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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (How to remove duplicate lines from files with awk)
+[#]: via: (https://opensource.com/article/19/10/remove-duplicate-lines-files-awk)
+[#]: author: (Lazarus Lazaridis https://opensource.com/users/iridakos)
+
+How to remove duplicate lines from files with awk
+======
+Learn how to use awk '!visited[$0]++' without sorting or changing their
+order.
+![Coding on a computer][1]
+
+Suppose you have a text file and you need to remove all of its duplicate lines.
+
+### TL;DR
+
+To remove the duplicate lines while _preserving their order in the file_, use:
+
+
+```
+`awk '!visited[$0]++' your_file > deduplicated_file`
+```
+
+### How it works
+
+The script keeps an associative array with _indices_ equal to the unique lines of the file and _values_ equal to their occurrences. For each line of the file, if the line occurrences are zero, then it increases them by one and _prints the line_, otherwise, it just increases the occurrences _without printing the line_.
+
+I was not familiar with **awk**, and I wanted to understand how this can be accomplished with such a short script (**awk**ward). I did my research, and here is what is going on:
+
+  * The awk "script" **!visited[$0]++** is executed for _each line_ of the input file.
+  * **visited[]** is a variable of type [associative array][2] (a.k.a. [Map][3]). We don't have to initialize it because **awk** will do it the first time we access it.
+  * The **$0** variable holds the contents of the line currently being processed.
+  * **visited[$0]** accesses the value stored in the map with a key equal to **$0** (the line being processed), a.k.a. the occurrences (which we set below).
+  * The **!** negates the occurrences' value:
+    * In awk, [any nonzero numeric value or any nonempty string value is true][4].
+    * By default, [variables are initialized to the empty string][5], which is zero if converted to a number.
+    * That being said:
+      * If **visited[$0]** returns a number greater than zero, this negation is resolved to **false**.
+      * If **visited[$0]** returns a number equal to zero or an empty string, this negation is resolved to **true**.
+  * The **++** operation increases the variable's value (**visited[$0]**) by one.
+    * If the value is empty, **awk** converts it to **0** (number) automatically and then it gets increased.
+    * **Note:** The operation is executed after we access the variable's value.
+
+
+
+Summing up, the whole expression evaluates to:
+
+  * **true** if the occurrences are zero/empty string
+  * **false** if the occurrences are greater than zero
+
+
+
+**awk** statements consist of a [_pattern-expression_ and an _associated action_][6].
+
+
+```
+`<pattern/expression> { <action> }`
+```
+
+If the pattern succeeds, then the associated action is executed. If we don't provide an action, **awk**, by default, **print**s the input.
+
+> An omitted action is equivalent to **{ print $0 }**.
+
+Our script consists of one **awk** statement with an expression, omitting the action. So this:
+
+
+```
+`awk '!visited[$0]++' your_file > deduplicated_file`
+```
+
+is equivalent to this:
+
+
+```
+`awk '!visited[$0]++ { print $0 }' your_file > deduplicated_file`
+```
+
+For every line of the file, if the expression succeeds, the line is printed to the output. Otherwise, the action is not executed, and nothing is printed.
+
+### Why not use the **uniq** command?
+
+The **uniq** command removes only the _adjacent duplicate lines_. Here's a demonstration:
+
+
+```
+$ cat test.txt
+A
+A
+A
+B
+B
+B
+A
+A
+C
+C
+C
+B
+B
+A
+$ uniq &lt; test.txt
+A
+B
+A
+C
+B
+A
+```
+
+### Other approaches
+
+#### Using the sort command
+
+We can also use the following [**sort**][7] command to remove the duplicate lines, but _the line order is not preserved_.
+
+
+```
+`sort -u your_file > sorted_deduplicated_file`
+```
+
+#### Using cat, sort, and cut
+
+The previous approach would produce a de-duplicated file whose lines would be sorted based on the contents. [Piping a bunch of commands][8] can overcome this issue:
+
+
+```
+`cat -n your_file | sort -uk2 | sort -nk1 | cut -f2-`
+```
+
+##### How it works
+
+Suppose we have the following file:
+
+
+```
+abc
+ghi
+abc
+def
+xyz
+def
+ghi
+klm
+```
+
+**cat -n test.txt** prepends the order number in each line.
+
+
+```
+1       abc
+2       ghi
+3       abc
+4       def
+5       xyz
+6       def
+7       ghi
+8       klm
+```
+
+**sort -uk2** sorts the lines based on the second column (**k2** option) and keeps only the first occurrence of the lines with the same second column value (**u** option).
+
+
+```
+1       abc
+4       def
+2       ghi
+8       klm
+5       xyz
+```
+
+**sort -nk1** sorts the lines based on their first column (**k1** option) treating the column as a number (**-n** option).
+
+
+```
+1       abc
+2       ghi
+4       def
+5       xyz
+8       klm
+```
+
+Finally, **cut -f2-** prints each line starting from the second column until its end (**-f2-** option: _Note the **-** suffix, which instructs it to include the rest of the line_).
+
+
+```
+abc
+ghi
+def
+xyz
+klm
+```
+
+### References
+
+  * [The GNU awk user's guide][9]
+  * [Arrays in awk][2]
+  * [Awk—Truth values][4]
+  * [Awk expressions][5]
+  * [How can I delete duplicate lines in a file in Unix?][10]
+  * [Remove duplicate lines without sorting [duplicate]][11]
+  * [How does awk '!a[$0]++' work?][12]
+
+
+
+That's all. Cat photo.
+
+![Duplicate cat][13]
+
+* * *
+
+_This article originally appeared on the iridakos blog by [Lazarus Lazaridis][14] under a [CC BY-NC 4.0 License][15] and is republished with the author's permission._
+
+--------------------------------------------------------------------------------
+
+via: https://opensource.com/article/19/10/remove-duplicate-lines-files-awk
+
+作者：[Lazarus Lazaridis][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://opensource.com/users/iridakos
+[b]: https://github.com/lujun9972
+[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/code_computer_laptop_hack_work.png?itok=aSpcWkcl (Coding on a computer)
+[2]: http://kirste.userpage.fu-berlin.de/chemnet/use/info/gawk/gawk_12.html
+[3]: https://en.wikipedia.org/wiki/Associative_array
+[4]: https://www.gnu.org/software/gawk/manual/html_node/Truth-Values.html
+[5]: https://ftp.gnu.org/old-gnu/Manuals/gawk-3.0.3/html_chapter/gawk_8.html
+[6]: http://kirste.userpage.fu-berlin.de/chemnet/use/info/gawk/gawk_9.html
+[7]: http://man7.org/linux/man-pages/man1/sort.1.html
+[8]: https://stackoverflow.com/a/20639730/2292448
+[9]: https://www.gnu.org/software/gawk/manual/html_node/
+[10]: https://stackoverflow.com/questions/1444406/how-can-i-delete-duplicate-lines-in-a-file-in-unix
+[11]: https://stackoverflow.com/questions/11532157/remove-duplicate-lines-without-sorting
+[12]: https://unix.stackexchange.com/questions/159695/how-does-awk-a0-work/159734#159734
+[13]: https://opensource.com/sites/default/files/uploads/duplicate-cat.jpg (Duplicate cat)
+[14]: https://iridakos.com/about/
+[15]: http://creativecommons.org/licenses/by-nc/4.0/