awk field extraction in scripts in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does $1 represent in an awk command?

$1 represents the first field (or column) of the current line being processed by awk.

beginner

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

Use awk '{print $3}' filename to print the third field of each line.

beginner

What is the default field separator in awk?

The default field separator is any whitespace (spaces or tabs).

intermediate

How can you change the field separator to a comma in awk?

Use the -F option like this: awk -F',' '{print $2}' filename to split fields by commas.

intermediate

In a script, how do you store the second field of a line into a variable using awk?

You can use command substitution: var=$(awk '{print $2}' filename). This stores the second field output into the shell variable var.

What does awk '{print $1}' file.txt do?

A Prints the whole line

B Prints the last field of each line

C Prints the first field of each line in file.txt

D Deletes the first field

This command prints only the first field of each line in the file.

How do you tell awk to split fields by a colon :?

A awk -S':' '{print $2}' file

B awk -F':' '{print $2}' file

C awk --field=':' '{print $2}' file

D awk -D':' '{print $2}' file

The -F option sets the field separator to colon.

If a line is apple orange banana, what does $3 represent?

A banana

B orange

C apple

D The whole line

$3 is the third field, which is 'banana'.

Which command prints the second and fourth fields separated by a space?

A awk '{print $2+$4}' file

B awk '{print $2.$4}' file

C awk '{print $2-$4}' file

D awk '{print $2, $4}' file

Using a comma in print outputs fields separated by a space.

What is the output of awk -F',' '{print $1}' on the line red,green,blue?

A red

B green

C blue

D red,green,blue

With comma as separator, $1 is 'red'.

Explain how awk extracts fields from a line and how you can change the field separator.

Think about how words in a sentence are split by spaces or other characters.

Describe how to use awk inside a bash script to save a specific field from a file into a variable.

Remember how you capture command output in bash.

Practice

1. What does awk '{print $2}' file.txt do in a bash script?

easy

A. Prints the second word (field) from each line of file.txt

B. Prints the entire line from file.txt

C. Prints the first word (field) from each line of file.txt

D. Prints the number of lines in file.txt

Solution

1. Step 1: Understand awk field numbering

   Awk splits each line into fields separated by spaces by default. $1 is the first field, $2 is the second, and so on.

2. Step 2: Analyze the command

   The command prints $2, which is the second field of each line in file.txt.

3. Final Answer:

   Prints the second word (field) from each line of file.txt -> Option A

4. Quick Check:

   Awk $2 prints second field [OK]

Hint: Remember $1 is first field, $2 second, etc. [OK]

Common Mistakes:

• Confusing $2 with line number
• Thinking it prints entire line
• Mixing up field numbers starting at 0

2. Which of the following is the correct syntax to print the third field using awk with a comma as the field separator?

easy

A. awk -F ',' '{print $3}' file.csv

B. awk '{print $3 -F ','}' file.csv

C. awk '{print $3, -F ","}' file.csv

D. awk -F ';' '{print $3}' file.csv

Solution

1. Step 1: Identify field separator option

   The -F option sets the field separator. For comma-separated files, use -F ','.

2. Step 2: Check print syntax

   The correct print syntax is '{print $3}' to print the third field.

3. Final Answer:

   awk -F ',' '{print $3}' file.csv -> Option A

4. Quick Check:

   Use -F ',' before script block [OK]

Hint: Use -F 'separator' before the awk script block [OK]

Common Mistakes:

• Placing -F inside the print statement
• Using wrong separator like semicolon for CSV
• Misplacing quotes around separator

3. Given a file data.txt with the following content:

apple 10 red
banana 5 yellow
cherry 20 red

What is the output of awk '{print $1, $3}' data.txt?

medium

A. apple 10
banana 5
cherry 20

B. apple red
banana yellow
cherry red

C. 10 red
5 yellow
20 red

D. apple banana cherry

Solution

1. Step 1: Identify fields in each line

   Each line has three fields: $1=fruit, $2=number, $3=color.

2. Step 2: Print first and third fields

   The command prints $1 and $3 separated by space, so output lines are fruit and color pairs.

3. Final Answer:

   apple red banana yellow cherry red -> Option B

4. Quick Check:

   Print $1 and $3 fields [OK]

Hint: Match $1, $2, $3 to columns in data [OK]

Common Mistakes:

• Printing wrong fields like $2 instead of $3
• Confusing field order
• Expecting numbers instead of colors

4. What is wrong with this awk command to print the first field from a colon-separated file?
awk '{print $1}' -F ':' file.txt

medium

A. The colon separator is invalid in awk

B. The $1 should be $0 to print the whole line

C. The -F option should come before the script block

D. The file name should come before the script block

Solution

1. Step 1: Check placement of -F option

   The -F option must come before the script block in awk commands.

2. Step 2: Analyze the command structure

   Correct syntax is: awk -F ':' '{print $1}' file.txt

3. Final Answer:

   The -F option should come before the script block -> Option C

4. Quick Check:

   -F before script block [OK]

Hint: Put -F before the '{print ...}' block [OK]

Common Mistakes:

• Placing -F after the script block
• Using wrong field variable
• Misordering file name and script

5. You have a file records.txt with lines like:

John|Doe|30|Engineer
Jane|Smith|25|Designer
Bob|Brown|40|Manager

Write an awk command to print only the names (first and last) of people older than 28.

hard

A. awk -F '|' '$3 < 28 {print $1, $2}' records.txt

B. awk -F ',' '$3 > 28 {print $1, $2}' records.txt

C. awk '{if ($3 > 28) print $1, $2}' records.txt

D. awk -F '|' '$3 > 28 {print $1, $2}' records.txt

Solution

1. Step 1: Set correct field separator

   The file uses '|' as separator, so use -F '|' to split fields correctly.

2. Step 2: Filter by age and print names

   Use condition $3 > 28 to select lines where age is greater than 28, then print first and second fields (first and last names).

3. Final Answer:

   awk -F '|' '$3 > 28 {print $1, $2}' records.txt -> Option D

4. Quick Check:

   Filter $3 > 28 and print $1, $2 [OK]

Hint: Use -F '|' and condition $3 > 28 to filter [OK]

Common Mistakes:

• Using wrong field separator
• Using wrong comparison operator
• Not filtering before printing