Using loops in awk

Let's learn about different types of loops that execute the same command multiple times.

An awk script has three main parts: BEGIN and END functions (both optional), and the function written by the user to be executed each time. In a way, the main body of awk is a loop because the commands in the function are executed once for each record. However, sometimes you want to execute multiple commands on a record, then you need to use a loop.

There are many types of loops, each suitable for different scenarios.

While Loop

A while loop tests an expression and executes commands if the expression is true. When the expression becomes false, the loop breaks.

#!/bin/awk -f
BEGIN {
    # Loop through 1 to 10
  i=1;
  while (i <= 10) {
    print i, "to the second power is ", i*i;
    i = i+1;
  }
exit;
}

In this simple example, awk prints the square of the integer value placed in the variable i. The while (i <= 10) statement tells awk to execute the loop only while the value of i is less than or equal to 10. After the loop is executed for the last time (the value of i is 10), the loop terminates.

do-while loop

The do-while loop executes the commands following the keyword do. A test expression is checked at the end of each iteration to determine whether to terminate the loop. The command is executed repeatedly only while the test expression returns true (that is, the condition that terminates the loop has not yet been reached). If the test expression returns false, the loop is terminated because the condition to terminate the loop is reached.

#!/usr/bin/awk -f
BEGIN {
    i=2;
    do {
        print i, "to the second power is ", i*i;
        i = i + 1
    }
    while (i < 10)
exit;
}

for loop

There are two kinds of for loops in awk.

A for loop initializes a variable, checks a test expression, increments the variable, and continues looping as long as the expression evaluates to true.

#!/bin/awk -f
BEGIN {
  for (i=1; i <= 10; i++) {
    print i, "to the second power is ", i*i;
  }
exit;
}

Another for loop sets an array variable with consecutive indices and executes a set of commands for each index. In other words, it uses an array to "collect" the results of each command execution.

This example implements a simplified version of the Unix command uniq. By adding a series of strings as keys to array a and incrementing the key value when the same key appears again, you can find out how many times a string appears (just like the --count option of uniq). If you print all the keys of the array, you will get all the occurrences of the string.

Take the demo file colours.txt (the file in the previous article) as an example:

name color amount
apple red 4
banana yellow 6
raspberry red 99
strawberry red 3
grape purple 10
apple green 8
plum purple 2
kiwi brown 4
potato brown 9
pineapple yellow 5

Here's a simple awk version of uniq -c:

#!/usr/bin/awk -f
NR != 1 {
  a[$2]++
}
END {
  for (key in a) {
        print a[key] " " key
  }
}

The third column of the example data file is a count of the entries listed in the first column. You can use an array and a for loop to count the entries in the third column by color.

#!/usr/bin/awk -f
BEGIN {
  FS=" ";
  OFS="\t";
  print("color\tsum");
}
NR != 1 {
  a[$2]+=$3;
}
END {
  for (b in a) {
    print b, a[b]
  }
}

You can see that we also need to print a column header in the BEFORE function (which is executed only once) before processing the file.

cycle

Loops are an important part of any programming language and awk is no exception. Using loops you can control how the awk script runs, what information it counts, and how it processes your data. We will discuss switch, continue, and next statements in our next article.

ps: Simple usage of awk-for loop

text:

[root@VM_0_84_centos ~]# cat sshd.txt
1 2 3
4 5 6
7 8 9

Loop to print the above text

Fixed format of for loop i=1 sets the initial variable of i i<=NF i variable is less than or equal to the value of NF variable (number of fields per row) i++ means i is incremented by +1,

[root@VM_0_84_centos ~]# cat sshd.txt |awk '{for(i=1;i<=NF;i++){print $i}}'
1
2
3
4
5
6
7
8
9

Summarize

The above is my introduction to using loops in awk. I hope it will be helpful to you. If you have any questions, please leave me a message and I will reply to you in time. I would also like to thank everyone for their support of the 123WORDPRESS.COM website!
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