So here is my problem: conceptually I know what how to do it, but I am still learning to script and hence dont quite know actually how to do this efficiently !

Any help with any of these options or a computationally better solution would be appreciated! I looked up quite a few options for one liners in awk, sed and grep etc but they mostly tip on getting common lines between multiple files and such. I am trying to get that one liner to work for contents of the same file across multiple lines. I have implemented option 2 but that is not very elegant, hence the post

I have this file-A with the following contents:

HWI-X00545:36:C1V15ACPX:2:1206:21033:18295
HWI-X00545:36:C1V15ACPX:2:2210:7893:55181
HWI-X00545:36:C1V15ACPX:2:2306:18502:73182

I want to get just the common parts of all the lines in file-A.

option 1: I could count the number of ":" and print everything before the fourth ":" not inclusive of the fourth ":" ( I could not figure how to do that!) and uniq

option 2: I could split lines at ":" and then awk the columns and join. ( this I feel is cumbersome,there must be a better way and easiest would be to option 3)

 sed 's/:/\t/g' trial_header.txt | awk '{print $1":"$2":"$3":"$4}' | uniq
 HWI-X00545:36:C1V15ACPX:2

option 3: just print the common parts of all lines in file-A and uniq it! ( not sure how to do this either! sort , uniq can work on when all the characters of all lines are same, not when partial lines are same??)

Note: BTW, I cannot do a character count, like cut -c -25 as this works for just this one file. I have other files with varying number of characters before the fourth ":" and I cant put it in a for loop due to that.

Thankyou in advance!