event. Of course, the exit status of the or-list was expected to be 1 indicating this error. Considering and-lists, we provide a similar example that will allow us to understand any differences between this command and the previously elaborated or-list command. 7 tiger% who cut -f1 -d' ' && echo "Both succeeded, thus and-list succeeded" w w getovv beusdul w svigkoi w w w w dowling xanthisl w Both succeeded, thus and-list succeeded 8 tiger% echo $status 0 9 tiger% who cat -f1 -d' ' && echo "Impossible to succeed" cat: illegal option -- f usage: cat [ -usvtebn ] [- file] tiger% echo $status 1 11 tiger% SAMPLE OUTPUT 7.12: OR-LIST PAGE 16

17 The execution of who cut -f1 -d' ' && echo "Both succeeded, thus and-list succeeded" add-list was successful. Therefore, both commands placed on the left hand and right hand side were executed. The exit status of this and-list was 0 which corresponds to successful execution. An attempt to execute who cat -f1 -d' ' && echo "Impossible to succeed had the expected outcome. The pipeline on the left hand side failed and returned exit status 1. Thus, the execution of this and-list was terminated and returns exit status equal to 1. Here, the general rule regarding or-lists is that if the first pipeline fails then the second will start running. The success of an or-list is guaranteed if one of the or-list parts is executed successfully. If both fail then the list fails. On contrast with or-lists, the rule for add-lists reflects that if one part fails then and-list fails. If both succeed then and-list succeeds. 7.3 ARITHMETIC OPERATIONS Being in front of our terminal window, we may sometimes need a calculator in order to perform some calculations. Moreover, when we develop scripts, we may need to perform various calculations in our scripts and display any results on a terminal window or store them into a file. The utility that is used in these cases in known as bc (basic calculator). Although, bc can perform some complex calculations in bases other than 10, in this section we concentrate in some basic calculations in base tiger% bc 10 * 2 20 scale = / * ^2 4 sqrt(800) tiger% SAMPLE OUTPUT 7.13: USING BC PAGE 17

18 As most of operations demonstrated are self explanatory, we omit any description. However, sqrt() is just a predefined function that calculates the square root of a given argument while scale = 6 reflects that any subsequent calculations should be displayed with 5 decimal numbers. OPERATION DESCRIPTION + Addition - Subtraction * Multiplication / Division % Integer remainder ^ Power sqrt Square root scale Scale length Number of decimal digits TABLE 7.2: OPERATORS AND FUNCTIONS IN BC Note that, we can also use parentheses, if we wish to perform more complex calculations. However, there are some rules i.e. multiplication and division take precedence over addition and subtraction. Moreover, we can use create pipelines using the bc utility. 30 tiger% bc 2 * * * (10 + (2*4)) tiger% echo "3 + 3" bc 6 34 tiger% echo "sqrt(9)" bc 3 35 tiger% SAMPLE OUTPUT 7.14: OPERATOR PRECEDENCE AND PIPELINING UNING BC The calculation of 2 * 10 2 gave us 18. This is correct as bc calculated first the multiplication part and then performed the subtraction. Similarly, bc performed the other operations entered according to some precedence over operators and parentheses. PAGE 18

19 7.4 EXERCISES 1. Create a directory called scripts. a. Change directory by moving into scripts. b. Execute an echo command with argument Hello & Have a nice day c. Did this command display the expected Hello & Have a nice day d. If NO, then use an appropriate character to override the functionality of &. 2. Repeat exercise one and instead of & symbol use the character >. Try to understand the output given by this command. After that use single quotes and try to understand the difference. 3. Define a variable called MYNAME and assign into it your name. a. Write a script called welcome that gives a similar output: Hello TOM. Your current directory is: /usr/eland/staff/user1/tom You can start working. NOTE: USE COMMAND PRINTF INSTEAD OF ECHO b. Execute this script. 4. Using back quotes define a variable MYWORKINGDIR and assign into it a value that corresponds to your current directory. [HINT: make use of pwd command] 5. Develop a script called myls that accepts one argument. This argument corresponds to the absolute path of a directory. The output of the script should be similar to: Script name: myls [USE $0 TO DISPLAY THE NAME OF THE SCRIPT] Absolute path: /user/eland/staff/user1/tome/scripts PAGE 19

20 The contents of this directory are: myfile1 myfile2 myfile3 [MAKE USE OF LS OR LS L COMMAND]] 6. Develop a script called calc that accepts three arguments (integers) and multiplies them. The output of the script should be similar to: Script name: Argument one: Argument two: Argument three: Performing multiplication The result is: ($1 * $2 * $3) =. 7. In scripts directory create two text files using the VI editor with names f1 and f2 with contents: [f1 Contents] This is my first file [f2 contents] This is my second file a. Write a list command that compares (diff) the two files. If f1 and f2 are different then the first file should be sent to yourself. [HINT: MAKE USE OF AN OR-LIST COMMAND, DIFF COMMAND AND MAILX COMMAND] b. Create a file called f3 with identical contents of file f1. Compare both files and if they are identical delete file f1. [HINT: MAKE USE OF AN A-LIST COMMAND, CMP COMMAND AND RM COMMAND] 8. Execute command ls l and then check its exit status. Enter another command that is typed incorrectly and then check its exit status. PAGE 20

21 9. Develop a script that accepts four arguments and using printf display its arguments as a word list. Make sure your script also displays the name of the script and the number of arguments given. PAGE 21