awk patterns and actions in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic structure of an awk command using patterns and actions?

An awk command has the form: awk 'pattern { action }' filename. The pattern selects lines, and the action tells awk what to do with those lines.

beginner

In awk, what happens if you omit the pattern but provide an action?

If the pattern is omitted, awk applies the action to every line of the input file.

beginner

What does the pattern /error/ mean in awk?

The pattern /error/ matches any line that contains the word 'error'. It uses regular expression matching.

beginner

How do you print the first field of each line using awk?

Use the action { print $1 }. For example: awk '{ print $1 }' filename prints the first word of each line.

intermediate

What does the pattern-action pair NR == 3 { print } do in awk?

It prints only the third line of the input. NR is the current line number, so when NR == 3, the action print runs.

What does this awk command do? awk '/fail/ { print $2 }' file.txt

A Prints all lines containing 'fail'

B Prints the second word of lines containing 'fail'

C Prints the second line of the file

D Prints the entire file

The pattern '/fail/' selects lines with 'fail'. The action '{ print $2 }' prints the second word of those lines.

What happens if you run awk '{ print $0 }' file.txt?

A Prints lines containing 'print'

B Prints only the first word of each line

C Prints nothing

D Prints the entire file line by line

$0 means the whole line. Without a pattern, awk prints every line.

Which pattern matches lines where the first field equals 'John'?

A NR == John

B /John/

C $1 == "John"

D John == $0

The pattern '$1 == "John"' matches lines where the first word is exactly 'John'.

What does this command do? awk 'NR > 5' file.txt

A Prints lines after the fifth line

B Prints the first five lines

C Prints lines containing '5'

D Prints nothing

NR > 5 is a pattern that matches lines after line 5. The default action is to print those lines.

How do you print the number of fields in each line using awk?

A { print NF }

B { print NR }

C { print $NF }

D { print $0 }

NF is a built-in variable holding the number of fields in the current line.

Explain how awk uses patterns and actions to process text files.

Think about how you pick certain lines and then decide what to do with them.

Describe how to print the third field of lines containing the word 'error' using awk.

Combine a pattern that finds 'error' and an action that prints a specific field.