Combine lines with matching first field

Using awk one liner

awk -F: -v ORS="" 'a!=$1{a=$1; $0=RS $0} a==$1{ sub($1":",";") } 1' file

Output:

apple:A fruit;Type of: piebanana:tropical fruitcherry:small burgundy fruit;1 for me to eat;bright red

setting ORS="" ; By default it is \n.
The reason why we have set ORS="" (Output Record Separator) is because we don't want awk to include newlines in the output at the end of each record. We want to handle it in our own way, through our own logic. We are actually including newlines at the start of every record which has the first field different from the previous one.

a!=$1 : When variable a (initially null) doesn't match with first field $1 which is for eg. applein first line, then set a=$1 and $0=RS $0 i.e $0 or simply whole record becomes "\n"$0 (basically adding newline at the beginning of record). a!=$1 will always satisfy when there is a different first field ($1) than the previous line's $1 and is thus a criteria to segregate our records based on first field.

a==$1: If it matches then it probably means you are iterating over a record belonging to the previous record set. In this case substitute first occurrence of $1: (Note the : ) for eg. apple: with ;. $1":" could also be written as $1FS where FS is :

If you have millions of line in your file then this approach would be fastest because it doesn't involve any pre-processing and also we are not using any other data structure say array for storing your keys or records.

Discover awk language:

awk -F':' '{ v=substr($0, index($0,":")+1); a[$1]=($1 in a? a[$1]";" : "")v }           END{ for(i in a) print i,a[i] }' OFS=':' infile.txt

The output:

apple:A fruit;Type of: piebanana:tropical fruitcherry:small burgundy fruit;1 for me to eat;bright red

I think this one do the job

 awk -F':' '$1!=a{if(b);print b;b=""}a=$1{$1="";if(!b)b=a;b=b$0}END{print b}' infile