Understanding `$0` in awk

In awk, $0 represents the entire current line being processed. Its behavior depends on how it’s used:

As a Pattern:

When used as a pattern (the condition in an awk script), $0 acts as a truthy/falsy test for whether the current line is non-empty.

• True (prints the line): If the line contains any text.
• False (skips the line): If the line is empty (contains only whitespace or nothing).

Example:

The command awk '$0' filename.txt effectively prints all non-empty lines from filename.txt. This is because the script implicitly prints the current line ($0) if the pattern $0 evaluates to true (i.e., the line is not empty).

Illustrative Example:

Let’s say filename.txt contains:

Hello

World

The output of awk '$0' filename.txt would be:

Hello
World

The blank line is omitted because $0 is false for that line.

Summary:

• $0 refers to the entire current line.
• As a pattern, $0 checks if the line is non-empty. If true, the line is typically printed (due to awk’s default action).
• awk '$0' provides a concise way to print only non-empty lines from a file.