Can I create a Bash script with persistent variable-values?
For example, I initialize a variable with 0 when the script runs for the first time (during a specific time limit), and the variable increases automatically with every time the script is running.
You can't, but you can use a file to do it
#!/bin/bash
valuefile="/tmp/value.dat"
# if we don't have a file, start at zero
if [ ! -f "$valuefile" ]; then
value=0
# otherwise read the value from the file
else
value=$(cat "$valuefile")
fi
# increment the value
value=$((value + 1))
# show it to the user
echo "value: ${value}"
# and save it for next time
echo "${value}" > "$valuefile"
expr is an artifact from the 1970s. All shells compliant with the POSIX sh standard support value=$((value + 1)), whereas expr is not part of the standard and not guaranteed to be present in new, POSIX-compliant shells and operating systems.
fork() for a command substitution to capture its output; you don't need to worry about whether implementations vary (and they do vary; I've seen some supporting regex features, for example, whereas others don't). And how expr handles overflow is OS-by-OS dependent...value=$((value + 1)), which will correctly handle any partition size in bytes that your OS can handle (contrary to your prior assertion, echo $((15904148959 + 1)) works fine). If you're on a 32-bit system that can't handle that, then value=$(bc <<<"$value + 1")busybox to provide expr (which isn't standard-mandated or even referenced), but not bc (which is both). Note that busybox is fully configurable -- it can provide bc, and it can fail to provide expr. But the one that fails to provide expr is still standards-compliant; the one that fails to provide bc isn't.I'm afraid you have to save the state in a file somewhere. The trick is to put it somewhere the user will be able to write to.
yourscriptvar=0
if [ -e "$HOME/.yourscriptvar" ] ; then
yourscriptvar=$(cat "$HOME/.yourscriptvar")
fi
# do something in your script
#save it output the file
echo $yourscriptvar > "$HOME/.yourscriptvar"
yourscriptvar=$(< "$HOME/.yourscriptvar"). Documented hereI had the same problem a few days ago and I've written my own tool to do the work because there is not other way to do something similar.
gvar is a pure Bash key-value store where each user has a different collection of data. The records are stored in the user's home directory.
Here it is the most interesting functions from the source code:
get_variable() {
cat $FILE | grep -w $1 | cut -d'=' -f2
}
set_variable() {
echo $1=$2 >> $FILE
}
remove_variable() {
sed -i.bak "/^$1=/d" $FILE
}
cat is unnecessary if you use grep since grep can already take a file as an argument. Besides, I wonder how this would work well in parallel execution if it doesn't use flock or something.