What does this script do?
n=23; i=0; while read in; do
nr=$(sed -r "s/(.)/\1\n/g" <<<$in | sort | uniq | wc -l)
[[ $nr == 5 ]] && i=$[$i+1]
[[ $nr == 5 && $n == $i ]] && out=$in && break
done <<<$(echo -e {1..4}{1..4}{1..4}{1..4}"\n")
echo $out #4312
Answer
This script is a Bash script that performs a specific task involving generating combinations of numbers, counting unique digits, and finding a particular combination based on certain conditions. Let's break down what each part of the script does:
Script Breakdown
Initialization:
bash
n=23
i=0
n is initialized to 23. This is the target count of combinations that meet a certain condition.
i is initialized to 0. This will be used to count how many combinations have met the condition so far.
While Loop:
bash
while read in; do
The while loop reads each line of input from a here-string provided by the <<< operator at the end of the script.
Generating Combinations:
bash
done <<<$(echo -e {1..4}{1..4}{1..4}{1..4}"\n")
echo -e {1..4}{1..4}{1..4}{1..4}"\n" generates all possible combinations of four digits where each digit can be 1, 2, 3, or 4. This results in 4^4 = 256 combinations.
The combinations are fed into the while loop one by one.
Counting Unique Characters:
bash
nr=$(sed -r "s/(.)/\1\n/g" <<<$in | sort | uniq | wc -l)
sed -r "s/(.)/\1\n/g" <<<$in takes the input string $in and inserts a newline after each character, effectively splitting the string into individual characters.
sort | uniq sorts the characters and removes duplicates.
wc -l counts the number of unique characters.
Checking Conditions:
bash
[[ $nr == 5 ]] && i=$[$i+1]
[[ $nr == 5 && $n == $i ]] && out=$in && break
The first condition checks if the number of unique characters ($nr) is 5. If true, it increments the counter i.
The second condition checks if the number of unique characters is 5 and if i equals n (23). If both conditions are met, it assigns the current combination to out and breaks the loop.
Output:
bash
echo $out #4312
Finally, it prints the value of out, which is the 23rd combination that has exactly 5 unique characters. However, since the maximum possible number of unique characters in a four-digit string composed of digits 1-4 is 4, this condition can never be met. Therefore, the script will never reach the echo $out line with a valid output, and the comment #4312 seems incorrect or misleading in this context.
Conclusion
The script seems to have a logical flaw because it is impossible for a four-digit combination of numbers from 1 to 4 to have 5 unique characters. Therefore, the condition [[ $nr == 5 ]] will never be true, and the script will not produce a valid output. The comment #4312 seems to be incorrect or based on a misunderstanding of the script's logic.
