Remove duplicate lines without sorting

I have a utility script in Python:

#!/usr/bin/env python
import sys
unique_lines = []
duplicate_lines = []
for line in sys.stdin:
  if line in unique_lines:
    duplicate_lines.append(line)
  else:
    unique_lines.append(line)
    sys.stdout.write(line)
# optionally do something with duplicate_lines

This simple functionality (uniq without needing to sort first, stable ordering) must be available as a simple UNIX utility, mustn't it? Maybe a combination of filters in a pipe?

Reason for asking: needing this functionality on a system on which I cannot execute Python from anywhere.

The UNIX Bash Scripting blog suggests:

awk '!x[$0]++'

This command is telling awk which lines to print. The variable $0 holds the entire contents of a line and square brackets are array access. So, for each line of the file, the node of the array x is incremented and the line printed if the content of that node was not (!) previously set.

A late answer - I just ran into a duplicate of this - but perhaps worth adding...

The principle behind @1_CR's answer can be written more concisely, using cat -n instead of awk to add line numbers:

cat -n file_name | sort -uk2 | sort -n | cut -f2-

• Use cat -n to prepend line numbers
• Use sort -u remove duplicate data (-k2 says 'start at field 2 for sort key')
• Use sort -n to sort by prepended number
• Use cut to remove the line numbering (-f2- says 'select field 2 till end')

To remove duplicate from 2 files :

awk '!a[$0]++' file1.csv file2.csv

Michael Hoffman's solution above is short and sweet. For larger files, a Schwartzian transform approach involving the addition of an index field using awk followed by multiple rounds of sort and uniq involves less memory overhead. The following snippet works in bash

awk '{print(NR"\t"$0)}' file_name | sort -t$'\t' -k2,2 | uniq --skip-fields 1 | sort -k1,1 -t$'\t' | cut -f2 -d$'\t'

Now you can check out this small tool written in Rust: uq.

It performs uniqueness filtering without having to sort the input first, therefore can apply on continuous stream.

There are two advantages of this tool over the top-voted awk solution and other shell-based solutions:

1. uq remembers the occurence of lines using their hash values, so it doesn't use as much memory use when the lines are long.
2. uq can keep the memory usage constant by setting a limit on the number of entries to store (when the limit is reached, there is a flag to control either to override or to die), while the awk solution could run into OOM when there are too many lines.

Thanks 1_CR! I needed a "uniq -u" (remove duplicates entirely) rather than uniq (leave 1 copy of duplicates). The awk and perl solutions can't really be modified to do this, your's can! I may have also needed the lower memory use since I will be uniq'ing like 100,000,000 lines 8-). Just in case anyone else needs it, I just put a "-u" in the uniq portion of the command:

awk '{print(NR"\t"$0)}' file_name | sort -t$'\t' -k2,2 | uniq -u --skip-fields 1 | sort -k1,1 -t$'\t' | cut -f2 -d$'\t'

I just wanted to remove all duplicates on following lines, not everywhere in the file. So I used:

awk '{
  if ($0 != PREVLINE) print $0;
  PREVLINE=$0;
}'

the uniq command works in an alias even http://man7.org/linux/man-pages/man1/uniq.1.html