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Counting unique lines in a file is a common task in Linux, and there are a number of different tools and methods that can be used to perform this operation. In general, the appropriate method depends on the specific requirements and constraints of the task, such as the size of the input file, performance and memory requirements, and the format and content of the data.

Count the number of unique lines in a file using the sort and uniq commands

One way to count the number of unique lines in a file in Linux is to use the sort and uniq commands. The sort command sorts the input data into a specified order, and the uniq command filters out duplicate lines from the sorted data.

The data.txt file contains the following content for this article's examples.

arg1
arg2
arg3
arg2
arg2
arg1

To count the number of unique lines in a file, you can use the following command:

sort data.txt | uniq -c | wc -l

Output:

3

This command sorts the data.txt file in ascending order (by default) and pipes the output to the uniq command. The uniq command filters out any duplicate lines from the sorted data and adds a count of the number of times each line appears in the input.

The output is then piped to the wc command, which counts the number of lines in the input and prints the results to the terminal.

The sort and uniq commands are simple and efficient tools for counting the number of unique lines in a file and are suitable for most common scenarios. However, they have some limitations and disadvantages, such as requiring the input data to be sorted, which can be slow for large files and use a lot of memory.

Also, the uniq command removes only adjacent duplicate lines from sorted data, so it might not give the expected results for some inputs.

Count the number of unique lines in a file using awk command

Another way to count the number of unique lines in a file in Linux is to use the awk command , which is a powerful text processing tool that can perform various operations on text files. The awk command has a built-in associative array data structure that can store and count the number of occurrences of each line in the input.

For example, to count the number of unique lines in a file named data.txt, you can use the following command:

awk '!a[$0]++' data.txt | wc -l

Output:

3

This command uses the awk command to read the data.txt file and applies a simple condition to each input line. The condition uses the !a[$0]++ expression, which increments the value of the a array for each line read.

This effectively counts the number of times each line occurs in the input and stores the counts in an array.

The awk command then applies ! a[$0]the operator of the expression, which negates the value of the array element. This means that only the rows in the array whose count is 0 will pass the condition and be printed to the output.

The output is then piped to the wc command, which counts the number of lines in the input and prints the results to the terminal.

The awk command also provides several options and features that you can use to control its behavior and customize its output. For example, you can use the -F option to specify a different field separator or use the -v option to define variables that you can use in your scripts.

You can also use the printf function to format the output of the awk command in various ways.

Here is an example of a more complex awk script that uses these functions to count the number of unique lines in a file called data.txt, where each line is a comma-delimited list of fields:

awk -F, '{a[$1]++} END {for (i in a) { printf "%s,%d\n", i, a[i] }}' data.txt | wc -l

Output:

3

The script uses the -F option to specify the , character as the field separator, which defines an array that stores and counts the occurrences of each field in the input.

The command then awkreads each line of the data.txt file and increments the value of the array for each field read. This effectively counts the number of times each unique field appears in the input.

The END block of the script is executed after all the input lines are read and the array is iterated using a for loop. The printf function is used to format the output of the awk command and print each unique field and its count to the output.

The output is then piped to the wc command, which counts the number of lines in the input and prints the results to the terminal.

In summary, there are several ways to count the number of unique lines in a file in Linux, and the appropriate method will depend on the specific requirements and constraints of the task. The sort and uniq commands are simple and effective tools for counting unique lines, while the awk command provides more advanced features and options to customize the output and behavior of the script.