Grouping records using shell

Question

Is it possible to transform this:

1 Alice
1 Bob
1 Cary
2 Dan
3 Eve
3 Fred 

into this:

1 Alice,Bob,Cary
2 Dan
3 Eve,Fred

using shell? (without Perl, Ruby, Python etc)

Answer 1

Yes, there's a few ways of doing this.

Sort is your friend: Sort Wiki.

As is Awk: Awk Howto.

Answer 2

Using associative arrays in bash version 4:

declare -A ary
while read line; do
  set -- $line
  ary[$1]+="$2,"
done < input_file
for key in ${!ary[@]}; do
  printf "%s %s\n" $key ${ary[$key]%,}  # the "%," strips the trailing comma
done

Answer 3

Crap I couldn't resist the challenge. Here's my solution in shell (bash to be exact) as a oneliner:

k=0; while read i j; do if [ $k -eq $i ]; then echo -n ",$j"; \
else [ $k -gt 0 ] && echo; echo -n "$i $j"; k=$i; fi; done \
</tmp/infile; echo

which produces the desired output, properly formatted:

1 Alice,Bob,Cary
2 Dan
3 Eve,Fred

Assumptions:

1. 0 is not a valid entry in the first column of the data.
2. the first column is already numerically sorted.

If #2 is not valid, you need to run the input through sort(1) first with a numeric sort to put it in the proper order.

If I find out I just answered your homework I will be extremely annoyed. My only consolation is that if your teacher sees what I wrote they will think you are nuts.

Answer 4

This one over at stackoverflow is very similar and has some good answers: http://stackoverflow.com/questions/380817/best-way-to-simulate-group-by-from-bash