cron is a program that, continuously sitting in the background, runs an executable at a set time of day, as set up in a configuration file crontab. anacron is a program, that, continually called by cron, runs an executable at a set day (week, month), as set up in a configuration file anacrontab.

An (ana)cron job is every such entry in the (ana)crontab, that tells (ana)cron which executable to run at which time/day. Whereas cron will skip a job if the computer was down at the set time, anacron will only skip a job if the computer was down during the whole set day (week, month). That is, anacron seizes every chance it gets to run the executable. Therefore, whereas cron is made for servers, that are always on, anacron it made for personal computers, such as desktops, laptops, tablets, that are sometimes turned off.

Here, we want to go one step further and make anacron only run an executable if the computer is online and/or on power supply: Instead of

  • configuring anacron by listing the executables and their frequency (day, week or month) in a configuration file,
  • we place them in appropriate folders, depending on their frequency (daily, weekly or monthly) and condition under which to be run (none, online, on power supply, or both).

Aim

Run the executables in the folders

~
| .config/
| | anacron.daily/
| | | on/
| | | on_ac/
| | | on_line/
| | | on_line_ac/
| | anacron.weekly/
| | | on/
| | | on_ac/
| | | on_line/
| | | on_line_ac/
| | anacron.monthly/
| | | on/
| | | on_ac/
| | | on_line/
| | | on_line_ac/

daily (in anacron.daily), weekly (in anacron.weekly) and monthly (in anacron.monthly) either

  • unconditionally (in on),
  • when on power supply (= AC/DC), for example, updating a file index (in on_ac),
  • when online, for example, updating an online backup (in on_line), or
  • when on power supply and online, for example, updating software (in on_line_ac).

Prerequisites

Besides having installed cron and anacron, the following programs have to be marked executable and placed into $PATH (note that we will add the folder ~/bin to $PATH, so that they all can be put inside ~/bin.):

  • The script run-parts.sh runs sequentially all executables in a folder.
  • The script on_ac_power tests whether the computer runs on battery or AC/DC.

  • The command-line tool nm-online tests whether the computer is online. However, the computer may be online, but behind a paywall or certain ports are blocked, such as the port 21 for SSH-connections. Therefore a better bet for checking connectivity is the program nc, though the check depends on a chosen server being up and running, say github.com. Both nm-online and nc are included in many Linux distributions.

  • The schedulers chrt and ionice run programs in the background, so that they will not hog the CPU respectively hard disc. Both chrt and ionice are included in many Linux distributions.
  • The auxiliary tool cronic to pass on (standard out and standard error) output only if the passed command (= cron job) fails (exits nonzero or crashes). For this error output (by default readable by mailx) to be most useful, the shell script of every cron job should begin with set -o xtrace -o errtrace -o errexit -o nounset -o pipefail to show debug output, exit on error or use of undeclared variable or pipe error. The tiny shell script cronic is available at Chuck Houpt’s homepage (and one of its many variants, such as chronic in Joey Hess’s moreutils, included in many Linux distributions).

Quick-Start

To readily set up the anacron as shown next step by step, clone this repository and follow its quick-start guide.

Implementation

We assume the user name konfekt, that is, the user’s home directory ~ is /home/konfekt.

Setting up the Executables

Create the folder ~/.config/cron.hourly. Inside it, put the four executables

anacrontab_on.sh
anacrontab_on_ac.sh
anacrontab_on_line.sh
anacrontab_on_line_ac.sh

First, anacrontab_on_line_ac.sh runs anacron only if the computer is online and on AC/DC, and reads

#!/bin/sh

# Do not run jobs when on battery power
(command -v on_ac_power >/dev/null /2> &1 && on_ac_power) || exit 0

# Do not run jobs when off-line
# (command -v nm-online >/dev/null /2> &1 && nm-online --timeout=10 --quiet) || exit 0
(nc -zw3 github.com 22) || exit 0

mkdir --parents "$XDG_DATA_HOME/anacron"
/usr/sbin/anacron -S "$XDG_DATA_HOME/anacron" -t "$XDG_CONFIG_HOME/anacrontab/on_line_ac" -s

Accordingly anacrontab_on_line.sh runs anacron only if the computer is online, and reads

#!/bin/sh

# Do not run jobs when off-line
# (command -v nm-online >/dev/null /2> &1 && nm-online --timeout=10 --quiet) || exit 0
(nc -zw3 github.com 22) || exit 0

mkdir --parents "$XDG_DATA_HOME/anacron"
/usr/sbin/anacron -S "$XDG_DATA_HOME/anacron" -t "$XDG_CONFIG_HOME/anacrontab/on_line_ac" -s

and anacrontab_on_ac.sh runs anacron only if the computer is on AC/DC, and reads

#!/bin/sh

# Do not run jobs when on battery power
(command -v on_ac_power >/dev/null /2> &1 && on_ac_power) || exit 0

mkdir --parents "$XDG_DATA_HOME/anacron"
/usr/sbin/anacron -S "$XDG_DATA_HOME/anacron" -t "$XDG_CONFIG_HOME/anacrontab/on_line_ac" -s

and anacrontab_on.sh runs anacron unconditionally, and reads

#!/bin/sh

mkdir --parents "$XDG_DATA_HOME/anacron"
/usr/sbin/anacron -S "$XDG_DATA_HOME/anacron" -t "$XDG_CONFIG_HOME/anacrontab/on_line_ac" -s

Setting up Anacron

Create the folder ~/.config/anacrontab. Inside it, put the four anacron configuration files

~/.config/anacrontab/on
~/.config/anacrontab/on_ac
~/.config/anacrontab/on_line
~/.config/anacrontab/on_line_ac

First, ~/.config/anacrontab/on_line_ac runs all due daily, weekly and monthly anacron jobs to be run when the computer is online and on AC/DC. It reads

SHELL=/bin/bash
USER=konfekt
HOME=/home/konfekt
BASH_ENV=/home/konfekt/.bash_profile
XDG_CONFIG_HOME=/home/konfekt/.config
XDG_CACHE_HOME=/home/konfekt/.cache
XDG_DATA_HOME=/home/konfekt/.local/share
PATH=/home/konfekt/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
# RANDOM_DELAY=5
# START_HOURS_RANGE=10-16

# period  delay  job-identifier         command
1         0      daily.on_line_ac       cronic chrt --idle 0 ionice -c2 -n7 run-parts.sh -v $XDG_CONFIG_HOME/anacron.daily/on_line_ac
7         5      weekly.on_line_ac      cronic chrt --idle 0 ionice -c2 -n7 run-parts.sh -v $XDG_CONFIG_HOME/anacron.weekly/on_line_ac
@monthly  10     monthly.on_line_ac     cronic chrt --idle 0 ionice -c2 -n7 run-parts.sh -v $XDG_CONFIG_HOME/anacron.monthly/on_line_ac

Accordingly,

  • ~/.config/anacrontab/on_line runs all due daily, weekly and monthly anacron jobs to be run when the computer is online.
  • ~/.config/anacrontab/on_ac runs all due daily, weekly and monthly anacron jobs to be run when the computer is on AC/DC.
  • ~/.config/anacrontab/on runs all due daily, weekly and monthly anacron jobs to be run unconditionally.

They read accordingly, that is, on_line_ac is replaced everywhere by on_line, on_ac, on respectively.

Setting up Cron

Add a file

~/.config/crontab

with content

SHELL=/bin/bash
USER=konfekt
HOME=/home/konfekt
BASH_ENV=/home/konfekt/.bash_profile
XDG_CONFIG_HOME=/home/konfekt/.config
XDG_CACHE_HOME=/home/konfekt/.cache
XDG_DATA_HOME=/home/konfekt/.local/share
PATH=/home/konfekt/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
# RANDOM_DELAY=5
# START_HOURS_RANGE=10-16

# period        command
* * * * * *     cronic chrt --idle 0 ionice -c2 -n7 run-parts.sh -v $XDG_CONFIG_HOME/cron.hourly

On the terminal, run

crontab ~/.config/crontab

This way, cron checks, via anacron, every minute for all due (daily, weekly and monthly) jobs to be run unconditionally or when the computer is either online, on AC/DC, or online and on AC/DC.

Extras

If you run jobs that need an SSH connection, for example an online backup over rsync, then the program keychain may interest you: A shell script that will cache your passphrase, so that cron jobs can access it.

To set it up, initialize it on start-up of the user’s session by

eval "$(keychain --agents ssh --quiet --eval ~/.ssh/id_rsa)"

(for example, KDE has the folder ~/.config/autostart-scripts for this purpose) and call

[ -z "$HOSTNAME" ] && HOSTNAME="$(uname -n)"
[ -f ~/.keychain/"$HOSTNAME"-sh ] && . ~/.keychain/"$HOSTNAME"-sh

before the cron job, for example one using rsync to back up files to a distant SSH server.

Example

For an example anacron job, here is a sample shell script which backs up all the files inside the home directory that are listed in files, except those listed in exclude, to a server; see this repository for an expanded version and full instructions:

#!/bin/bash

set -o xtrace -o errtrace -o errexit -o nounset -o pipefail

# set up keychain to pass passphrase to ssh in cron jobs
[ -z "$HOSTNAME" ] && HOSTNAME="$(uname -n)"
[ -f ~/.keychain/"$HOSTNAME"-sh ] && . ~/.keychain/"$HOSTNAME"-sh
# CONFIG
FROM_FOLDER=$HOME
TO_FOLDER="$USER@rsync.server.com:/users/$USER"

FILES_FILE=$XDG_CONFIG_HOME/backup/files
EXCLUDE_FILE=$XDG_CONFIG_HOME/backup/exclude

LOG_FILE=$XDG_CACHE_HOME/backup/cloud/log

# because --files-from disables --recursive in --archive it must enable explicitly
RSYNC_BKP_ARGS="--recursive --archive --hard-links --ignore-errors --modify-window=1 --delete --compress --partial --human-readable --info=progress2 "
SSH_ARGS="-v -P"

# BACKUP Local -> Server:
LOG_FILE_DIR=$(dirname "${LOG_FILE}")
mkdir --parents "$LOG_FILE_DIR"

rsync $RSYNC_BKP_ARGS --rsh="ssh $SSH_ARGS" --log-file="$LOG_FILE" --files-from="$FILES_FILE" --exclude-from="$EXCLUDE_FILE" "$FROM_FOLDER" "$TO_FOLDER"

To run it daily as soon as the computer is online, put it into the folder :

~
| .config/
| | anacron.daily/
| | | on_line/