Getting the count of unique values in a column in bash

To see a frequency count for column two (for example):

awk -F '\t' '{print $2}' * | sort | uniq -c | sort -nr

fileA.txt

z    z    a
a    b    c
w    d    e

fileB.txt

t    r    e
z    d    a
a    g    c

fileC.txt

z    r    a
v    d    c
a    m    c

Result:

  3 d
  2 r
  1 z
  1 m
  1 g
  1 b

Here is a way to do it in the shell:

FIELD=2
cut -f $FIELD * | sort| uniq -c |sort -nr

This is the sort of thing bash is great at.

The GNU site suggests this nice awk script, which prints both the words and their frequency.

Possible changes:

• You can pipe through sort -nr (and reverse word and freq[word]) to see the result in descending order.
• If you want a specific column, you can omit the for loop and simply write freq[3]++ - replace 3 with the column number.

Here goes:

 # wordfreq.awk --- print list of word frequencies
 
 {
     $0 = tolower($0)    # remove case distinctions
     # remove punctuation
     gsub(/[^[:alnum:]_[:blank:]]/, "", $0)
     for (i = 1; i <= NF; i++)
         freq[$i]++
 }
 
 END {
     for (word in freq)
         printf "%s\t%d\n", word, freq[word]
 }

Perl

This code computes the occurrences of all columns, and prints a sorted report for each of them:

# columnvalues.pl
while (<>) {
    @Fields = split /\s+/;
    for $i ( 0 .. $#Fields ) {
        $result[$i]{$Fields[$i]}++
    };
}
for $j ( 0 .. $#result ) {
    print "column $j:\n";
    @values = keys %{$result[$j]};
    @sorted = sort { $result[$j]{$b} <=> $result[$j]{$a}  ||  $a cmp $b } @values;
    for $k ( @sorted ) {
        print " $k $result[$j]{$k}\n"
    }
}

Save the text as columnvalues.pl
Run it as: perl columnvalues.pl files*

Explanation

In the top-level while loop:
* Loop over each line of the combined input files
* Split the line into the @Fields array
* For every column, increment the result array-of-hashes data structure

In the top-level for loop:
* Loop over the result array
* Print the column number
* Get the values used in that column
* Sort the values by the number of occurrences
* Secondary sort based on the value (for example b vs g vs m vs z)
* Iterate through the result hash, using the sorted list
* Print the value and number of each occurrence

Results based on the sample input files provided by @Dennis

column 0:
 a 3
 z 3
 t 1
 v 1
 w 1
column 1:
 d 3
 r 2
 b 1
 g 1
 m 1
 z 1
column 2:
 c 4
 a 3
 e 2

.csv input

If your input files are .csv, change /\s+/ to /,/

Obfuscation

In an ugly contest, Perl is particularly well equipped.
This one-liner does the same:

perl -lane 'for $i (0..$#F){$g[$i]{$F[$i]}++};END{for $j (0..$#g){print "$j:";for $k (sort{$g[$j]{$b}<=>$g[$j]{$a}||$a cmp $b} keys %{$g[$j]}){print " $k $g[$j]{$k}"}}}' files*

Ruby(1.9+)

#!/usr/bin/env ruby
Dir["*"].each do |file|
    h=Hash.new(0)
    open(file).each do |row|
        row.chomp.split("\t").each do |w|
            h[ w ] += 1
        end
    end
    h.sort{|a,b| b[1]<=>a[1] }.each{|x,y| print "#{x}:#{y}\n" }
end

Here is a tricky one approaching linear time (but probably not faster!) by avoiding sort and uniq, except for the final sort. It is based on... tee and wc instead!

$ FIELD=2
$ values="$(cut -f $FIELD *)"
$ mkdir /tmp/counts
$ cd /tmp/counts
$ echo | tee -a $values
$ wc -l * | sort -nr
9 total
3 d
2 r
1 z
1 m
1 g
1 b
$

Pure-Bash version:

FIELD=1
declare -A results
while read -a line; do
    results[${line[$FIELD]:-(empty)}]=$((results[${line[$FIELD]:-(empty)}]+1));
done < file.txt
echo ${results[@]@A}

The key logic is to fill an associative array which keys are the values found in the file and the array's value is the number of occurrence:

• $FIELD is the selected column number
• ${line[$FIELD]} is the column value from that line in the file
• ${...:-(empty)} is a special case for empty values (what happens if there is less columns than expected?)

To have the output sorted in the expected OP format, a little more work is needed:

sort -rn < <(
    for k in "${!results[@]}"; do
        echo "${results[$k]} $k";
    done
)

Warning: it works well for tab-delimited and space-delimited files, but works bad for values with spaces in it.