crontab permissions

[prasadb]

hello all,

will anybody please tell me how to assign the permission to modify the crontab to some another user..?

and also let me know how to recover the crontab file is it is deleted accidently.

thanx in advance.

[prasadb]

in addition to above ..please tell me what could happend if i execute only this command..

#crontab

(without any options).i have not made any experiment as i came to know it will remove all the cron jobs..please let me know how much it is true...

thanx in advance..

[Yogeeraj_1]

hi,

You recover your crontab from backup.

Path would be:
/var/spool/cron/crontabs/

hope this helps!
kind regards
yogeeraj

[Yogeeraj_1 · Accepted solution]

hi again,

>(without any options).i have not made any experiment as i came to know it will remove all the cron jobs..please let me know how much it is true...


Yes. you will lose all the crontab entries if you do this (followed by: control+D).

In fact, when you launch "crontab", the program will wait for your inputs. Pressing Control+D will commit the changes made. But, control+C will discard all changes and the current crontab settings will remain the same.


hope this help!

kind regards
yogeeraj

[Dennis Handly]

>will anybody please tell me how to assign the permission to modify the crontab to some another user?

You can't. Only you and root can modify it.
In fact on my 11.23 system, the original user can't read it (except by crontab -l):
$ ll /var/spool/cron/crontabs
-r-------- 1 root GRP-name USR-name

You could use chmod a+r on that file so anyone can read it. I suppose you could allow group write: chmod g+w

Why do you want to do this?

>please tell me what could happen if I execute only this command..
>#crontab
>(without any options). ... as I came to know it will remove all the cron jobs.

This will empty the crontab entry for the user.

[prasadb]

Thanks Yogiraj..i got it...

but tell me what will happen if i run #crontab without any option...

[Yogeeraj_1]

Tests carried out here:

SRV2>crontab -e
#*******************************************************************************
# min|hour |day |month|day | |script
# | |of mo| |of wk| |
#----|-----|-----|-----|-----|-------|-----------------------------------
#*******************************************************************************
#
#
#*******************************************************************************
# END OF TABLE day0->Sunday day6->Saturday
#*******************************************************************************

~
~
~
~
"/tmp/crontab3Zqcya" 11 lines, 541 characters
SRV2>crontab -l
#*******************************************************************************
# min|hour |day |month|day | |script
# | |of mo| |of wk| |
#----|-----|-----|-----|-----|-------|-----------------------------------
#*******************************************************************************
#
#
#*******************************************************************************
# END OF TABLE day0->Sunday day6->Saturday
#*******************************************************************************

SRV2>crontab



^D <==========control+D pressed
SRV2>crontab -l
SRV2>crontab
#*******************************************************************************
# min|hour |day |month|day | |script
# | |of mo| |of wk| |
#----|-----|-----|-----|-----|-------|-----------------------------------
#*******************************************************************************
#
#
#*******************************************************************************
# END OF TABLE day0->Sunday day6->Saturday
#*******************************************************************************

SRV2>crontab -l
#*******************************************************************************
# min|hour |day |month|day | |script
# | |of mo| |of wk| |
#----|-----|-----|-----|-----|-------|-----------------------------------
#*******************************************************************************
#
#
#*******************************************************************************
# END OF TABLE day0->Sunday day6->Saturday
#*******************************************************************************
SRV2>crontab
^C <==========control+C pressed
SRV2>crontab -l
#*******************************************************************************
# min|hour |day |month|day | |script
# | |of mo| |of wk| |
#----|-----|-----|-----|-----|-------|-----------------------------------
#*******************************************************************************
#
#
#*******************************************************************************
# END OF TABLE day0->Sunday day6->Saturday
#*******************************************************************************
SRV2>

[Yogeeraj_1]

>but tell me what will happen if i run #crontab without any option...

Mentioned it above.

#crontab

Will cause the program to interact with you. It just hangs waiting for your inputs.

If you
Control+C => discard changes and original content unchanged.

Control+D => commit changes and original content is LOST!!.


hope this helps!

kind regards
yogeeraj

[MarkSyder]

To recover the crontab file in case of accidental deletion, add the following line to your crontab:

0 5 * * 0 crontab -l > /tmp/cron.safe # Backup crontab file in case of accidental deletion

This creates a backup copy in /tmp every Sunday at 5 o'clock in the morning. To make the backup current type:

crontab /tmp/cron.safe

If you have more than one user doing this, make sure they use different names for the safe copy: e.g. /tmp/cron.safe.username.

Mark Syder (like the drink but spelt different)

[Dennis Handly]

>Mark: crontab /tmp/cron.safe

If you are fiddling with crontabs, always use crontab -e.
And as you suggested, use crontab -l to create a backup, every so often.

[Matti_Kurkela]

Dennis already told you to always use "crontab -e" for editing the crontab file.

The reason for this is: the "crontab" command does more than just start the editor. It also signals the cron daemon after any changes are made. Upon receiving the signal, the cron daemon knows to re-read your crontab file.

If this signalling is not done, the cron daemon will not notice your changes: it will keep executing the old version of your crontab information.

I've seen this happen: it really confuses anyone who does not know how cron works.

The best way to allow User A to edit User B:s crontab is to use sudo (or some other similar tool) to allow A to execute "crontab -e" as user B.

The sudo configuration syntax for this would be:

userA hostname=(userB) crontab -e

and the userA would have to execute this command:

sudo -u userB crontab -e

MK

[prasadb]

many many thanks to allwho took time to help solve my problem..

[prasadb]

many many thanks to allwho took time to help solve my problem..