Monitoring WAL lag in PostgreSQL

pg_xlog_diff.sh

#!/usr/bin/env bash
# set -x

PG_PRIMARY='db1.example.com'
PG_STANDBY='db2.example.com'
PG_ROLE='user'
PG_DB='test'
PASSWORD='password'

primary_xloc=$(PGPASSWORD=$PASSWORD psql -U$PG_ROLE -d $PG_DB -h $PG_PRIMARY -q -t -A -c 'select * from pg_current_xlog_location()' | cut -c 3- | bc)
standby_xloc=$(PGPASSWORD=$PASSWORD psql -U$PG_ROLE -d $PG_DB -h $PG_STANDBY -q -t -A -c 'select * from pg_last_xlog_replay_location()' | cut -c 3- | bc)

if [ $(( $primary_xloc - $standby_xloc )) -eq 0 ]; then
    exit 0
else
    exit 1
fi