Backup PostgreSQL DB running in Docker container

backup.sh

#!/bin/zsh

# Constants {
postgres_container_name=""
main_user=""
# }

# Make a copy of the DB files within the container

postgres_home_dir=$(docker exec --user postgres $postgres_container_name bash -c "echo \$HOME")
postgres_backup_container_dir="$postgres_home_dir/backup"

cmd="rm -rf $postgres_backup_container_dir ; pg_basebackup -U \$POSTGRES_USER -D $postgres_backup_container_dir --format=plain --write-recovery-conf --checkpoint=fast --progress && echo SUCCESS"

docker exec --user postgres -t $postgres_container_name bash -c $cmd

# Copy the DB files to the host

main_user_home_dir=$(cat /etc/passwd | grep $main_user | cut -d : -f 6)
host_backup_dir="$main_user_home_dir/backup"

docker cp $postgres_container_name:$postgres_home_dir/backup $host_backup_dir

# (If you wonder why not copy the DB files right away then know that doing
# so poses a risk of data corruption. pg_basebackup is a utility for safely
# copying DB files. Then, the copied files are safe to read since they are
# backup, i.e. not being read by the DB.)