Crontab: Ubuntu running script (noob)

Question

I'm new to crontab and would like to run the following script from /etc/crontab:

0 15    * * *   root    bash-c 'for i in /home/dell/Downloads/*.{pdf,docx,png,jpg,PDF,DOCX}; do shred -zvu "$i" -n20; done'

I have tried with and without bash-c option, yet the script doesn't run.

My objective is:

Help is appreciated.

Consider putting this in your user crontab (i.e. `crontab -e` as user `dell`) instead of in the system crontab as root. — marcelm, Feb 27 '19 at 14:09
Thanks for all comments. Executing the command in a script and adding this all to crontab -e, instead of trying to modify /etc/crontab seems to resolve the issues. One question remains: Running -> 31 15 * * * /home/dell/shred.sh -> works. Running -> @reboot /home/dell/shred.sh -> does not work. One of my main frustrations here is that there are no error messages, no indication what is done incorrectly. Appreciate your input how I can run this script on startup. — orrp, Mar 02 '19 at 13:40

pLumo · Accepted Answer · 2019-02-27T08:33:12.707

13

You're missing a space after the command bash and the argument -c.

This should work:

0 15    * * *  root bash -c 'for i in /home/dell/Downloads/*.{pdf,docx,png,jpg,PDF,DOCX}; do shred -zvu "$i" -n20; done'

Some additional hints:

Don't run a crontab as user root if you don't need to.
You wrote that you put it in /etc/crontab file. Don't edit crontab files directly, rather use crontab -e command or sudo crontab -e for commands which need root rights. Note, that you don't put the user field in the "other" crontab files.
If you have more than one command you can use bash -c as you do, but I'd rather put the commands in a script and execute this from crontab.
To run a script on startup, you can use @reboot instead of 0 15 * * *.

edited Feb 27 '19 at 08:33

answered Feb 27 '19 at 08:25

pLumo

5

_“I'm not sure what the word root is about here.”_ See the header of `/etc/crontab`: _Unlike any other crontab you don't have to run the `crontab` command to install the new version when you edit this file and files in /etc/cron.d. These files also **have username fields**, that none of the other crontabs do._ However, in this case, only the user’s home folder is involved, so `root` should be not used in this case. – Melebius Feb 27 '19 at 08:30
1

thanks, as I never edited `/etc/crontab` directly I didn't even know this... :-D – pLumo Feb 27 '19 at 08:32
2

See also [Two Other Types of Crontab](https://help.ubuntu.com/community/CronHowto#Two_Other_Types_of_Crontab) – steeldriver Feb 27 '19 at 08:32
1

I suspect there may be a more fundamental issue with running a `bash -c` command from cron, since IIRC each command is implicitly executed by `/bin/sh -c` (or, more exactly, `$SHELL -c`). It *may* be necessary to set `SHELL=/bin/bash` (to suport brace expansion etc.) and then run the command *without* the `bash -c` wrapper. But really the best approach (as you note) is to put the bash code in an external script file. – steeldriver Feb 27 '19 at 09:18

1 Answers1