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How Can I Eliminate Duplicates in a Linux File?

The uniq command reads the input file and compares adjacent lines. Any line that is the same as the one before it will be discarded. In other words, duplicates are discarded, leaving only the unique lines in the file. Let's say you're a publisher with an inventory of all your books in the my.books file shown here:

Atopic Dermatitis for Dummies
Atopic Dermatitis for Dummies
Chronic Rhinitis Unleashed
Chronic Rhinitis Unleashed
Chronic Rhinitis Unleashed
Learn Nasal Endoscopy in 21 Days

To remove all the duplicates from the list of books, use this command:

uniq my.books
Atopic Dermatitis for Dummies
Chronic Rhinitis Unleashed
Learn Nasal Endoscopy in 21 Days

If you want to print only the book titles that are not duplicated (to find out which books you have one copy of), add the -u flag, like this:

uniq -u my.books
Learn Nasal Endoscopy in 21 Days

Conversely, you might want to exclude the titles that appear only once. If so, add the -d flag, like this:

uniq -d my.books
Atopic Dermatitis for Dummies
Chronic Rhinitis Unleashed

Now let's take inventory. To summarize the list of books and add a count of the number of times each one appears in the list, add the -c flag, like this:

uniq -c my.books
2 Atopic Dermatitis for Dummies
3 Chronic Rhinitis Unleashed
1 Learn Nasal Endoscopy in 21 Days

Note that the uniq command does not sort the input file, so you may want to use the sort command to prepare the data for uniq in advance. (See the end of this section for an example.)

Here's a recap of the flags you can use with the uniq command:

-u Print only lines that appear once in the input file.

-d Print only the lines that appear more than once in the input file.
Precede each output line with a count of the number of times it was found.

Previous Lesson: Sorting Data
Next Lesson: Selecting Columns



Comments - most recent first
(Please feel free to answer questions posted by others!)

Samit     (07 Nov 2014, 09:39)
Hi Bob,

I need to eliminates duplicates records in the file based on there date for e.g. see below file
so from below file i need to keep the record with uniq value from coloumn 1 and which are occured recently i.e by latest date

what command should be best suited

112 absdsds 2014:03:01 03:49
112 absdsds 2014:03:01 03:50
113 absdsds 2014:03:01 03:32
113 absdsds 2014:03:01 03:32
112 absdsds 2014:03:01 03:49
112 absdsds 2014:06:01 03:49
112 absdsds 2014:04:01 03:49
AdelaideSEO     (02 Apr 2013, 08:27)
Hey thanks for the Tut.
Is it possible or how do I deduplicate a list of items in a .txt file without sorting them - they are already in the correct order but I know there are duplicates. cheers Steve

Andrew     (02 Jan 2013, 01:18)
Hi, I admire and love the content of this tutorial. But did you change your site a little bit? I cannot reach or read your articles on the new style with some water in the background, just the links, but the main article stays invisible.
MikeQ     (14 Nov 2012, 19:32)
This is nice and all but what if you don't want to sort the data because it messes things up ? why can't I use uniq with just cat or something.?
Tevera     (15 Feb 2012, 06:57)
simple and gud
Saif     (06 Jan 2012, 18:51)
Short & Sweet tutorial.. Thanks Author..
beterlatethannotatall     (12 Dec 2011, 14:38)
Nisa your guess/assumption is correct. It eliminates dupes that are consecutive. I.e. 111223334444111555 would be 123415, sorting first does fix that.

Biko all of these cmds work on stdin and stdout unless directed to do otherwise. So when you use a file as input, stdout is still where the modified data will end up unless you redirect stdout to the file to save the output. I.e. The sample data above in a file nums.

$ echo '111223334444111555' > nums
$ cat nums | sort | uniq > nums

see no output between prompts is what you should get. the > nums is the redirect for stdout, putting the output into nums file for saving.

$ cat nums

This is what should be in the nums file now.
Rockin     (31 May 2011, 00:27)
awesome !
fiyas     (16 Mar 2011, 01:11)
Superb Tutorial.... Thanks
Nisa     (22 Sep 2010, 07:11)
@Biko It is said that only consecutive lines will be tested for duplicity. You will have to sort the file first so that the duplicate lines are adjacent. Don't forget to save the output of the sorting to the same file
biko     (29 Jul 2010, 09:23)
it just brings the list out but doesnt merge o eliminate the duplicates and wen i use the -c flag it just counts them all as 1.......
Bob Rankin     (28 Jul 2010, 08:55)
@biko - What response do you get? I sure hope it's not "uniq is not recognized as an internal or external command, operable program or batch file." :-)
biko     (28 Jul 2010, 07:46)
hello i tried the uniq command and it did not work alone or with either of the flags why?
umar ayaz     (27 Jun 2010, 15:58)
Excellent simple tutorial about cut
Dennis     (16 Mar 2010, 09:21)
evrica! it works grate with piping :))
Dennis     (16 Mar 2010, 08:49)
does uniq work for consecutive lines only?

(that means we need to sort first, isn't ?)

Bob Rankin     (05 Mar 2010, 09:36)
Did you sort first?
ramya     (05 Mar 2010, 06:15)
i cannot remove the duplicate entries using uniq command why?

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