UNIX for Beginners Questions & Answers

awk -F. '{A[$1]=A[$1] q "," q $2} END{for(i in A) print q i A[i] q}' q=\' file

awk -F. -vFMT="'%s'" '                          # split line at ".", use FMT variable for printing as single
                                                # quote handling is difficult within the awk script
$1 != LAST      {printf QRS FMT, $1             # on changing first field (LAST is the empty string in the first record
                                                # so pattern is TRUE) print it after a temporary record separator (which
                                                # is empty, too, on the 1. record)
                 QRS = ORS                      # now set temp sep to line feed for all remaining records
                 LAST = $1                      # keep track of the first field for the upcoming pattern checksy
                }
                {printf "," FMT, $2             # now keep adding the quoted second fields until first field changes
                }
END             {printf ORS                     # final line feed 
                }
' file

awk                                            #### Starting awk here.
-F"."                                          #### Mentioning -F as .(DOT) here so e could make custom delimiters in awk as per our requirement so making DOT as a delimiter.
-vs1="'"                                       #### defining a variable named s1 whose value is '. So in awk we could define variables by using -v variable_name="value".
'FNR==NR                                       #### mentioning here FNR==NR condition. Where FNR and NR both are awk's in-built keywords and tell us the number of lines in any 
                                                    Input_file but a major difference between FNR and NR is FNR's value will be RESET each time a new Input_file is being read 
                                                    and NR's value will keep on increase till the last Input_file being read(as in awk we could mention mutiple files as an Input).
{A[$1]=A[$1]?A[$1] OFS s1 $NF s1:s1 $NF s1;    #### So creating here an array named A whose index is $1(first field) and whose value is s1 $NF s1 if that element's index is NOT registered in array A.
                                               #### Where s1 as mentioned before is a variable with value of ' and $NF denoted the last field of a line which is being read. So if a value is find in array A
                                                    which is already registered(mentioned in code by ?) then A's that specific index's value will be A[$1] OFS s1 $NF s1, which means appending the current value
                                                    into the previous A's value of current index(which is $1 from each line).
next}                                          #### Now using next which is a in-built keyword of awk. So by mentioning this we are skipping all further statements now.
A[$1]{                                         #### A[$1], so this is a condition which will be executed when 2nd time Input_file is being read and it make sure if any first field is present in array A then execute following statements.
print s1 $1 s1 OFS A[$1];                      #### printing here s1 $1 s1 OFS A[$1], where s1 kis a variable mentioned above, $1 is first field of line and OFS is Output field separator(awk's in-built keyword, whose default value is a space) then A[$1], which will print array A's value whose index is $1 of current line.
delete A[$1]                                   #### deleting the array A's element whose index is $1 so that we will not execute already executed $1 as we are reading Input_file twice.
}' OFS=,   Input_file  Input_file              #### Mentioning the value of OFS as a comma(,) which is awk's in-built keyword denotes as Ouptut field separator, mentioning Input_file 2 times too here.