Lab - 8 Awk Programming

Transcription

1 Lab - 8 Awk Programming AWK is another interpreted programming language which has powerful text processing capabilities. It can solve complex text processing tasks with a few lines of code. Listed below are just a few of the uses of AWK: Text processing, Producing formatted text reports, Performing arithmetic operations, Performing string operations, and many more. Most of the programs mentioned in the shell scripting section can also be done with awk as it also has similar conditional structures, loops, functions, etc. Awk has more of a C like syntax. We will focus more on text processing capabilities of awk. As you are already familiar with structure and basics of awk program and there is no ambiguity in the syntax as it was there with shell scripting, we will start with simple awk programs and move to complex ones towards the end. Instructions: Read the awk programming lecture slides carefully. Revisit lab-5 in which you learnt how to run awk in one line. Read this link for built-in variables in awk. They are very helpful.

2 Running awk Programs If the program is short, then it can be included in the same line on the terminal. awk 'program' input-file1 input-file2... If the program is long, it should be written in a file and saved with.awk extension. Actually, you can just save the file w/o any extension as well. The program is run with awk -f program-file input-file1 input-file2... Awk Working Methodology 1. Awk reads the input files one line at a time. 2. For each line, it matches with given pattern in the given order, if matches performs the corresponding action. 3. If no pattern matches, no action will be performed. 4. In the above syntax, either search pattern or action are optional, But not both. 5. If the search pattern is not given, then Awk performs the given actions for each line of the input. 6. If the action is not given, print all that lines that matches with the given patterns which is the default action. 7. Empty braces without any action does nothing. It won t perform default printing operation. 8. Each statement in Actions should be delimited by semicolon.

3 1. lets first start with one line programs. Consider the following data: The above data is stored in marks.txt 1.1) Display just the names, genders and endsem marks of: (a) all students (b) the first 3 students only (c) the last student Sol: (a) awk '{print $1 "\t" $2 "\t" $5}' marks.txt (b) head 4 marks.txt awk '{print $1 "\t" $2 "\t" $5}' (c) tail 1 marks.txt awk '{print $1 "\t" $2 "\t" $5}' 1.2. Store the male students in males.txt and the females in females.txt. Sol: awk '$2 == "M" {print} ' marks.txt cat > males.txt awk '$2 == "F" {print} ' marks.txt cat > females.txt 1.3. Compute the total mark for each student and display it along with their names. Sol: awk 'NR>1 { print $1"\t" $3 + $4 + $5 }' marks.txt 1.4. Who is the topper? Which students scored above the class average? Sol: awk 'NR>1 { print $1"\t" $3 + $4 + $5 }' marks.txt cat > total sort k 2 total tail 1 awk '{print $1}' 2. Program to assign grades to students using if-else blocks

4 3. Printing the fields in reverse order using for loop. One good property of awk arrays is that they can use strings as subscripts, which we will use below. 4. Removing duplicates from your history For this, first use the following command to write your history into a file. history awk 'FS="[0 9]+[\" \"]" {print $2}' > history.txt Then run the following script: The data array is indexed by the text of each line ($0). It is incremented by one when it encounters a line. lines array holds all the unique lines. You can also print number of times each command was executed by using: print data[lines[i]], lines[i] [Exercise: Use the above program, sort the output by the number of times each command was executed and write the output to a new file]

5 5. Using gsub function and regular expressions to remove punctuation characters in the text. 6. Little XYZ has joined our IIITD and he is very new to the grading system present here. He has finished his first semester and has got some grades. But he doesn t know how to compute the GPA. Below is a list of subjects which he has taken, grade he obtained and the subject credits. Can you help him to compute his GPA using awk script? (Hint : calculate the GPA based off of the equation sum of grade * credit hours / sum of credit hours)