How to Filter Command Output Using Awk and STDIN

In the earlier segments of the Awk command series, our focus was primarily on reading input from files. However, what if you need to read input from STDIN?

In Part 7 of the Awk series, we will explore several examples where you can use the output of other commands as input, rather than reading from a file.

List Files Owned by User in Directory

Let's begin with the dir command, which functions similarly to the ls command. In our initial example, we utilize the output of the dir -l command as input for awk to display the owner's username, group name, and the files they own in the current directory:

<code>dir -l | awk '{print $3, $4, $9;}'</code>

List Files Owned by Root User

Consider another example where we use awk expressions. Here, we aim to list files owned by the root user by filtering strings with the awk command below:

<code>dir -l | awk '$3=="root" {print $1,$3,$4, $9;} '</code>

The command above employs the (==) comparison operator to filter out files in the current directory owned by the root user, using the expression $3==”root”.

Use Awk Comparison Operator to Match String

Let's examine another example where we use an awk comparison operator to match a specific string.

We used the cat command to view the contents of a file named tecmint_deals.txt and wanted to see only deals of type Tech, so we executed the following commands:

<code>cat tecmint_deals.txt
cat tecmint_deals.txt | awk '$4 ~ /tech/{print}'
cat tecmint_deals.txt | awk '$4 ~ /Tech/{print}'</code>

In the above example, we used the ~ /pattern/ comparison operator. Note that there are two commands to highlight an important point.

When running the command with the pattern tech, nothing is displayed because there are no deals of that type, but with Tech, you see deals of type Tech.

Therefore, be cautious when using this comparison operator, as it is case-sensitive as demonstrated above.

You can always use the output of another command as input for awk instead of reading from a file, which is straightforward as shown in the examples above.

Filter Files Modified Today

To filter files modified today, you can use awk to extract and display these files:

<code>ls -l | awk '$6 == "Aug" && $7 == "9" {print $9}'</code>

This command filters files modified on August 9.

Count Files of a Specific Type

To count the number of .txt files in a directory, use:

<code>ls -l | awk '$9 ~ /\.txt$/ {count  } END {print count}'</code>

Extract Specific Fields from Command Output

If you want to extract specific fields from a ps command output, such as the process ID and command name:

<code>ps aux | awk '{print $1, $2, $11}'</code>

I hope these examples were clear and helpful for you. If you have any questions, feel free to leave them in the comment section below, and don't forget to check out the next part of the series, where we will delve into awk features like variables, numeric expressions, and assignment operators.

For those looking for a comprehensive resource, we've compiled all the Awk series articles into a book that includes 13 chapters and spans 41 pages, covering both basic and advanced Awk usage with practical examples.

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