Shell Programming and Scripting

#! /bin/sh
# count1 appends an increment to a file 200 times
# note that a file called numbers must be initialised to contain 0

count=0
while [ $count -lt 200 ] # loop 200 times 
do
count=`expr $count + 1` # increment counter
n=`tail -1 numbers` # get last line from numbers file
expr $n + 1 >> numbers # add 1 to it and append it back
done

#! /bin/sh
# count2 also increments and appends a value to the numbers file
# but only when it can successfully create a new hard link to the numbers file

count=0
while [ $count -lt 200 ] # still loop 200 times
do
count=`expr $count + 1` # increment the loop counter
if ln numbers numbers.lock # try to enter the critical region
then # in this case, ln is similar to TSL
n=`tail -1 numbers` # get the last number to increment
expr $n + 1 >> numbers # increment it and append it to the file
rm numbers.lock # exit the critical region
fi # Note that if the ln was unsuccessful, we don't
# do busy waiting, but just continue looping
done

echo `expr $n + 1` >> numbers # add 1 to it and append it back

#! /bin/ksh  or bash or dash or ...
# count1 appends an increment to a file 200 times
# note that a file called numbers must be initialised to contain 0
echo 1 > numbers # empty file 1st and put the 1st value.
count=0
while (( count < 200 )) # loop 200 times
do
   ((count+=1)) # increment counter
   n=$(tail -1 numbers) # get last line from numbers file
   # why not use count ?
   ((n+=1))
   echo $n  >> numbers # add 1 to it and append it back
done

count=0
touch numbers
echo $count > numbers
while [ $count -lt 200 ] # loop 200 times