global variable in bash

Using shared memory

 

#] echo 0 > /dev/shm/foo

 

Increase `foo`'s value

 

#] echo $(($(</dev/shm/foo)+1)) > /dev/shm/foo

 

decrease `foo`'s value

 

#] echo $(($(</dev/shm/foo)-1)) > /dev/shm/foo