In /usr/bin/pamstretch-gen line 19:
if ! cat $2 >$tempfile 2>/dev/null; then
         ^-- SC2086: Double quote to prevent globbing and word splitting.
             ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
if ! cat "$2" >"$tempfile" 2>/dev/null; then


In /usr/bin/pamstretch-gen line 26:
width=`pnmfile $tempfile 2>/dev/null|cut -d " " -f 3`
      ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.
               ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
width=$(pnmfile "$tempfile" 2>/dev/null|cut -d " " -f 3)


In /usr/bin/pamstretch-gen line 34:
target_width=`awk 'BEGIN{printf("%d",'0.5+"$width"*"$1"')}'`
             ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
target_width=$(awk 'BEGIN{printf("%d",'0.5+"$width"*"$1"')}')


In /usr/bin/pamstretch-gen line 38:
int_scale=`awk '
          ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
int_scale=$(awk '
BEGIN {
int_scale=1;int_width='"$width"'
while(int_width<'"$target_width"')
  {
  int_scale++
  int_width+='"$width"'
  }
print int_scale
}')


In /usr/bin/pamstretch-gen line 50:
  pnmscale "$1" $tempfile
                ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
  pnmscale "$1" "$tempfile"


In /usr/bin/pamstretch-gen line 52:
  pamstretch "$int_scale" $tempfile | pnmscale -xsi "$target_width"
                          ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
  pamstretch "$int_scale" "$tempfile" | pnmscale -xsi "$target_width"


In /usr/bin/pamstretch-gen line 55:
rm -f $tempfile;
      ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
rm -f "$tempfile";

For more information:
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...