Unix shell script sed command error

Below script runs successfully

a=0
b=1
r=10
while [ $a -lt $ncnt ]
do
   echo $a
sed -n $b,$10p test_ids.txt > test_ids_1.txt
echo $b
a=`expr $a + 1`
done


But below code is not work it gives error
I am passing variable r instead of 10
Please help what am I wrong ?

a=0
b=1
r=10
while [ $a -lt $ncnt ]
do
   echo $a
sed -n $b,$rp test_ids.txt > test_ids_1.txt
echo $b
a=`expr $a + 1`
done
digs developerAsked:
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woolmilkporcCommented:
You should separate the variable "r" from  from the "p" subcommand by means of the curly braces:

sed -n $b,${r}p test_ids.txt >> test_ids_1.txt

Use ">>" instead of ">" alone, otherwise the output file "test_ids_1.txt" will get overwritten at each iteration - although it will contain the same first ten lines of "test_ids.txt" over and over, depending on the value of "ncnt" (a variable you shouldn't forget to fill, by the way).
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