Script for counting number of Postgres txs performed since starting the script

count_txs.sh

#!/bin/sh

# Requires `psql`. The script effectively wraps a `psql` invocation.
# This is what I use for installing psql: `apt update && apt install postgresql-contrib -y`.

# Checks Postgres's tx counter, then blocks until ctrl-c is pressed, then checks tx counter again,
# prints difference, and exits. Start it before performing some database interactions, then press
# Ctrl-C to get the number of transaction commits that were performed since starting script.
#
# Should be called with the psql command as argument:
#
# Examples:
# ./count_txs.sh psql -E postgresql://a:b@c:1234/d
# ./count_txs.sh docker exec -it $SOME_CONTAINER psql -E postgresql://user:pass@host:1234/db

psql_command=$@

get_second_line() {
  sed -sn 2p
}

trim_whitespace() {
  tr -d ' '
}

print_tx_counter_value() {
  $psql_command --no-align \
    -c 'SELECT sum(xact_commit+xact_rollback) FROM pg_stat_database;' \
    | get_second_line \
    | trim_whitespace
}

counter_at_start="$(print_tx_counter_value)"

calc_difference() {
  # using python, because expr and bc complain about non-integer output and
  # whitespace trimming doesn't seem to help
  python3 -c "print($1 - $2)"
}

print_change_since_start() {
  counter_at_exit="$(print_tx_counter_value)"
  echo ""
  echo "counter_at_start $counter_at_start"
  echo "counter_at_exit $counter_at_exit"
  calc_difference $counter_at_exit $counter_at_start
}

finish_script() {
  print_change_since_start && exit
}

trap finish_script 2

echo "ctrl-c to stop counting"
sleep infinity