Difference between revisions of "Linux-AWK/C2/Loops-in-awk/English"

• while
• do-while
• for
• and more looping constructs in awk

We will do this through some examples.

• Ubuntu Linux 16.04 OS and
• gedit text editor 3.20.1

• You can use any text editor of your choice.

• You should have familiarity with any programming language like C or C++
• If not, then please go through the corresponding tutorials on our website.

Please download and extract them.

while, do-while and for are the available loops in awk.

While loop first checks whether the specified condition is true.

If yes, then it executes the code within the body.

This loop will be repeated as long as the specified while condition is true.

The same is available in the Code Files link of this tutorial.

Variable f represents the field counter or the position of the fields for each record.

Which means the first 3 fields will get printed for each record.

awk -f while_loop.awk awkdemo.txt

[Enter]

awk space minus small f while_loop.awk space awkdemo.txt

Press Enter

The do-while loop always executes the code inside the body once.

Then it checks the specified condition.

And repeats the code inside the body, as long as the specified condition is true.

The same is available in the Code Files link.

This is the condition that will be checked.

After that, the statements inside the loop will be executed repeatedly as long as the condition is true.

Which means the first 3 fields will get printed for all the records.

awk -f do_loop.awk awkdemo.txt

[Enter]

Now type:

awk space -f do_loop.awk awkdemo.txt

and press Enter

Then why do we have both while and do-while loops?

Let us understand the difference.

[code uploaded as while_loop_mod.awk]

Now, change the value of loop counter i from 1 to 4.

(i<=3)

So this means, we should not get any output.

[Enter]

Now press Enter.

For each record in the awkdemo.txt file, blank lines are getting printed in the output.

[code uploaded as do_loop_mod.awk]

Change the value of i from 1 to 4

Save the file and switch to the terminal.

[Enter]

Now press Enter.

What is the reason?

Then the condition is checked.

Since the value of the loop counter i is 4, the condition is false.

Hence, the loop is terminated there only, for that record.

Which means the first field for each record will get printed.

Use the do-while loop, for a job to be executed at least once irrespective of any other condition.

The for statement starts by executing initialization.

Then, as long as the condition is true, it repeatedly executes the statements within and then increments. 

Assuming your familiarity with a language like C or C++, I am not explaining the syntax in detail.

Here, initialization, condition checking and variable incrementation are done in the same line.

Try this out by yourself.

• break
• continue
• exit

We will see some relevant examples on these in further tutorials.

Here notice that the single line comments are declared with single hash (#) symbol.

The multiline comments are declared with the help of double hash (##) symbol.

We have to skip the lines starting with hash (##) symbol.

How can we do this?

We will use the same example for skipping the comments.

If the pattern is found, the keyword next tells awk to skip the current line immediately.

Then awk will start processing from the next line in the file.

This will save the processing time.

awk -F "|" -f next.awk awkdemo_mod.txt

[Enter]

press Enter.

Say in awkdemo_mod.txt and awkdemo2.txt

It also has comments preceeded with hash # sysmbol.

And it has large text at the end with double hash ## symbol.

So, awk should process both the files to give an increment to all the students.

Once we reach double hash(##) symbol of the first file,

awk should stop processing that file entirely.

Then it has to start the execution from the next file.

This will save the processing time.

Highlight $0~/^#/ {nextfile;}

I have added dollar zero tilde slash caret symbol double hash slash within curly brackets nextfile semicolon below the begin statement.

This will search for double hash # symbol at the beginning of a line.

If found, awk will skip the current file to process the next file.

awk -F "|" -f next.awk awkdemo_mod.txt awkdemo2.txt

[Enter]

awk space -F "|" space -f space next.awk space awkdemo_mod.txt space awkdemo2.txt

Press Enter

Let us summarize.

In this tutorial we learnt about-

• while
• do… while
• for
• next
• nextfile in awk

• print only even fields(i.e. field 2, field 4 etc.
• irrespective of how many fields are there in the input file.

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