Comparing two lists

Rolf Nielsen listreader at
Sat May 7 11:15:03 UTC 2011

2011-05-07 07:28, Robert Bonomi skrev:
>>  From listreader at  Fri May  6 20:14:09 2011
>> Date: Sat, 07 May 2011 03:13:39 +0200
>> From: Rolf Nielsen<listreader at>
>> To: Robert Bonomi<bonomi at>
>> CC: freebsd-questions at
>> Subject: Re: Comparing two lists
>> 2011-05-07 02:54, Robert Bonomi skrev:
>>>>    From owner-freebsd-questions at  Fri May  6 19:27:54 2011
>>>> Date: Sat, 07 May 2011 02:09:26 +0200
>>>> From: Rolf Nielsen<listreader at>
>>>> To: FreeBSD<freebsd-questions at>
>>>> Subject: Comparing two lists
>>>> Hello all,
>>>> I have two text files, quite extensive ones. They have some lines in
>>>> common and some lines are unique to one of the files. The lines that do
>>>> exist in both files are not necessarily in the same location. Now I need
>>>> to compare the files and output a list of lines that exist in both
>>>> files. Is there a simple way to do this? diff? awk? sed? cmp? Or a
>>>> combination of two or more of them?
>>> If the files have only 'minor' differences -- i.e. no long runs of lines
>>> that are in only one fie -- *and* the common lines are  in the same order
>>> in each file, you can use diff(1), without any other shennigans.
>>> If the above is -not- true, and If you need _only_ the common lines, AND
>>> order is not important, then sort(1) both files, and use diff(1) on the
>>> two sorted versions.
>>> Beyond that it depends on what you mean by 'extensive' ones.  megabytes?
>>> Gigabytes? or what??
>> Some 10,000 to 20,000 lines each. I do need only the common lines. Order
>> is not essential, but would make life easier. I've tried a little with
>> uniq, as suggested by Polyptron, but I guess 3am is not quite the right
>> time to do these things. Anyway, thanks.
> Ok, 20k lines is only a medium-size file. There's no problem in fitting
> the entire file 'in memory'.  ('big' files are ones that are larger than
> available memory. :)

By "quite extensive" I was refering to the number of lines rather than 
the byte size, and 20k lines is, by my standards, quite a lot for a 
plain text file. :P
But that's beside the point. :)

> Using uniq:
>     sort  {{file1}} {{file2}} |uniq -d

Yes, I found that solution on
which is mainly about comm, but also lists other ways of doing things. I 
also found
grep -xF -f file1 file2
there, and I've tested that one too. Both seem to be doing what I want.

> to maintain order, put the following in a file, call it 'common.awk'
>       NR==FNR   { array[$0]=1; next; }
>                 { if (array[$0] == 1) print $0; }
> then use the command:
>    awk -f common.awk {{file1}} {{file2}}
> This will output common lines, in the order they occur in _file2_.

I took the liberty of sending a copy of this to the list although you 
replied privately.

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