Shell Programming and Scripting

 #!/bin/bash

if [ -e number.txt ]; then
        echo "File exists"
else
        echo "File does not exists. Create file now."
        echo 1 > number.txt
fi

lockfile script.lock

a=$(/usr/bin/tail -n 1 number.txt)

if [ $a -gt 1 ]; then
        a=$(/usr/bin/tail -n 1 number.txt)
        /usr/bin/expr $a - 1 >> number.txt
fi

if [ $a -lt 2 ]; then
        a=$(/usr/bin/tail -n 1  number.txt)
        /usr/bin/expr $a + 1 >> number.txt
fi

rm -f script.lock

for a in {1..1000}; do bash script.sh & done