UNIX for Dummies Questions & Answers

Hi,

From time to time, I have a cron job that I have to enable/disable/enable/disable. I just want to know if there is any way to insert cron entries or do this to enable/disable bit via a script instead.

It seems 'safer' to do this via a script rather than manual? I think or is this such a bad idea

Rather than inserting and deleting lines in your crontab file, I'd suggest manually adding the line originally. Then you can disable it by adding a # to the start of the line and enable it by removing the #.

Note that the default editor used by crontab -e is vi (which isn't really suitable for scripting), but you can use EDITOR=ed crontab -e or EDITOR=ex crontab -e to specify an editor that is easy to use in a script.

Or you could put something at the start of the script to 'test' for the presence of a file and, if not present, just exit.

You could then stop the job running by deleting the file and reschedule the job by using 'touch' to create the file.

The safest way is to have your script do it. [ -f /path/to/myscript-lock ] && exit 0

If you must edit cron automatically, I put a unique ID in the cron line to make it easy to match and difficult for a script to mistake.

Code:

0 * * * * /path/to/myscript.sh # 1234123132

The way I would do this would be to have two crontab files.

When you want to enable the (new) cron entries, use the script to copy the "enabled" crontab file over to crontab and restart crond.

When you want to disable the (new) cron entries, use the script to copy the "disabled" crontab file over to crontab and restart crond.

For the insane hackers here are some script commands that edit the crontab.
Say the entry (with a unique identifier) is

Code:

0 * * * * /path/to/myscript.sh # 1234123132

then one can disable it with

Code:

echo "/1234123132/ s/^/#/
w
q" | EDITOR=ed crontab -e

and enable with

Code:

echo "/1234123132/ s/^#//
w
q" | EDITOR=ed crontab -e

An alternative method, works with Unix cron:
disable

Code:

crontab -l | sed '/1234123132/ s/^/#/' | crontab

enable

Code:

crontab -l | sed '/1234123132/ s/^#//' | crontab

Vixie cron (most Linux distros) does not allow crontab alone but needs crontab -.

I would look for /# ID$/, not just /ID/, in case you're unfortunate enough to find another ID ending in the right digits but beginning with something else.