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vladimir pavloski
Asked: 2019-04-05 21:22:23 +0800 CST

I need help to get crontab working in Xubuntu 16.04.6

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I have set a crontab job like this : 

@reboot  xmodmap -e \"keycode 105=Delete\"
@reboot  xmodmap -e \"keycode 66=Home\"

and saved on default directory suggested by NANO (/tmp/..../.../cron), but it's not working. I can list cron job, cron service is running, the output of journalctl -b 0 _SYSTEMD_UNIT=cron.service displays the jobs without errors and so on. Besides that, I also can't run a script at boot: @reboot /home/user/scripts/myscript.sh

Why is cron not working as I expect?

scripts cron xubuntu

2 Answers

  1. xmodmap modifies the keycode mapping for the current session. So if you run it in crontab, it opens a session, modifies the mapping and closes the session again - and its effect is gone.

    You should instead run the xmodmap commands in your profile $HOME/.profile (or in your $HOME/.xinitrc if you are starting the X session only at login).

    man xmodmap says:

    It is usually run from the user's session startup script to configure the keyboard according to personal tastes.

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  2. Take a look at man cron, or other examples on the Internet. I believe that the problem you are having is you're not using the full path the the binaries. 

    | melon@pc ~>$ which xmodmap
/usr/bin/xmodmap

    In your cron -e, instead of xmodmap, you would put /usr/bin/xmodmap.

    But, even with this change, you won't see any key changes.

    Linux is great in that it provides various solutions to problems, but look into how others have solved similar problems. For example, xmodmap is typically invoked in .xinitrc, which is executed when your X session starts. xmodmap really won't work the way you're trying, at least for your normal user.

    Since you're interested in xmodmap, I recommend you look at documentation and examples and forget about cron completely for this current issue.

    If you are unwilling to look at documentation and other examples, or want a real understanding of how things work, you can look up the man pages:

    XMODMAP(1)                            General Commands Manual                            XMODMAP(1)

NAME
       xmodmap - utility for modifying keymaps and pointer button mappings in X

SYNOPSIS
       xmodmap [-options ...] [filename]

DESCRIPTION
       The  xmodmap  program is used to edit and display the keyboard modifier map and keymap table
       that are used by client applications to convert event keycodes into keysyms.  It is  usually
       run  from  the user's session startup script to configure the keyboard according to personal
       tastes.

    This last sentence has the answers to your problem.

    At this point, you don't know if cron isn't working (this is your suspicion), or if xmodmap fired, but you don't see the effects. The latter is the likely scenario. Your user needs to execute xmodmap within the current X session for xmodmap changes to be effective.

    Once you get xmodmap working without cron, you can solve the next problem. As mentioned earlier, you need full system paths, but you also need to tell cron how to run the file. You've simply pointed to the file, which works on your terminal, as it uses the shebang/interpreter (ie: #!/bin/bash). So, for cron to run a bash file...

    | melon@nifflheim ~>$ which bash
/usr/bin/bash
| melon@nifflheim ~>$ crontab -e
...
@reboot /usr/bin/bash /home/user/scripts/myscript.sh

    Though, do you really want root running a script found in your user directory? With what you've learned so far, maybe you add /usr/bin/bash /home/user/scripts/myscript.sh (or source it) in ~/.xinitrc instead ^_~.

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