Netyjnd

This question is strongly related to this and this question. I have a file that contains several lines where each line is a path to a file. Now I want to pair each line with each different line (not itself). Also a pair A B is equal to a B A pair for my purposes, so only one of these combinations should be produced.

Example

files.dat reads like this in a shorthand notation, each letter is a file path (absolute or relative)

a
b
c
d
e

Then my result should look something like this:

a b
a c
a d
a e
b c
b d
b e
c d
c e
d e

Preferrably I would like to solve this in bash. Unlike the other questions, my file list is rather small (about 200 lines), so using loops and RAM capacity
pose no problems.

Use this command:

awk ' name[$1]++ 
 END PROCINFO["sorted_in"] = "@ind_str_asc"
 for (v1 in name) for (v2 in name) if (v1 < v2) print v1, v2 
 ' files.dat

PROCINFO may be a gawk extension. 
If your awk doesn’t support it,
just leave out the PROCINFO["sorted_in"] = "@ind_str_asc" line
and pipe the output into sort (if you want the output sorted).

(This does not require the input to be sorted.)

$ join -j 2 -o 1.1,2.1 file file | awk '!seen[$1,$2]++ && !seen[$2,$1]++'
a b
a c
a d
a e
b c
b d
b e
c d
c e
d e

This assumes that no line in the input file contains any whitespace. It also assumes that the file is sorted.

The join command creates the full cross product of the lines in the file. It does this by joining the file with itself on a non-existing field. The non-standard -j 2 may be replaced by -1 2 -2 2 (but not by -j2 unless you use GNU join).

The awk command reads the result of this and only outputs results that are pairs that has not yet been seen.

If you have ruby installed:

$ ruby -0777 -F'n' -lane '$F.combination(2) ' ip.txt
a b
a c
a d
a e
b c
b d
b e
c d
c e
d e

• 
  -0777 slurp entire file (should be okay as it is mentioned in OP that file size is small)


• 
  -F'n' split based on newline, so each line will be an element in $F array


• 
  $F.combination(2) generate combinations 2 elements at a time


• 
  print as required


• if input file can contain duplicates, use $F.uniq.combination(2)
  

for 3 elements at a time:

$ ruby -0777 -F'n' -lane '$F.combination(3) ' ip.txt
a b c
a b d
a b e
a c d
a c e
a d e
b c d
b c e
b d e
c d e

With perl (not generic)

$ perl -0777 -F'n' -lane 'for $i (0..$#F) 
 for $j ($i+1..$#F) 
 print "$F[$i] $F[$j]n" ' ip.txt
a b
a c
a d
a e
b c
b d
b e
c d
c e
d e

With awk

$ awk ' a[NR]=$0 
 END for(i=1;i<=NR;i++)
 for(j=i+1;j<=NR;j++)
 print a[i], a[j] ' ip.txt 
a b
a c
a d
a e
b c
b d
b e
c d
c e
d e

A python solution.
The input file is fed to itertools.combinations from the standard library, which generates 2-length tuples that are formatted and printed to standard output.

python3 -c 'from itertools import combinations
with open("file") as f:
 lines = (line.rstrip() for line in f)
 lines = (" ".format(x, y) for x, y in combinations(lines, 2))
 print(*lines, sep="n")
'

Here's one in pure shell.

a=$1
shift
for f in "$@"
do
 echo $a $f
done
test $# -gt 1 && /bin/sh $0 "$@"

Example:

~ (137) $ sh test.sh $(cat file.dat)
a b
a c
a d
a e
b c
b d
b e
c d
c e
d e
~ (138) $