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Deploying cron programmaticaly the unix way

Written by Solène, on 31 May 2018.
Tags: #unix

Comments on Fediverse/Mastodon

Here is a little script to automatize in some way your crontab deployment when you don’t want to use a configuration tool like ansible/salt/puppet etc… This let you package a file in your project containing the crontab content you need, and it will add/update your crontab with that file.

The script works this way:

$ ./install_cron crontab_solene

with crontab_solene file being an actual crontab correct, which could looks like this:

## TAG ##
MAILTO=""
*/5 * * * *  ( cd ~/dev/reed-alert && ecl --load check.lisp )
*/10 * * * * /usr/local/bin/r2e run
1 * * * * vacuumdb -azf -U postgres
## END_TAG ##

Then it will include the file into my current user crontab, the TAG in the file is here to be able to remove it and replace it later with the new version. The script could be easily modified to support the tag name as parameter, if you have multiple deployments using the same user on the same machine.

Example:

$ crontab -l
0 * * * * pgrep iridium | xargs renice -n +20
$ ./install_cron crontab_solene
$ crontabl -l 
0 * * * * pgrep iridium | xargs renice -n +20
## TAG ##
MAILTO=""
*/5 * * * *  ( cd ~/dev/reed-alert && ecl --load check.lisp )
*/10 * * * * /usr/local/bin/r2e run
1 * * * * vacuumdb -azf -U postgres
## END_TAG ##

If I add to crontab_solene the line 0 20 * * * ~/bin/faubackup.sh I can now reinstall the crontab file.

$ crontabl -l 
0 * * * * pgrep iridium | xargs renice -n +20
## TAG ##
MAILTO=""
*/5 * * * *  ( cd ~/dev/reed-alert && ecl --load check.lisp )
*/10 * * * * /usr/local/bin/r2e run
1 * * * * vacuumdb -azf -U postgres
## END_TAG ##
$ ./install_cron crontab_solene
$ crontabl -l 
0 * * * * pgrep iridium | xargs renice -n +20
## TAG ##
MAILTO=""
*/5 * * * *  ( cd ~/dev/reed-alert && ecl --load check.lisp )
*/10 * * * * /usr/local/bin/r2e run
1 * * * * vacuumdb -azf -U postgres
0 20 * * * ~/bin/faubackup.sh
## END_TAG ##

Here is the script:

#!/bin/sh
    
if [ -z "$1" ]; then
    echo "Usage: $0 user_crontab_file"
    exit 1
fi
    
VALIDATION=0
grep "^## TAG ##$" "$1" >/dev/null
VALIDATION=$?
grep "^## END_TAG ##$" "$1" >/dev/null
VALIDATION=$(( VALIDATION + $? ))
    
if [ "$VALIDATION" -ne 0 ]
then
    echo "file ./${1} needs \"## TAG ##\" and \"## END_TAG ##\" to be used"
	exit 2
fi
    
crontab -l | \
    awk '{ if($0=="## TAG ##") { hide=1 };  if(hide==0) { print } ; if($0=="## END_TAG ##") { hide=0 }; }' | \
    cat - "${1}" | \
    crontab -