UNIX for Advanced & Expert Users

#!/bin/ksh
# Assume we get the input from the command line

prevval=$1                         # Read first item and assign as "previous value"
shift                              # Remove first from input line
seqflag="N"

for currval in $*                  # For every subsequent item
do
   ((nextval=$prevval+1))
   if [ $currval -eq $nextval ]    # We are in sequence
   then
      if [ "$seqflag" = "N" ]      # We were not previously in sequence
      then
         seqflag="Y"
         seqstart=$prevval
      fi                           # No action for continuing sequence
   else                            # We are not in sequence
      if [ "$seqflag" = "N" ]      # We were not previously in sequence
      then
         output="${output},$prevval"
      else                         # We were previously in sequence
         seqflag="N"
         output="${output},${seqstart}-${prevval}"
      fi
   fi
   prevval=$currval                # Set previous value to current value ready to loop round again
done


# So we have read all the input through so we end the output and display or file it

if [ "$seqflag" = "N" ]            # We were not previously in sequence
then
   output="${output},$currval"
else                               # We were previously in sequence
   output="${output},${seqstart}-${currval}"
fi

# Chop off leading comma (maybe better logic would avoid this!)
output="${output#,}"

echo "$output"

$ sequencer.ksh 2 6 7 8 11 12 18 25 26 27
2,6-8,11-12,18,25-27
$ sequencer.ksh 4 5 6 7 11 12 13 14 19 22 57 199 200 201 202
4-7,11-14,19,22,57,199-202

$ sequencer.ksh 1 2 3 4 5 6 7 8 9
1-9
$ sequencer.ksh 1 2 3 5 4 6 7 8 9
1-3,5,4,6-9

((nextval=$prevval+1))

echo "\$nextval=$nextval"