Docker cron daemon pattern example

app-cron

# /etc/cron.d/app-cron
0 0 * * * root bash -l -c '/usr/bin/rake.sh rake:task1 >> /dev/console 2>&1'
*/5 * * * * root bash -l -c '/usr/bin/rake.sh rake:task2 >> /dev/console 2>&1'
*/10 * * * * root bash -l -c '/usr/bin/rake.sh rake:task3 >> /dev/console 2>&1'

cron-env.sh

#!/bin/bash

# /usr/bin/cron-env.sh

set -e

echo "[`date`] Generateing /etc/environment"

if [ ! -f /usr/src/app/.cron-env-prepared ]; then
  echo "[`date`] Preparing environment ..."
else
  echo "[`date`] Environment has been previously prepared."
  exit 0
fi

BLACKLIST=(
  LS_COLORS
  HOSTNAME
  TERM
  PWD
  SHLVL
  HOME
  LESSOPEN
  LESSCLOSE
  _
  no_proxy
)

for VAR in `compgen -e`; do
  eval unique_value=\$$VAR

  if [[ ${BLACKLIST[*]} =~ "$VAR" ]]; then
    echo "== Skipping blacklisted: $VAR"
  else
    sed_escaped_value="$(echo "$unique_value" | sed 's/"/\\"/g')"
    echo "$VAR=\"$sed_escaped_value\"" >> /etc/environment
  fi
done

touch /usr/src/app/.cron-env-prepared
echo "[`date`] Generated /etc/environment"
exit 0

docker-entrypoint.sh

#!/bin/bash
set -e

VALUE="$1"

echo "Using input value: $VALUE"

case $VALUE in
cron)
  echo "Starting cron..."
  /usr/bin/cron-env.sh
  cron -L 15 -f
  ;;
*)
  if [ ! -f /usr/src/app/.app-prepared ]; then
    echo "-- NOTE: Backend Environment has NOT been previously prepared. --"
    echo "-- Backend Environment is prepared by executing the /usr/bin/prepare-app.sh script. --"
  else
    echo "Environment has previously been prepared."
  fi

  echo "Executing: $@"
  exec "$@"
  ;;
esac

echo "Goodbye!"

prepare-app.sh

#!/bin/bash

# /usr/bin/prepare-app.sh

set -e

echo "Verifying app environment configured..."

if [ ! -f /usr/src/app/.app-prepared ]; then
  echo "Preparing environment ..."
else
  echo "Environment has been previously prepared. Skipping execution of /usr/bin/prepare-app.sh"
  echo "Backend ready to start."
  exit 0
fi

REQUIRED=(
  A_REQUIRED_ENV_VAR
  ANOTHER_REQUIRED_ENV_VAR
)
for unique in "${REQUIRED[@]}"; do
  eval unique_value=\$$unique
  if [ "$unique_value" ]; then
    echo "Required Value Set: $unique"
  else
    echo >&2 "Required Value '$unique' must be set"
    echo >&2 "Goodbye!"
    exit 1
  fi
done

touch /usr/src/app/.app-prepared
echo "Backend ready to start."
exit 0

rake.sh

#!/bin/bash

# /usr/bin/rake.sh

set -e

VALUE="$1"

/usr/bin/prepare-app.sh | while read -r line; do echo "[$VALUE] $line"; done

echo "[$VALUE] Executing rake task: $VALUE"
cd /usr/src/app && /usr/local/bin/bundle exec rails $VALUE | while read -r line; do echo "[$VALUE] $line"; done