Determining a character in a specific position in a string

Assignment Detail:- Goal The main purpose of this practical is to give you further familiarity with UNIX shell scripts- 1- UNIX Shell Scripting1-2 Hands on exercises: A Create a script named emp3-sh, with the following content: #! /bin/sh echo -e "Enter the pattern to be searched: \c" read pnameecho -e "Searching for $pname\n" if grep "$pname" e-lst; thenecho "Pattern found in e-lst" elif grep "$pname" f-lst; thenecho "Pattern found in f-lst" else echo "Pattern not found"fi Save the script- Assign execute permission to the script-B Create a text file named e-lst, with the following content:2000, John Warren, NSW 2001, Adam Davis, NSW 3000, John Smith, ACT Create a text file named f-lst, with the following content:2000, Jack Williams, NSW 2001, Adam Davis, NSW3000, Jack Swan, ACTC Ensure that your emp3-sh, e-lst, and f-lst are all stored under the current directory- Then run the emp3-sh script by specifying the pattern as John, Jack, and Andrew, respectively-Observe the outputs and ensure that they are correctly generated- 2- UNIX Shell Scripting - test The test command uses certain operators to evaluate a condition and returns either a true or falseexit status - which can then used by if for making decisions- It works in three ways:• Compare two numbers• Compares two strings or a single string for a null value -the empty string, ""-• Checks a file's attributesThe test command does not display any output but simply returns a value that sets the exit status variable $????- In addition to the test command, some other syntax is very useful for strings:Determining the size of a string• Method 1: Assign the string to a variable, then use ${#variableName}e-g- 1 e-g- 2 $ s1="someString"; theSize=${#s1}$ echo $theSize 10 $ mystring="this is the test"$ echo ${#mystring} 16 • Method 2: Assign the string to a variable, use the length keyword for the expr commandSee the expr command in section 4-1e-g-$ s1="someString"; theSize=`expr length "$s1"` note the use of backquotes -`- here$ echo $theSize 10• Method 3: Use the echo command in combination with wc e-g$ s1="someString"$ theSize=`echo -n "$s1" | wc -m` -n suppresses adding a new line, -m counts characters$ echo $theSize 10 Determining a character in a specific position in a stringA string is just a sequence of characters, one after the other, numbered from position 0 -left to right-- One or more characters can be determined from the following syntax: Syntax: ${varname:position:count} the $ here is not the prompt, it is required e-g e-g e-g Start from where???? How many characters???? $ s1="someString"$ someChar=${s1:3:1}$ echo $someChar e $ s1="someString"$ someChar=${s1:4:3}$ echo $someChar Str $ s1="someString"; start=1; num=4$ someChar=${s1:start:num}$ echo $someChar omeS Note the variable names inside the ${} do not need to be prefixed by ' e-g- num and not $num Hands on exercises: A Perform the following actions on your shell- For each command line, the first $ is the shell prompt- Remember $???? has the value of 0 if the command succeeds, and a non-zero -normally 1 or 2- value if it fails- $ x=5; y=7; z=8 $ test $x -eq $y; echo $???? $ test $x -lt $y; echo $???? $ test $z -gt $y; echo $???? $ test $z -eq $y; echo $????B Try to understand the following statements: if test $# -ne 3; thenecho "You did not enter three arguments" elseecho "You entered the right number" fi Remember from a previous tutorial that, $# refers to the number of arguments in a command line-C Repeat step A by changing the test commands to --: $ - $x -eq $y -; echo $???? $ - $x -lt $y -; echo $???? $ - $z -gt $y -; echo $???? $ - $z -eq $y -; echo $???? D The above examples show how to compare numbers on a UNIX shell- The following example demonstrates how to compare strings-Create the following script and name it as compile-sh: #! /bin/shif - $# -eq 1 -; thenif - $1 = "j" -; thenfile=`ls -t *-java | head -1`echo -e "The last modified Java file is $file\n" elif - $1 = "c" -; thenfile=`ls -t *-c | head -1`echo -e "The last modified C file is $file\n" elseecho "Invalid file type"fi elseecho -e "Usage: $0 file_type\nValid file types are c and j"fi This script stores the last modified C or Java program filename in the variable file- It then displays the file name- You have to provide one argument to the script - the file type, which must be c -for C program- or j -for Java program-- If a user does not enter a file type or enters a file type which is neither c nor j, then the script usage information is displayed- The usage information shows the correct way to run a script- Remember from last week's tutorial that, a shell script can read in command-line arguments- The first argument is referred to as $1, the second argument is referred to as $2, and so on- $0 refers to the script file name- $# refers to the number of arguments in the command line- The ls with option -t displays file names by modification time- The last modified file is displayed first- The head -1 -digit one, not ell- gets the file name listed at the top -or in the beginning--E Prepare some testing C and Java files before you attempt to run this script- Following exercises in the previous section, make a copy of e-lst and name it as t1-c- Make a copy of f-lst and name it as p1-java- Make a copy of t1-c and name it as t2-c- Make a copy of p1-java andname it as p2-java-F Assign execute permission to compile-sh- Run the script as follows- Ensure that you understand the outputs- $ -/compile-sh$ -/compile-sh k$ -/compile-sh c$ -/compile-sh j G Create a script named emp4-sh, with the following content: if - -f $1 -; then echo "File exists"elseecho "File does not exist"fi Save it- Assign execute permission to it-H Run the script as follows: $ emp4-sh t1-c$ emp4-sh t2-c$ emp4-sh t3-cI Modify the content of emp4-sh, so that it becomes the following: if - ! -f $1 -; thenecho "File does not exist" elif - ! -r $1 -; thenecho "File is not readable" elif - ! -w $1 -; thenecho "File is not writable" elseecho "File is readable and writable"fi ???? Note: ! reverses a condition -it is the equivalent to not- • - ! -f $1 - is true if the ordinary file $1 does NOT exist-• - ! -r $1 - is true if the file $1 is NOT readable• - ! -w $1 - is true if the file $1 is NOT writable• - ! -x $1 - is true if the file $1 is NOT executableJ Run the script again as follows:$ emp4-sh t1-c$ emp4-sh t2-c$ emp4-sh t3-cK Remove the read permission of t1-c and the write permission of t2-c, and then run the above command lines again to observe the outputs- 3- UNIX Shell Scripting - The case conditional3-1 Hands on exercises A Remove all the temporary C or Java files from your current directory- Create a scrip which is named emp5-sh, with the following content: #! /bin/sh tput clearecho -e "\n 1- Find files modified in last 24 hours" echo -e "\n 2- The free disk space"echo -e "\n 3- Space consumed by this user" echo -e "\n 4- Exit\n\n"echo -e "SELECTION: \c" read choice case $choice in1- find $HOME -mtime -1 -print ;;2- df ;;3- du -s $HOME ;; 4- exit ;;*- echo "Invalid option" ;;esacHere the first 4 echo commands generate a menu- The 5th echo prompts the user to make a selection -which needs to be a number in the range of 1 - 4, inclusive-- Depending on the user's selection -which is stored in the variable choice-, a corresponding command line is run- If the user's selection is a character other than 1, 2, 3, 4, then the message "Invalid option" is displayed-B Run emp5-sh, and enter a number in the range of 1 - 4 -inclusive- to observe its output- Run the script again by entering another number- Also try to enter a number which is not in therange -such as 5- to see the output-C The case conditional is able to match multiple patterns or using wild cards as described in previous tutorials- Add the following statements to the end of emp5-sh: echo -e "Wish to continue???? -y/n-: \c" read answercase $answer inY|y- echo -e "I will do something later\n";;N|n- echo "OK, no longer continuing!" exit;;*- echo "Invalid option" ;;esac Here Y|y matches Y or y, N|n matches N or n- Save the change and run emp5-sh again-D You can also use wildcards in case conditional: echo -e "Wish to continue???? -y/n-: \c" read answercase $answer in-Yy--Ee-*- ;; # matches YES, yes, Yes, etc-Nn--Oo-- exit ;; # matches NO, No, no, nO*- echo "Invalid option" ;; esac 4- Arithmetic expressions and the sleep command 4-1-1 Hands on exercises A Run the following command line to see the effect of sleep- $ sleep 3; echo "3 seconds have elapsed"B Try the following arithmetic operations: $ expr 3 + 5$ x=3; y=5$ expr $x - $y$ expr 3 \* 5$ expr $y / $x$ expr $x % $y The operators -+, -, *, /, %- must be enclosed on both sides by whitespace- ???? Note: The expr command only handles integers- In the 4th command line above, why is there a \ character required before *???? The most common use of expr is for incrementing the value of a variable e-g-:$ x=5$ x=`expr $x + 1` -note the back-quotes-$ echo $x$ x=5; y=2; z=`expr $x + $y` -note the back-quotes- Double parenthesesThe double parentheses -- and -- are also often used to perform basic arithmetic -think of them as a shortcut for the expr command, although there are some differences--e-g-$ x=3; y=5$ echo $-- $x + $y --$ z=$-- $x * $y -- -note the * does not need to be escaped here!- 5- UNIX Shell Scripting - LoopingLike in C or Java programming, loops -or iteration- allow you to perform a set of instructions repeatedly - repeating a block of commands over and over again either until some condition is no longer true -the while loop-, or repeating commands for each item in a sequence -the for loop - next week-- Hands on exercises A Under your kit501 directory, make a new directory named Looping- Change into this new directory-B Create a shell script which is named as colour-sh, with the following content: echo "Guess my favourite colour: " read guesswhile - "$guess" != "red" - doecho "No, not that one- Try again-" read guessdoneecho "Well done" Here the read command stores the user input into variable guess- As long as the user input is not "red", the loop body is entered, which displays a message and then prompts the user to enter a guess again- By the time the user input is "red" -has guessed correctly-, the loop is broken and exited- Assign execute permission to the script, and run it- To break the loop, you have to enter "red"- Alternatively, you can press CTRL-C to stop-C Copy the script emp5-sh into the current Looping folder- The script is from a previous section of this tutorial- Rename emp5-sh as emp6-sh- Modify the exiting content of emp6-sh so that its new content are as follows -note the -mtime option for find here is -1 -minus one--: #! /bin/sh tput clear answer=ywhile - $answer = y - doecho -e "\n 1- Find files modified in last 24 hours" echo -e "\n 2- The free disk space"echo -e "\n 3- Space consumed by this user" echo -e "\n 4- Exit\n\n"echo -e "SELECTION: \c" read choice case $choice in1- find $HOME -mtime -1 -print ;;2- df ;;3- du -s $HOME ;;4- exit ;;*- echo "Invalid option" ;; esac done Note the use of a variable named answer which is used to control the loop- Run the script- Can you see the purpose of the loop for this script????D The above script needs to be terminated by entering 4 as the user input -or pressing CTRL-C-- Another way to control a loop is to use the continue command and the break command- • break: break out of the current loop• continue: start the current loop again Add the following statements to the end of the loop body in emp6-sh -ie, between the esacline and the done line-: echo -e "Wish to continue???? -y/n-: \c" read answer2case $answer2 in -yY-- continue ;;*- break ;; esac Save the change and run emp6-sh again-E Sometimes it is necessary to set up an infinite loop- The following is such an example: while true doecho "message repeated every 2 seconds" sleep 2done Use the above statements to make a new script named emp7-sh- Assign execute permission to it- Run the script- You have to press CTRL-C to stop it- An infinite loop can be used for more useful purposes, eg, a system administrator can set up an infinite loop to monitor the available space in disks every few minutes-F What does the following program do???? while - ! -r invoice-lst - dosleep 30 doneecho "That file can be read now!" Attachment:- UNIX shell scripts-rar

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