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If I have n files in a directory, for example;

a

b

c

How do I get pairwise combinations of these files (non-directional) to pass to a function?

The expected output is

a-b

a-c

b-c

so that it can be passed to a function like

fn -file1 a -file2 b

fn -file1 a -file2 c

...

This is what I am trying out now.

for i in *.txt

 do

  for j in *.txt

   do

    if [ "$i" != "$j" ]

     then

      echo "Pairs $i and $j"

     fi

   done

 done

Output

Pairs a.txt and b.txt

Pairs a.txt and c.txt

Pairs b.txt and a.txt

Pairs b.txt and c.txt

Pairs c.txt and a.txt

Pairs c.txt and b.txt

I still have duplicates (a-b is same as b-a) and I am thinking perhaps there is a better way to do this.

Put the file names in an array and run through it manually with two loops.

You get each pairing only once if if j < i where i and j are the indexes used in the outer and the inner loop, respectively.

$ touch a b c d

$ f=(*)

$ for ((i = 0; i < ${#f[@]}; i++)); do 

      for ((j = i + 1; j < ${#f[@]}; j++)); do 

          echo "${f[i]} - ${f[j]}"; 

      done;

  done 

a - b

a - c

a - d

b - c

b - d

c - d

You're very close in your script, but you want to remove duplicates; i.e a-b is considered a duplicate of b-a.

We can use an inequality to handle this; only display the filename if the first file comes before the second file alphabetically. This will ensure only one of each matches.

for i in *.txt

do

  for j in *.txt

  do

    if [ "$i" < "$j" ]

    then

     echo "Pairs $i and $j"

    fi

  done

done

This gives the output

Pairs a.txt and b.txt

Pairs a.txt and c.txt

Pairs b.txt and c.txt

This isn't an efficient algorithm (it's O(n^2)) but may be good enough for your needs.

With join trick for filenames without whitespace(s):

Sample list of files:

$ ls *.json | head -4

1.json

2.json

comp.json

conf.json

$ join -j9999 -o1.1,2.1 <(ls *.json | head -4) <(ls *.json | head -4) | awk '$1 != $2'

1.json 2.json

1.json comp.json

1.json conf.json

2.json 1.json

2.json comp.json

2.json conf.json

comp.json 1.json

comp.json 2.json

comp.json conf.json

conf.json 1.json

conf.json 2.json

conf.json comp.json

• 
  -j option points to a common field position to join on; but -j9999 will provoke mixed joining resembling cartesian product.

for i in *.txt ; do

  for j in *.txt ; do

    if [ "$i" '<' "$j" ] ; then

      echo "Pairs $i and $j"

    fi

  done

done