Dealing with duplicates

Often, you need to eliminate duplicates from an input file. This could be based on the entire line content or based on certain fields. These are typically solved with the sort and uniq commands. Advantages with awk include regexp based field and record separators, input doesn't have to be sorted, and in general more flexibility because it is a programming language.

The example_files directory has all the files used in the examples.

Whole line duplicates

awk '!a[$0]++' is one of the most famous awk one-liners. It eliminates line based duplicates while retaining the input order. The following example shows it in action along with an illustration of how the logic works.

$ cat purchases.txt
coffee
tea
washing powder
coffee
toothpaste
tea
soap
tea

$ awk '{print +a[$0] "\t" $0; a[$0]++}' purchases.txt
0       coffee
0       tea
0       washing powder
1       coffee
0       toothpaste
1       tea
0       soap
2       tea

# only those entries with zero in first column will be retained
$ awk '!a[$0]++' purchases.txt
coffee
tea
washing powder
toothpaste
soap

See also huniq, a faster alternative for removing line based duplicates.

Column wise duplicates

Removing field based duplicates is simple for a single field comparison. Just change $0 to the required field number after setting the appropriate field separator.

$ cat duplicates.txt
brown,toy,bread,42
dark red,ruby,rose,111
blue,ruby,water,333
dark red,sky,rose,555
yellow,toy,flower,333
white,sky,bread,111
light red,purse,rose,333

# based on the last field
$ awk -F, '!seen[$NF]++' duplicates.txt
brown,toy,bread,42
dark red,ruby,rose,111
blue,ruby,water,333
dark red,sky,rose,555

For multiple fields comparison, separate the fields with , so that SUBSEP is used to combine the field values to generate the key. As mentioned before, SUBSEP has a default value of \034 non-printing character, which is typically not used in text files.

# based on the first and third fields
$ awk -F, '!seen[$1,$3]++' duplicates.txt
brown,toy,bread,42
dark red,ruby,rose,111
blue,ruby,water,333
yellow,toy,flower,333
white,sky,bread,111
light red,purse,rose,333

Duplicate count

In this section, how many times a duplicate record is found plays a role in determining the output.

First up, printing only a specific numbered duplicate.

# print only the second occurrence of duplicates based on the second field
$ awk -F, '++seen[$2]==2' duplicates.txt
blue,ruby,water,333
yellow,toy,flower,333
white,sky,bread,111

# print only the third occurrence of duplicates based on the last field
$ awk -F, '++seen[$NF]==3' duplicates.txt
light red,purse,rose,333

Next, printing only the last copy of duplicates. Since the count isn't known, the tac command comes in handy again.

# reverse the input line-wise, retain first copy and then reverse again
$ tac duplicates.txt | awk -F, '!seen[$NF]++' | tac
brown,toy,bread,42
dark red,sky,rose,555
white,sky,bread,111
light red,purse,rose,333

To get all the records based on a duplicate count, you can pass the input file twice. Then use the two file processing trick to make decisions.

# all duplicates based on the last column
$ awk -F, 'NR==FNR{a[$NF]++; next} a[$NF]>1' duplicates.txt duplicates.txt
dark red,ruby,rose,111
blue,ruby,water,333
yellow,toy,flower,333
white,sky,bread,111
light red,purse,rose,333

# all duplicates based on the last column, minimum 3 duplicates
$ awk -F, 'NR==FNR{a[$NF]++; next} a[$NF]>2' duplicates.txt duplicates.txt
blue,ruby,water,333
yellow,toy,flower,333
light red,purse,rose,333

# only unique lines based on the third column
$ awk -F, 'NR==FNR{a[$3]++; next} a[$3]==1' duplicates.txt duplicates.txt
blue,ruby,water,333
yellow,toy,flower,333

Summary

This chapter showed how to work with duplicate contents for records and fields. If you don't need regexp based separators and if your input is too big to handle, then specialized command line tools like sort and uniq will be better suited compared to awk.

Next chapter will show how to write awk scripts instead of the usual one-liners.

Exercises

The exercises directory has all the files used in this section.

1) Retain only the first copy of a line for the input file lines.txt. Case should be ignored while comparing the lines. For example, hi there and HI TheRE should be considered as duplicates.

$ cat lines.txt
Go There
come on
go there
---
2 apples and 5 mangoes
come on!
---
2 Apples
COME ON

$ awk ##### add your solution here
Go There
come on
---
2 apples and 5 mangoes
come on!
2 Apples

2) Retain only the first copy of a line for the input file twos.txt. Assume space as the field separator with exactly two fields per line. Compare the lines irrespective of the order of the fields. For example, hehe haha and haha hehe should be considered as duplicates.

$ cat twos.txt
hehe haha
door floor
haha hehe
6;8 3-4
true blue
hehe bebe
floor door
3-4 6;8
tru eblue
haha hehe

$ awk ##### add your solution here
hehe haha
door floor
6;8 3-4
true blue
hehe bebe
tru eblue

3) For the input file twos.txt, create a file uniq.txt with all the unique lines and dupl.txt with all the duplicate lines. Assume space as the field separator with exactly two fields per line. Compare the lines irrespective of the order of the fields. For example, hehe haha and haha hehe should be considered as duplicates.

$ awk ##### add your solution here

$ cat uniq.txt 
true blue
hehe bebe
tru eblue

$ cat dupl.txt 
hehe haha
door floor
haha hehe
6;8 3-4
floor door
3-4 6;8
haha hehe