Shell Programming and Scripting

$ echo "a b
b a
e f
c d
f e
d c
" | awk '{
        # get a b order even if we get b a
        if($1 > $2) { tmp=$1; $1=$2; $2=tmp; }
        if(A[$1] != $2)  { print ; A[$1]=$2; }
}' | sort # sort is necessary for your example data, otherwise we get a b \n e f \n ...
a b
c d
e f
$

awk '{
   n = split($0,a," ")
   for (i=1; i<n; i++)
     for (j=1; j<=n-i; j++)
       if (a[j] > a[j+1]) {
          t=a[j]
          a[j]=a[j+1]
          a[j+1]=t
       }
   for (i=1; i<=n; i++)
       v=sprintf("%s",v?v""a[i]:a[i])
   if (v in x); else print $0
   x[v]=v; v=""
}' file

awk '{
  if(a[$1]==$2 || a[$2]==$1) next; 
  a[$1]=$2;  print
}' data

fold -w 1 inputfile | sort | uniq | xargs -n2

fold -w 1 inputfile | sort -u | xargs -n2

$ cat data
a b
b a
e f
c d
f e
d c
a c
$ fold -w 1 data | sort | uniq | xargs -n2
a b
c d
e f