Find strings that appear consecutively in a file

Question

All,
I have a file where strings appear on each line in following format:

STRING1  
STRING2  
STRING1  
STRING1  
STRING1   
STRING3  
STRING4  
STRING4  
STRING5  
STRING6  
STRING5 

I want to find which strings appear consecutively in the file, using bash script. For e.g., in the above example, STRING1 and STRING4 appear consecutively and should be there in the output. Note, even though STRING5 appears twice, it isn't appearing consecutively.

I don't want to sort the file as the file size would be normally large and that would add overhead. The user would supply the string and the script would says if the string appears consecutively in the file.

FYI, the strings might have unwanted spaces after them.

Answer 1

$ uniq -d <<< 'STRING1
> STRING2
> STRING1
> STRING1
> STRING1
> STRING3
> STRING4
> STRING4
> STRING5
> STRING6
> STRING5'
STRING1
STRING4

Answer 2

awk -v "key=STRING4" '
    $1 == key && $1 == prev {
        print key " appears on consecutive lines on line " NR
        found=1
        exit 0
    } 
    {prev = $1}
    END {if (! found) {print key " does not appear on consecutive lines"; exit 1}}
' filename

Answer 3

What about read and string compares? Seems like the easiest solution to me.

while read line; do 
    if [ "$line" == "$temp" ]; then 
        echo "$line"; 
    fi; 
    temp=$line; 
done < test.txt