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Given a file like this:


I would like to list all rows (in bash terminal) such that the value in column 1 appears at least twice (in column 1). The result should be

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To try and avoid storing the whole file in memory, you could do:

awk -F , '
  !count[$1]++ {save[$1] = $0; next}
  count[$1] == 2 {
    print save[$1]
    delete save[$1]
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Perl solution:

perl -F, -ane ' $h{ $F[0] } .= $_
                $h{$_} =~ tr/\n// >= 2 and print $h{$_} for keys %h
              ' < input-file
  • -n reads the input line by line
  • -a splits each line on -F, i.e. comma, into the @F array.
  • lines are stored in the %h hash keyed by the first field ($F[0]). They are concatenated together (.=).
  • at the end of the file ("Eskimo greeting" }{), we loop over the keys and count the number of newlines (using the tr operator). If its at least 2, we print the stored lines.

You can feed the output to | sort -n if you want the first column to be numerically sorted.

Attention: if the last line didn't end in a newline, its group would report its size - 1. You can chomp each line and add the newlines yourself to fix it, or use array of arrays of lines instead of array of strings.

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With awk (GNU awk for multi-dimensional arrays)

gawk -F, '
    { line[NR] = $0; count[$1]++; found[$1][NR] = 1}
    END {
        for (id in count)
            if (count[id] > 1)
                for (nr in found[id]) 
                    print line[nr]
' file

The order of the output may not be the same as the input file.

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I believe that's GNU AWK 4. Earlier versions handle (fake) multi-dimensional arrays differently. – Dennis Williamson Jan 15 at 21:05
for i in $(cat given | cut -d, -f1)
  linect=$(grep ^"${i}," given | wc -l)
  if [ ${linect} -gt 1 ]
    grep ^"${i}," given >> result
sort result |uniq > desiredoutput

as long as the fields are delimited by comma and you are seeking duplicates in the column 1 and column 1 only, this should work.

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can be written as grep -f <(cut -d, -f1 file | sort | uniq -c | awk '$1>1 {print "^"$2","}') file – glenn jackman Jan 15 at 16:59
one of the many ways it can be written, is in my example. I just wanted to make it easier to understand. – MelBurslan Jan 15 at 17:02
understood. Just making it more efficient: one grep call versus 2n calls. – glenn jackman Jan 15 at 17:08
@glennjackman I like your solution. however, it appears to not work with huge files. I'm not sure if that's an issue with grep. – Bob Jan 18 at 10:56

Another variant (where test.txt is your input file):

FILE=test.txt ; for n in $(cat ${FILE} | awk -F"," '{count[$1]++} END {for (i in count) print i":"count[i]}'|grep -v ':1'|awk -F: '{print $1}');do grep ^${n} ${FILE} ;done
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That's just about the ugliest thing I've ever seen. – Dennis Williamson Jan 15 at 21:05

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