Question

I’ve made a bash script which I run every hour with crontab, and I need to store one variable so that I can access it the next time I run it. The script changes the variable every time it runs, so I can’t hardcode it in. Right now I am writing it to a txt file and then reading it back. Is there a better way to do it than this? And the way I am reading the txt file is something I found on here, I don’t understand it, and it’s kinda clunky. Is there not a built in command for this? Anyway, here’s the applicable code, with some of the variables changed to make it easier to read.

while read x; do
  var=$x
done < var.txt

# Do some stuff, change var to a new value

echo $var > var.txt

The variable is only a single integer, so the text file feels overkill.

Answer 1

There’s no need to use var; x will be in scope for the current shell. Alternately,

read var < var.txt
# do stuff with var
echo $var > var.txt

---

I recommend using a simple text file to store the variable. However, there is the (highly questionable) option of a self-modifying script. FOR ENTERTAINMENT PURPOSES ONLY!

#!/bin/bash

read val < <( tail -n 1 "$0" )

(( val++ ))
echo "$val"

tmp=$(mktemp /tmp/XXXXXXX)
sed '$s/.*/'$val'/' "$0" > "$tmp"
mv "$tmp" "$0"

exit
0

The key is to have the next-to-last line be the exit command, so nothing after it will execute. The last line is the variable value you want to persist. When the script runs, it reads from its own last line. Before it exits, it uses sed to write a copy of itself toa temp file, with the last line modified with the current value of the persistent value. Then we overwrite the current script with the temp file (assuming we will have permission to do so).

But seriously? Don’t do this.