why subtraction return - symbol

Question

I have a problem with a simple subtraction but I don\'t understand what\'s wrong.

My code :

start= date +%s%N | cut -b1-13
#Treatment...
end= date +%s%N         


        

Answer 1

The command:

a= b

(note the space) will set a to an empty string while it runs the command b. It's a way to temporarily set environment variables for a single command, things like:

PATH=/path/to/somwhere gcc whatever  # Here, PATH has the modified value.
echo $PATH                           # Here, PATH has its original value.

So the command line:

start= date +%s%N | cut -b1-13

sets start temporarily to nothing and runs the date command. Hence both start and end are still empty when you use them, which is why you only get the -, since expr - just gives you -.

If you want to get the results of the date command into a variable, use:

start=$(date +%s%N | cut -b1-13)

Answer 2

You didn't assign to the variables. You must not have spaces around the equals sign.

Also, you're doing it wrong.

start=$(date +%s%N | cut -b1-13)