[SOLVED] A bash script that remembers variable contents from previous runs.

stf92 · 08-14-2015, 07:42 PM

Hi: suppose I have a bash script that is run every minute by cron. Suppose further the script is this:

Code:

 
#!/bin/bash             
let COUNT=COUNT+1

I would like the script to remember old values of COUNT, so that COUNT can reach the value 2. If I could somehow export, from the bash instance running this script, the COUNT variable to the parent procces (this is another bash instance) and parent variables where available to children, then my problem is solved. But the export mechanism (export sentence) doesn't seem to work that way. Adding an 'export COUNT' to the script doesn't work. The first question would be: affordmentioned problem solvable with bash scripts? Or should I forget about cron and make a program that works on the background, with a sentence like sleep 60?

astrogeek · 08-14-2015, 07:55 PM

How about this...

Code:

echo 1 > /path/to/count.txt

Then create cron script...

#!/bin/bash
COUNT=$(</path/to/count.txt)

... do something with COUNT ...

((COUNT = $COUNT + 1))
echo "$COUNT" > /path/to/count.txt

*** Updated per HMW's post #4 below... thanks!

stf92 · 08-14-2015, 08:17 PM

Of course! Making a file work like a variable. But now the variable is non volatile. The other solution would be to write a daemon, that is

Code:

[program body]
[Interrupt service routine]
Hang the ISR from some signal

I decide on the first approach. The initialization can be done at boot time, /etc/profile for example.
Thanks.

HMW · 08-15-2015, 10:33 AM

Quote:

Originally Posted by astrogeek

How about this...

Code:

echo 1 > /path/to/count.txt

Then create cron script...

#!/bin/bash
COUNT = $(</path/to/count.txt)

... do something with COUNT ...

((COUNT = $COUNT + 1))
echo "$COUNT" > /path/to/count.txt

Let me first of all say that this is an excellent solution to this problem. And rep has been given accordingly, and I realize the code is a mockup of sorts.

However, just to avoid anybody becoming disgruntled. This:

Code:

COUNT = $(</path/to/count.txt)

Will not work due to the spaces, it should be:

Code:

COUNT=$(</path/to/count.txt)

So, again, no critique against the code. Just correcting a minor error.

Best regards,
HMW

astrogeek · 08-15-2015, 12:23 PM

Quote:

Originally Posted by HMW

However, just to avoid anybody becoming disgruntled. This:

Code:

COUNT = $(</path/to/count.txt)

Will not work due to the spaces, it should be:

Code:

COUNT=$(</path/to/count.txt)

Thanks! Of course, you are correct.

I usually write a test case before posting code, and did so with this one... but then I typed it in instead of copy/paste and had some sort of brain glitch... a recurring problem lately!

Thanks for the correction (also fixed in my post to prevent problems for others trying to use it)!

HMW · 08-15-2015, 12:27 PM

Quote:

Originally Posted by astrogeek

Thanks for the correction (also fixed in my post to prevent problems for others trying to use it)!

No problem astro, I get those glitches too!

Best regards,
HMW