Cron is an application available on many platforms designed to allow one to schedule invocations of applications at set calendar date and times on a computer.When an application is invoked, it appears to run as a particular user, BUT with a minimal environment variable setup and typically in a preset command shell.The limited environmental shell setup is the first problem users typically encounter - they expect that they will find various variables that are set during their machine's startup environment during login, or their own personalized login environment - but those are not present.Thus one must decide to take one of two courses of action:
- write the application in such a way that it needs no special variables
- set the special variables up that may be needed
- create some sort of wrapper application that presets the operational environment
- how to invoke Tk apps who do not need an X display - perhaps some sort of image manipulation extension
Another connection with cron - tkcron is at least one Tk application that allows a GUI interface to setting up entries in cron.
See also cron and Tcl.
Tcl related applications that relate to cron:
What: cron interface Where: ftp://avahi.inria.fr/tcl/tkcron/ ftp://dslab.csie.ncu.edu.tw/pub/tcl/ ftp://kolal.inria.fr/tcl/ ftp://ftp.procplace.com/pub/tcl/sorted/packages-7.6/misc/tkcron-2.12/tkcron-2.12.tcl.gz Description: TkCron is a Tk 4 simple interface to crontab creation. Updated: 02/1997 Contact: mailto:[email protected] (Cedric Beust - TkCron v1 author) mailto:[email protected] (Wann-Ban Chi - TkCron v2 author) mailto:[email protected] (Colin Lee - TkCron v2 author) What: cronjob Where: ftp://ftp.procplace.com/pub/tcl/sorted/packages-8.0/apps/cronjob/1.0/cronjob-1.0.tar.gz Description: Tcl application that provides cron like functionality. Updated: 12/2001 Contact: mailto:[email protected] (Arjen Markus) What: Gnome Moleskine Where: http://www.micampe.it/software/moleskine/ Description: Source code editor for the GNOME desktop. Makes use of Scintilla. Has lexers for ASP, Baan, Bullant, Eiffel KW, LISP, NN Crontab, PHP Ruby, Tcl, VB Script, and more. Currently at version 0.7.7 . Updated: 02/2002 Contact: See web site What: LARD Where: http://apt.cs.manchester.ac.uk/projects/tools/lard/ Description: Language for Asynchronous Research and Development (LARD) is a hardware description language, which uses Tcl as its scripting language. Updated: 07/2003 Contact: See web site What: MAT Where: http://www.ee.ryerson.ca/%7Esblack/mat Description: Enterprise Monitoring and Administration Tool (MAT), aka MATtool. Used to manage a heterogeneous UNIX network. The monitoring daemon maintains historical info on several monitored parameters with graphing and event triggering when thresholds are exceeded. Also has self-upgrade ability. Allows you to add/modify/delete users, groups, hosts, email aliases, filesystems, crontabs, DNS configs, domains and records, login messages, services, NIS records, syslogd, syslog files, routes, processes, lastlog, disk space, and exported file systems, as well as network connectivity, required processes, disk usage, CPU usage, run queues, logins, ftp server status, and SMTP server status. Available for HPUX 10.20/11.00, IRIX 6, Linux 2.x, Solaris 5.x, using Tcl/Tk 8.x (Perl 5.x is optional). Currently at version 0.29 . Updated: 06/2002 Contact: mailto:[email protected] (S. Mark Black) What: Newsspy Where: ftp://18.104.22.168/pub/newsspy/newsspy-1.0.tgz ??? Description: A Tcl program to connect to a NNTP news server and scan article headers for special keywords. If found, read article and append to a file. Can be invoked from cron. Updated: Contact: mailto:[email protected] (hofmann) What: vcron Where: http://www.linux-kheops.com/pub/vcron/vcronGB.html http://www.linux-kheops.com/pub/vcron/vcron-1.7.tgz Description: A visual cron/at interface, allowing one to view, modify, delete and add actions. Written in Tcl/Tk 8.0 and has an inline manual. Updated: 09/2001 Contact: mailto:[email protected] (Daniel Roche)
Expect applications are notorious for behavior that turns mysterious in a cron context. Here  and here  are comments about the situation.