How to remove duplicate lines inside a text file?

Assuming you can afford to keep as much as the de-duplicated file in memory (if your data is indeed duplicated by a factor of 100, that should be about 20MiB + overhead), you can do this very easily with Perl.

$ perl -ne 'print unless $dup{$_}++;' input_file > output_file

This preserves the order too.

You could extract the number of occurrences of each line from the %dup hash if you so wished, as an added free bonus.

If you prefer awk, this should do it too (same logic as the perl version, same ordering, same data gathered in the dup variable):

$ awk '{if (++dup[$0] == 1) print $0;}' input_file > output_file

There’s a simple (which is not to say obvious) method using standard utilities which doesn’t require a large memory except to run sort, which in most implementations has specific optimizations for huge files (a good external sort algorithm). An advantage of this method is that it only loops over all the lines inside special-purpose utilities, never inside interpreted languages.

<input nl -b a -s : |           # number the lines
sort -t : -k 2 -u |             # sort and uniquify ignoring the line numbers
sort -t : -k 1n |               # sort according to the line numbers
cut -d : -f 2- >output          # remove the line numbers

If all lines begin with a non-whitespace character, you can dispense with some of the options:

<input nl | sort -k 2 -u | sort -k 1n | cut -f 2- >output

For a large amount of duplication, a method that only requires storing a single copy of each line in memory will perform better. With some interpretation overhead, there’s a very concise awk script for that (already posted by enzotib):

<input awk '!seen[$0]++'

Less concisely: !seen[$0] {print} {seen[$0] += 1}, i.e. print the current line if it hasn’t been seen yet, then increment the seen counter for this line (uninitialized variables or array elements have the numerical value 0).

For long lines, you can save memory by keeping only a non-spoofable checksum (e.g. a cryptographic digest) of each line. For example, using SHA-1, you only need 20 bytes plus a constant overhead per line. But computing digests is rather slow; this method will only win if you have a fast CPU (especially one with a hardware accelerator to compute the digests) and not a lot of memory relative to the size of the file and sufficiently long lines. No basic utility lets you compute a checksum for each line; you’d have to bear the interpretation overhead of Perl/Python/Ruby/… or write a dedicated compiled program.

<input perl -MDigest::MD5 -ne '$seen{Digest::MD5::md5($_)}++ or print' >output

Python One liners :

python -c "import sys; lines = sys.stdin.readlines(); print ''.join(sorted(set(lines)))" < InputFile

With bash 4, a pure-bash solution that takes advantage of associative arrays can be used. Here is an example

unset llist; declare -A llist;
while read -r line; do
if [[ ${llist[$line]} ]]; then
  continue
else 
  printf '%sn' "$line"
  llist[$line]="x"
fi
done < file.txt

sort -u big-csv-file.csv > duplicates-removed.csv

Note that the output file will be sorted.

As no other answer provided inplace support, here is one:

gawk -i inplace '!a[$0]++' file

You can use uniq http://www.computerhope.com/unix/uuniq.htm

uniq reports or filters out repeated lines in a file.

None of the answers here worked for me on my Mac so I wrote a simple python script that works for me. I am ignoring leading/trailing whitespace and also don’t care about memory consumption.

import sys

inputfile = sys.argv[1]
outputfile = sys.argv[2]

with open(inputfile) as f:
    content = f.readlines()

content = [x.strip() for x in content]

my_list = list(set(content))

with open(outputfile, 'w') as output:
    for item in my_list:
        output.write("%sn" % item)

Save the above to unique.py and run like this:

python unique.py inputfile.txt outputfile.txt

SOLUTION WITHOUT MAINTAINING THE ORIGINAL SEQUENCE ORDER

I did it with the following code piece.

sort duplicates.txt | uniq > noDuplicates.txt

The sort command sorts the lines alphabetically, and the uniq command removes the duplicates.

NOTE: Why we sorted the lines first is that uniq does not detect duplicate lines unless they are adjacent.