awk- An Advanced Filter

awk- An Advanced Filter byProf. Shylaja S S Head of the Dept. Dept. of Information Science & Engineering, P.E.S Institute of Technology, Bangalore-560085 shylaja.sharath@pes.edu

Session Objectives • Decision making using if • for loop • while loop

Control Flow- The If Statement: • awk has conditional structures (the if statement) and loops (while or for). • These control structures execute the body of statements depending on the success or failure of the control command. • A control command is a condition that is specified in the first line of the construct.

Contd.. • The if statement can be used when the && and || are found to be inadequate for certain tasks. • Syntax: • If (condition is true) • { Statement } • else • { Statement }

Contd.. • For example, to select lines where the basic pay exceeds 8000, the selection criteria is: • $4 > 8000 • An alternative form of this requires the if statement: • awk –F “|” ‘{ if ($4 > 8000) printf ………. • if can be used with the comparison operators.

Contd.. • It can be used with special symbols ~ and !~ to match a regular expression. • When used in combination with the logical operators || and &&, awk programming becomes quite easy and powerful. • Examples: if ( NR > = 2 && NR <= 5 ) • if ( $2 == “manager” || $2 == “dm” ) • if ( $4 ~ /^d.g.m/ ) • if ( $2 !~ / [aA]gg?[ar]+wal/ )

Contd.. if-else structure: The if-else structure in awk is similar to C if-else. Example: if ( $6 < 6000 ) da = 0.25*$6 else da = 1000

Contd.. • The above if-else can be replaced by if construct with a compact conditional structure as: • $6 < 6000 ? da = 0.25*$6 : da = 1000

Looping with for • awk supports two looping constructs namely for and while. • The for and while execute the loop body as long as the control command returns a true value. • The for loop has two forms. • First is a simple for that resembles its C counterpart. • Example: for (count=0; count<10; count++)

Contd.. This simple form consists of three components: • Initialization, which initializes the value of k, • Conditional Expression, which checks the condition in every iteration • Increment, while the sets the increment used for every iteration. Note: for is useful for centering text.

Contd.. Example: $ echo “ >Salary Statement \n for\n this Month” | >awk ‘ { for (i=1 ; i < (55 –length($0)) /2 ; i++) >printf “%s”,” “ >printf $0}’emp.lst The above examples uses awk for loop with echo in a pipeline to centre the text.

Contd.. • Output: • Salary Statement • for • this Month • Here the for loop uses the first printf statement to print the required number of spaces (page width assumed to be 55 ). • The line is then printed with the second printf statement, which falls outside the loop.

Contd.. • Using for with an Associative Array: • The second form of the for loop uses the associative feature of awk’s arrays. • This form is similar to that of perl. • The loop selects each index of an array. • Syntax: • for ( k in array ) • commamds

Contd.. • Here, k is the subscript of the array arr. • Since k can be strings, all environment variables can also be printed. • Example: • $ awk ‘BEGIN { • >for ( key in ENVIRON ) • >print key “=” ENVIRON [key] • >}’

Contd.. Output: LOGNAME = ISEDEPT MAIL=/var/mail/ISEDEPT PATH=/usr/bin::/usr/local/bin::/usr/bin TERM=xterm HOME=/home/ISEDEPT SHELL=/bin/bash

Contd.. • Since the index is a string, any field can be used as index. • Elements of the array can be used as counters. • Example: • $awk –F ’|’ ‘{ count[$3]++ } • >END { for ( desig in kount) • >print desig, kount[desig] }’ emp.lst

Contd.. • Output: • manager 4 • chairman 1 • executive 2 • director 3 • Note: • Here the employee databases is break up and grouped on their designation.

Contd.. • The array count[] takes as its subscript non-numeric values manger, chairman, executive, director. • for is invoked in the END section to print the subscript (desg) and the number of occurrence of the subscript (count[desg]).

LOOPING with while • The while loop like for loop repeatedly iterates the loop until the command succeeds. • Example: • k=0 • while (k < (55 – length($0))/2) { • printf “%s”,“ ” • k++ • } • print $0

Contd.. • Here the while loop prints a space and increments the value of k with every iteration. • The condition (k < (55 – length($0))/2) is tested at the beginning of every iteration, and the loop body only if the test succeeds.

Conclusion • In this session we saw topics of awk like:. • Use of if decision making structure helpful in writing awk programs. • Looping constructs like for and while. • Two varieties of using for

awk- An Advanced Filter