Add unique value from first column before each group - bash

Question

I have following file contents:
T12 19/11/19 2000
T12 18/12/19 2040
T15 19/11/19 2000
T15 18/12/19 2080
How to get following output with awk,bash and etc, I searched for similar examples but didn't find so far:
T12
19/11/19 2000
18/12/19 2040
T15
19/11/19 2000
18/12/19 2080
Thanks,
S

Answer 1

Could you please try following. This code will print output in same order in which first field is occurring in Input_file.
awk '
!a[$1]++ && NF{
b[++count]=$1
}
NF{
val=$1
$1=""
sub(/^ +/,"")
c[val]=(c[val]?c[val] ORS:"")$0
}
END{
for(i=1;i<=count;i++){
print b[i] ORS c[b[i]]
}
}
' Input_file
Output will be as follows.
T12
19/11/19 2000
18/12/19 2040
T15
19/11/19 2000
18/12/19 2080
Explanation: Adding detailed explanation for above code here.
awk ' ##Starting awk program from here.
!a[$1]++ && NF{ ##Checking condition if $1 is NOT present in array a and line is NOT NULL then do following.
b[++count]=$1 ##Creating an array named b whose index is variable count(every time its value increases cursor comes here) and its value is first field of current line.
} ##Closing BLOCK for this condition now.
NF{ ##Checking condition if a line is NOT NULL then do following.
val=$1 ##Creating variable named val whose value is $1 of current line.
$1="" ##Nullifying $1 here of current line.
sub(/^ +/,"") ##Substituting initial space with NULL now in line.
c[val]=(c[val]?c[val] ORS:"")$0 ##Creating an array c whose index is variable val and its value is keep concatenating to its own value with ORS value.
} ##Closing BLOCK for this condition here.
END{ ##Starting END block for this awk program here.
for(i=1;i<=count;i++){ ##Starting a for loop which runs from i=1 to till value of variable count.
print b[i] ORS c[b[i]] ##Printing array b whose index is i and array c whose index is array b value with index i.
}
} ##Closing this program END block here.
' Input_file ##Mentioning Input_file name here.

Answer 2

Here is a quick awk:
$ awk 'BEGIN{RS="";ORS="\n\n"}{printf "%s\n",$1; gsub($1" +",""); print}' file
How does it work?
Awk knows the concept records and fields.
Files are split in records where consecutive records are split by the record separator RS. Each record is split in fields, where consecutive fields are split by the field separator FS.
By default, the record separator RS is set to be the <newline> character (\n) and thus each record is a line. The record separator has the following definition:
RS:
The first character of the string value of RS shall be the input record separator; a <newline> by default. If RS contains more than one character, the results are unspecified. If RS is null, then records are separated by sequences consisting of a <newline> plus one or more blank lines, leading or trailing blank lines shall not result in empty records at the beginning or end of the input, and a <newline> shall always be a field separator, no matter what the value of FS is.
So with the file format you give, we can define the records based on RS="".
By default, the field separator is set to be any sequence of blanks. So $1 will point to that particular word we want on the separate line. So we print it with printf, and then we remove any reference to it with gsub.

Answer 3

awk is very flexible and provides a number of ways to solve the same problem. The answers you have already are excellent. Another way to approach the problem is to simply keep a single variable that holds the current field 1 as its value. (unset by default) When the first field changes, you simply output the first field as the current heading. Otherwise you output the 2nd and 3rd fields. If a blank-line is encountered, simply output the newline.
awk -v h= '
NF < 3 {print ""; next}
$1 != h {h=$1; print $1}
{printf "%s %s\n", $2, $3}
' file
Above are the 3-rules. If the line is empty (checked with number of fields less than three (NF < 3), then output the newline and skip to the next record. The second checks if the first field is not equal to your current heading variable h -- if not, set h to the new heading and output it. All non-empty records have the 2nd and 3rd fields output.
Result
Just paste the command above at the command line and you will get the desired result, e.g.
awk -v h= '
> NF < 3 {print ""; next}
> $1 != h {h=$1; print $1}
> {printf "%s %s\n", $2, $3}
> ' file
T12
19/11/19 2000
18/12/19 2040
T15
19/11/19 2000
18/12/19 2080