04. Patterns and Actions print
In the previous lesson, we looked at records & fields, and saw how AWK is able to parse and manipulate them. While learning this, you may have noticed that each action performed is enclosed within braces, which then applies to all records.
Patterns
But what if we just wanted to apply actions to lines that matched a specific pattern? We can do this by preceding actions by a regular expression pattern.
/pattern/ { action } // Action is applied only those records that match pattern
/pattern/ // Print all lines matching /pattern/
{ action } // Apply actions on all linesThis allows us to select which lines to apply our actions to. If you do not specify a pattern, then the action is applied to all lines. On the other hand, if there is no action, then all lines with the specified pattern are printed out.
Actions
Actions tell AWK how to process a specific record or part of its fields. Let's look at the print action, as it's the most basic thing you can do with a record.
Printing
When print is called, it will print out the record with an output record separator (ORS), the default of which is a newline character. In the following example, all record will be printed. We have already seen how we can specify the entire record (with $0) and the specific field n ($n) with the dollar symbol.
$ echo ' uno dos tres ' | gawk -F' ' '{ print $0 }'
uno dos tres
# Default is to print the record
$ echo ' uno dos tres ' | gawk -F' ' '{ print }'
uno dos tres
# Print a specific field only
$ echo ' uno dos tres ' | gawk -F' ' '{ print $2 }'
dosPrinting by pattern
Now we can follow a certain pattern and print only those that match. For this example, we'll use grades.txt, which is a file containing grade reports of five students.
$ gawk '$6 ~ /B/ { print $0 }' grades.txt
Vern Wynne 85 78 93 B
Ingram Dannie 84 85 94 B+
Johnnie Adair 78 94 87 B
# Print last name, first name for students with a B
$ awk '/B/ {print $2 "\t" $1}' grades.txt
Wynne Vern
Dannie Ingram
Adair JohnnieHere, we can use the ~ to select those that match field #6.
That's all for now...
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