How to store state between two consecutive runs of a bash script

Question

I have bash script which runs using a cron job every minute. I would like to save the script's state for reuse in the next run.

What would be the best way to save the state (in this case a variable which is assigned a number); so in the next run, the number can be compared with the value from previous runs.

Answer 1

Example of saving and reloading variable value from file

#!/usr/bin/env bash

# The file holding persistent variable value
statefile='/var/statefile'

# Declare our variable as holding an integer
declare -i persistent_counter

# If the persistence file exists, read the variable value from it
if [ -f "$statefile" ]; then
  read -r persistent_counter <"$statefile"
else
  persistent_counter=0
fi

# Update counter
persistent_counter="$((persistent_counter + 1))"

# Save value to file
printf '%d\n' "$persistent_counter" >"$statefile"

Another way with sourcing:

#!/usr/bin/env bash

# The file holding persistent variable value
statefile='/var/statefile'

# Source the statefile
. "$statefile" 2>/dev/null || :

# Declare our variable as holding an integer
declare -i persistent_counter

# Assign default value if unset
: ${persistent_counter:=0}

# Update counter
persistent_counter="$((persistent_counter + 1))"

# Persist variable to file
declare -p persistent_counter >"$statefile"

Persisting multiple variables (an integer, a string and an array):

#!/usr/bin/env bash

# The file holding persistent variable value
statefile='/tmp/statefile'

# Initialize variables even if they are later state restored
persistent_counter=0
persistent_last_date="never"
persistent_array=()

save_state () {
  typeset -p "$@" >"$statefile"
}

# Source the statefile to restore state
. "$statefile" 2>/dev/null || :

# Set save_state call on script exit to automatically persist state
trap 'save_state persistent_counter persistent_last_date persistent_array' EXIT

# Display restored variables
printf 'Last time script ran was: %s\n' "$persistent_last_date"
printf 'Counter was: %d\n' "$persistent_counter"
printf 'Array contained %d elements:\n' "${#persistent_array[@]}"
printf '%s\n' "${persistent_array[@]}"

# Update counter
persistent_counter="$((persistent_counter + 1))"

# Update Array
if ! [ "$persistent_last_date" = 'never' ]; then
  persistent_array+=("$persistent_last_date")
fi

# Update last run time
persistent_last_date="$(date -R)"

# Now that the script exit, it will automatically trap call save_state