9.1. Directing Script Execution

Control flow statements are essential for creating non-trivial programs that can make decisions and perform repetitive tasks. This section covers AWK’s conditional statements (if) for decision-making and its looping constructs (for, while) for iteration.

9.2. Conditional Statements

The Statement

The if statement executes an action only if a specified condition is true.

• Syntax: if (condition) action
• Block Syntax: if (condition) { action-1; action-2; … }

This example checks if a number is even:

[jerry]$ awk ‘BEGIN {num = 10; if (num % 2 == 0) printf “%d is even number.\n”, num }’

# Output:

# 10 is even number.

The Statement

The if-else statement provides an alternative action to execute when the condition is false.

• Syntax: if (condition) action-1 else action-2

This example checks if a number is even or odd:

[jerry]$ awk ‘BEGIN { num = 11; if (num % 2 == 0) printf “%d is even number.\n”, num; else printf “%d is odd number.\n”, num }’

# Output:

# 11 is odd number.

The Ladder

Multiple conditions can be chained together to test for various possibilities.

[jerry]$ awk ‘BEGIN { a = 30; if (a==10) print “a = 10”; else if (a == 20) print “a = 20”; else if (a == 30) print “a = 30”; }’

# Output:

# a = 30

9.3. Looping Constructs

The Loop

The for loop provides a classic structure for iteration, consisting of an initializer, a condition, and an incrementor.

• Syntax: for (initialization; condition; increment/decrement) action

This example prints the numbers from 1 to 5:

[jerry]$ awk ‘BEGIN { for (i = 1; i <= 5; ++i) print i }’

# Output:

# 1

# 2

# 3

# 4

# 5

The Loop

The while loop repeats an action as long as its condition remains true. The condition is tested at the beginning of each iteration.

• Syntax: while (condition) action

[jerry]$ awk ‘BEGIN {i = 1; while (i < 6) { print i; ++i } }’

# Output:

# 1

# 2

# 3

# 4

# 5

The Loop

The do-while loop is similar, but it tests the condition at the end of the loop. This guarantees the loop body will execute at least once.

• Syntax: do action while (condition)

[jerry]$ awk ‘BEGIN {i = 1; do { print i; ++i } while (i < 6) }’

# Output:

# 1

# 2

# 3

# 4

# 5

9.4. Loop Control Statements

The Statement

The break statement immediately terminates the innermost for, while, or do-while loop it is in.

[jerry]$ awk ‘BEGIN { sum = 0; for (i = 0; i < 20; ++i) { sum += i; if (sum > 50) break; else print “Sum =”, sum } }’

# Output:

# Sum = 0

# Sum = 1

# Sum = 3

# Sum = 6

# Sum = 10

# Sum = 15

# Sum = 21

# Sum = 28

# Sum = 36

# Sum = 45

The Statement

The continue statement skips the remainder of the current loop iteration and proceeds to the next one. This example prints only even numbers.

[jerry]$ awk ‘BEGIN { for (i = 1; i <= 20; ++i) { if (i % 2 == 0) print i ; else continue } }’

# Output:

# 2

# 4

# 6

# 8

# 10

# 12

# 14

# 16

# 18

# 20

The Statement

The exit statement terminates the entire AWK script immediately, not just a loop. It can optionally return a status code to the shell.

[jerry]$ awk ‘BEGIN { sum = 0; for (i = 0; i < 20; ++i) { sum += i; if (sum > 50) exit(10); else print “Sum =”, sum } }’

After the script exits, you can check its status code:

[jerry]$ echo $?

# Output:

# 10

As programs grow in complexity, it becomes necessary to organize code into reusable blocks, which is the purpose of functions.