Before I start I'm not asking for any code to be written just to enlighten and the behavior I am having. I have this snippet of code.
NOW=$(date +"%H")
While [ true ]; do
echo $NOW
done
I would of expected so that when it would be printed to the screen the time would update since I am storing the date command and formatting it into the variable NOW , but instead all it does is keep printing the same date that the script was started at. Will someone enlighten me on why it does that.
It behaves as it should. What you expect, could be achieved this way:
while [ true ]; do
NOW=$(date +"%H")
echo $NOW
done
Now the variable is updated at each iteration of the loop.
Compare both with this one:
NOW=date
while [ true ]; do
$NOW
done
Here date is recalculated at each iteration, as the command itself is stored in the variable, not its result.
$NOW be used with +"%H" ? Maybe worth updating your example with this.
A variable is given a value when it is assigned.
For example
x=1
will set the value of the variable x to 1. The value doesn't change unless you assign a new value to it.
When you do
NOW=$(date +"%H")
then the shell will run the date command and put the results into the NOW variable. Again, the value doesn't change unless you assign a new value. It doesn't matter that date is an external command; it's run once when the variable is assigned the value.
So you either need to assign the value inside the loop... or use a function.
NOW()
{
date +"%H"
}
This doesn't defined a variable, but a function. You can now use this inside your loop
while [ true ]; do
echo $(NOW)
done
Note that the way you call it is different.
In this limited case we can make it simpler:
while [ true ]; do
NOW
done
So the final script would be
#!/bin/bash
NOW()
{
date +"%H"
}
while [ true ]; do
NOW
done
while [ true ]isn't doing what you think it is -- the[ ]command is checking the string "true" to see if it's non-blank, and since it is it returns true. Butwhile [ false ],while [ wibble ]etc would all also return true (and hence run the loop forever). It makes much more sense to usewhile true-- without the[ ], around ittrueis interpreted as a command, and there is a command namedtruethat always succeeds (i.e. returns true). (Compare withwhile false, which runs the commandfalse, which always fails/returns false.)