How to find common elements among multiple arrays in Shell script?

Finding Common Elements Among Multiple Arrays in Shell Script

In the world of shell scripting, working with arrays is a common task, and sometimes you may need to find the common elements among multiple arrays. This can be useful in various scenarios, such as comparing user inputs, merging data from different sources, or identifying shared elements in a set of configurations.

Approach 1: Using the comm Command

One efficient way to find the common elements among multiple arrays in a shell script is to use the comm command. The comm command compares two sorted files line by line and produces three columns of output: lines that are unique to the first file, lines that are unique to the second file, and lines that are common to both files.

Here's an example script that demonstrates how to use comm to find the common elements among multiple arrays:

#!/bin/bash

# Define the arrays
array1=(apple banana cherry)
array2=(banana cherry orange)
array3=(cherry grape)

# Convert the arrays to sorted files
printf '%s\n' "${array1[@]}" | sort > file1.txt
printf '%s\n' "${array2[@]}" | sort > file2.txt
printf '%s\n' "${array3[@]}" | sort > file3.txt

# Find the common elements
comm -12 file1.txt file2.txt | comm -12 - file3.txt

In this script, we first define the three arrays we want to compare. Then, we convert each array to a sorted file using the printf command and the sort utility. Finally, we use the comm command to find the common elements among the three files.

The -12 options in the comm command tell it to suppress the output of the unique lines from the first and second files, respectively, and only display the common lines.

The output of this script would be:

cherry

This shows that the only common element among the three arrays is cherry.

Approach 2: Using the awk Command

Another way to find the common elements among multiple arrays in a shell script is to use the awk command. awk is a powerful text processing tool that can be used to manipulate and analyze data in a structured way.

Here's an example script that demonstrates how to use awk to find the common elements among multiple arrays:

#!/bin/bash

# Define the arrays
array1=(apple banana cherry)
array2=(banana cherry orange)
array3=(cherry grape)

# Find the common elements using awk
awk 'BEGIN{
    for (i in ARGV) {
        if (i > 0) {
            while (getline < ARGV[i]) {
                arr[++n] = $0
            }
            close(ARGV[i])
        }
    }
}
{
    count[$0]++
}
END {
    for (i in arr) {
        if (count[arr[i]] == length(ARGV)-1) {
            print arr[i]
        }
    }
}' file1.txt file2.txt file3.txt

In this script, we first define the three arrays we want to compare. Then, we use the awk command to process the data.

The BEGIN block reads the contents of the files into an array arr, and the count array keeps track of the number of times each element appears across all the files.

The END block then iterates through the arr array and prints the elements that appear in all the files (i.e., the common elements).

The output of this script would be:

cherry

Again, the only common element among the three arrays is cherry.

Visualizing the Concept with Mermaid

Here's a Mermaid diagram that illustrates the process of finding common elements among multiple arrays in a shell script:

This diagram shows the high-level steps involved in the process: defining the arrays, converting them to sorted files, using either the comm or awk command to find the common elements, and finally outputting the common elements.

By using a visual representation, you can help your students better understand the overall workflow and the different approaches they can use to solve this problem in a shell script.

In conclusion, finding common elements among multiple arrays in a shell script can be accomplished using either the comm command or the awk command. Both approaches have their own advantages and can be useful in different scenarios. By understanding these techniques, your students will be better equipped to tackle similar problems in their shell scripting endeavors.