Title : Deploying cron programmaticaly the unix way
Author: Solène
Date  : 31 May 2018
Tags  : unix

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 -