How to use Unix Shell to show only the first n columns and last n columns?

awk -F, '{print $1, $2, $(NF-1), $NF}'  < input

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More generally (per the Question's title), to print the first and last n columns of the input -- without checking to see whether that means printing some columns twice --

awk -v n=2 '{ 
  for(i=1; i <= n && i <= NF; i++)
      printf "%s%s", $i, OFS
    for(i=NF-n+1; i <= NF && i >= 1; i++)
      printf "%s%s", $i, OFS
    printf "%s", ORS
  }' < input

(using -F as needed for the delimiter)