Linux awk Command Questions & Answers

 Top 20 Linux awk Command Questions & Answers

Question 1: Basic Field Printing

Question: What does the command awk '{print $1, $3}' file.txt do?

 * A) Prints the 1st and 3rd lines of the file

 * B) Prints the 1st and 3rd columns (fields) separated by space

 * C) Prints character 1 and character 3 of every line

 * D) Replaces column 1 with column 3

   Answer: B

   Explanation: By default, awk splits lines by whitespace and maps each segment to $1, $2, $3, etc. Comma separation in print outputs the Output Field Separator (default is space).

Question 2: Entire Record Selector

Question: Which built-in variable represents the entire current input line or record?

 * A) $NF

 * B) $0

 * C) $1

 * D) $*

   Answer: B

   Explanation: $0 holds the full line/record, whereas $1, $2, ... hold individual fields.

Question 3: Dynamic Last Field

Question: How do you print the last field of every row, regardless of how many columns are present?

 * A) awk '{print $LAST}' file.txt

 * B) awk '{print $(NF-1)}' file.txt

 * C) awk '{print $NF}' file.txt

 * D) awk '{print $(END)}' file.txt

   Answer: C

   Explanation: NF holds the number of fields in the current record. Referencing $ before NF ($NF) resolves to the value of the last field.

Question 4: Field Delimiter Flag

Question: Which command-line option changes the input field delimiter to a colon (:)?

 * A) awk -d: '{print $1}' /etc/passwd

 * B) awk -F: '{print $1}' /etc/passwd

 * C) awk -s: '{print $1}' /etc/passwd

 * D) awk -t: '{print $1}' /etc/passwd

   Answer: B

   Explanation: -F sets the Input Field Separator (FS). -F: tells awk to treat : as the column delimiter.

Question 5: Total Records vs File Records

Question: What is the primary difference between NR and FNR?

 * A) NR counts fields; FNR counts rows

 * B) NR is cumulative line count across all files; FNR resets to 1 for each new file

 * C) FNR counts characters; NR counts lines

 * D) There is no difference; they are aliases

   Answer: B

   Explanation: NR (Number of Records) tracks total lines processed across the entire run, while FNR (File Number of Records) tracks the line number relative to the file currently being processed.

Question 6: Output Delimiter Modification

Question: Given awk -F: 'BEGIN{OFS=" - "} {print $1, $7}' /etc/passwd, what does OFS do?

 * A) Omits the first separator

 * B) Sets the Output Field Separator between fields to " - "

 * C) Filters out matching fields

 * D) Specifies an output file stream

   Answer: B

   Explanation: OFS controls how fields printed with commas (like $1, $7) are joined in the output.

Question 7: Pre-Processing Blocks

Question: When does the BEGIN block execute?

 * A) Once for each line read

 * B) Before any input files or records are read

 * C) Only when an error occurs

 * D) After the last line is processed

   Answer: B

   Explanation: BEGIN { ... } runs once before any input stream processing begins, commonly used to initialize variables or headers.

Question 8: Summary Calculations

Question: How can you print the total sum of column 2 after reading an entire file?

 * A) awk '{sum += $2} END {print sum}' file.txt

 * B) awk 'BEGIN {sum += $2} {print sum}' file.txt

 * C) awk '{print sum($2)}' file.txt

 * D) awk 'END {sum += $2; print sum}' file.txt

   Answer: A

   Explanation: The main body {sum += $2} accumulates the sum for every line, and the END block runs once after reading the entire file to display the final result.

Question 9: Regular Expression Matching

Question: Which operator tests if field 1 contains the pattern error (case-sensitive)?

 * A) $1 == /error/

 * B) $1 ~ /error/

 * C) $1 =~ /error/

 * D) $1 matches "error"

   Answer: B

   Explanation: ~ is the regular expression match operator in awk. !~ tests for non-matching.

Question 10: Filtering Specific Line Ranges

Question: What does awk 'NR >= 5 && NR <= 10' file.txt do?

 * A) Prints lines that have between 5 and 10 columns

 * B) Prints lines 5 through 10 of the file

 * C) Deletes lines 5 to 10

 * D) Skips lines 5 through 10

   Answer: B

   Explanation: NR evaluates line numbers. Since no action {...} is provided, awk applies the default action: print the matching record.

Question 11: Emulating wc -l

Question: Which one-liner prints the total line count of a file, equivalent to wc -l?

 * A) awk '{count++}' file.txt

 * B) awk 'END {print NR}' file.txt

 * C) awk 'BEGIN {print NR}' file.txt

 * D) awk '{print NF}' file.txt

   Answer: B

   Explanation: At the end of processing, NR holds the total count of records processed.

Question 12: Column Value Filtering

Question: How do you print lines where the 3rd field is numeric and greater than 50?

 * A) awk '$3 > 50 {print $0}' file.txt

 * B) awk '{if ($3 > 50)}' file.txt

 * C) awk 'WHERE $3 > 50' file.txt

 * D) awk 'FILTER($3, 50)' file.txt

   Answer: A

   Explanation: Conditions outside braces act as pattern filters: $3 > 50 selects records where field 3 exceeds 50, triggering the {print $0} action (or default print).

Question 13: In-Place Substitution

Question: What does the built-in function sub(/foo/, "bar", $0) do?

 * A) Replaces all occurrences of foo with bar in the line

 * B) Replaces only the first occurrence of foo with bar in the line

 * C) Splits $0 at every foo

 * D) Reverses foo into bar across all files

   Answer: B

   Explanation: sub() replaces only the first match. To replace all occurrences globally, use gsub().

Question 14: String Length Calculation

Question: Which command outputs only records whose overall line length exceeds 80 characters?

 * A) awk 'size($0) > 80' file.txt

 * B) awk 'length($0) > 80' file.txt

 * C) awk 'chars($0) > 80' file.txt

 * D) awk 'strlen > 80' file.txt

   Answer: B

   Explanation: length() calculates the string length in characters (or defaults to $0 if no argument is passed).

Question 15: Removing Duplicate Consecutive/Non-consecutive Lines

Question: What classic idiom prints unique lines while preserving original order?

 * A) awk '!seen[$0]++' file.txt

 * B) awk 'unique($0)' file.txt

 * C) awk 'distinct {print $0}' file.txt

 * D) awk '{dedup($0)}' file.txt

   Answer: A

   Explanation: Associative array seen tracks lines. For the first occurrence of a line, seen[$0] evaluates to 0 (false), which ! flips to true, executing the default print action; the post-increment ++ sets it to 1, causing future occurrences to evaluate to false.

Question 16: Custom Record Separator

Question: By default, what is the Record Separator (RS) set to?

 * A) Space

 * B) Tab (\t)

 * C) Newline (\n)

 * D) Comma (,)

   Answer: C

   Explanation: RS defaults to a newline character (\n), meaning each line is treated as an independent record.

Question 17: String Splitting

Question: What does split("user@domain.com", arr, "@") produce?

 * A) arr[0]="user", arr[1]="domain.com"

 * B) arr[1]="user", arr[2]="domain.com"

 * C) A syntax error because arrays cannot hold strings

 * D) arr["@"]="user domain.com"

   Answer: B

   Explanation: In awk, array indexes created by functions like split() start at 1, not 0.

Question 18: Line Inversion / Pattern Inversion

Question: How do you print all lines that do not contain the word DEBUG?

 * A) awk '!/DEBUG/' file.txt

 * B) awk 'NOT /DEBUG/' file.txt

 * C) awk '/^DEBUG/' file.txt

 * D) awk 'drop(/DEBUG/)' file.txt

   Answer: A

   Explanation: The logical NOT operator ! negates pattern matches; !/pattern/ selects lines that do not match.

Question 19: External Shell Variables

Question: What is the recommended flag to pass an external bash/environment variable val="abc" into an awk variable x?

 * A) awk -e x="$val" '{print x}' file.txt

 * B) awk -v x="$val" '{print x}' file.txt

 * C) awk --set x="$val" '{print x}' file.txt

 * D) awk -var x="$val" '{print x}' file.txt

   Answer: B

   Explanation: The -v flag assigns a value to an awk variable before execution starts.

Question 20: Built-in printf Formatting

Question: What does awk '{printf "%-15s %5.2f\n", $1, $2}' data.txt do?

 * A) Prints field 1 centered and field 2 in hexadecimal

 * B) Left-aligns field 1 within 15 characters and formats field 2 as a floating point number with 2 decimal places

 * C) Truncates field 1 to 15 chars and divides field 2 by 5.2

 * D) Throws a syntax error because printf does not accept format specifiers in awk

   Answer: B

   Explanation: awk supports standard C-style printf formatting: %-15s means a left-justified string of width 15, and %5.2f specifies a float with minimum width 5 and 2 decimal points.

Quick Answer Key

| # | Answer | Key Topic |

|---|---|---|

| 1 | B | Field extraction & default IFS |

| 2 | B | $0 (current record) |

| 3 | C | $NF (dynamic last column) |

| 4 | B | -F delimiter option |

| 5 | B | NR vs FNR distinction |

| 6 | B | OFS (Output Field Separator) |

| 7 | B | BEGIN block lifecycle |

| 8 | A | END block summation |

| 9 | B | ~ regex matching operator |

| 10 | B | NR line range selection |

| 11 | B | END {print NR} line count |

| 12 | A | Numeric field condition |

| 13 | B | sub() vs gsub() |

| 14 | B | length() string function |

| 15 | A | !seen[$0]++ deduplication |

| 16 | C

 | Default RS (\n) |

| 17 | B | 1-based indexing in split() |

| 18 | A | !/regex/ pattern negation |

| 19 | B | -v argument passing |

| 20 | B | printf string & float formatting |