|  Hi Readers,
Cron, Batch And At
By: A.P. Lawrence
The "batch" command really just calls at with special flags set:
at -q b -m now
but "cron" is truly a totally separate command.
One of the common problems posted to the Unix newsgroups goes something like:
"I have a command script. If I run it from the command line or with "at", it
works, but if I run it with "cron" it fails. Why?"
Not only is this a common question, but amazingly enough, it usually
generates three or four wrong answers every time it appears. The correct answer
is that it fails because "cron" runs with a different environment than what you
have. You have a certain PATH, you have other environment variables set, and "at"
deliberately notices all that and makes sure that when your command runs, all
those things will be in place. The "cron" utility does not: it has its own environment,
probably very different from yours.
Very often, it's just the PATH that is different. For example, root's environment
usually has
PATH=/bin:/etc:/usr/bin:/tcb/bin,
but cron (in SCO OSR5) sets its environment to
PATH=:/bin:/usr/bin:/usr/lbin
Note: the man page for crontab says that the PATH will be "/bin:/usr/bin:"
but the above PATH is what actually happens on my 5.0.5 system. The difference
is especially significant because of the placement of the lone ":", which adds
"." to cron's PATH. According to the manual, the "." would end up at end of the
path, and thus would be the last place searched. It actually ends up at the beginning.
As the first command found is the command executed, this can cause unexpected
results. Cron does cd to the home directory of the user whose cron job is being
run, so "." will always refer to that directory- unless the command script itself
changes directories and then issues another command.
The other likely cause for failure is a missing environment variable. Cron
only has:
HOME=/ (or the home directory of the user whose job is running)
HZ=100
IFS= <TAB><LF> (not actually set to this; edited for viewing)
LOGNAME=root (or whomever's job is running)
MAILCHECK=600
OPTIND=1
PATH=:/bin:/usr/bin:/usr/lbin
SHELL=/bin/sh
TZ=EST5EDT
Linux cron has the very nice feature of being able to set environment variables
directly in the crontab file ( see man 5 crontab on a Linux system). It also allows
you to control who mail is sent to when the commands generate output. Linux crontab
files are found in non-standard (non-standard for Unix) places, so you should
read the man pages ( or "info cron" ) carefully- it's a bit different than Unix.
See Bofcusm/1880.html for more on crontab and /etc/default/cron.
If your command script requires anything else, it will fail. If your script
required Korn shell, it would also fail (if cron notices that you are not running
/bin/sh when you change a crontab, it will remind you that it plans to use /bin/sh).
A simple way to fix this is to use "at" to set up your environment.You can,
for example, type:
at now + 1 hour <ENTER>
fakecommand <ENTER>
<CTRL-D>
"at" will spit back a job number. Change directories to /usr/spool/cron/atjobs
and you'll see that same number listed. Notice that it is owned by root with group
of cron, and that the permissions are: ---Sr-S---. Copy that file somewhere else,
then rm the file in the spool area, and take a look at the copy you made. It will
look something like this:
: at job
export _; _='/usr/bin/at'
export HZ; HZ='100'
export PATH; PATH=':/bin:/etc:/usr/bin:/tcb/bin:/usr/local/bin:/usr/local/etc'
export HUSHLOGIN; HUSHLOGIN='FALSE'
export LOGNAME; LOGNAME='root'
export MAIL; MAIL='/usr/spool/mail/root'
export SHELL; SHELL='/bin/sh'
export HOME; HOME='/'
export TERM; TERM='scoansi'
export PWD; PWD='/tmp'
export TZ; TZ='EST5EDT'
export ENV; ENV='/.kshrc'
:
# @(#) proto 23.2 91/08/29
#
# Copyright (C) 1988-1991 The Santa Cruz Operation, Inc.
# All Rights Reserved.
# The information in this file is provided for the exclusive use of
# the licensees of The Santa Cruz Operation, Inc. Such users have the
# right to use, modify, and incorporate this code into other products
# for purposes authorized by the license agreement provided they include
# this notice and the associated copyright notice with any such product.
# The information in this file is provided "AS IS" without warranty.
#
#ident "@(#)adm:.proto 1.2"
cd /tmp
ulimit 4194303
umask 22
fakecommand
Notice what great pains "at" has gone to to match your environment. Not only
do you have all your environment variables, but your ulimit and umask have been
matched and it has even cd'd you to the directory you were in when you issued
the command. So now use that script from crontab rather than "fakecommand" directly.
That is, if you copied the at script to /usr/local/bin/runfake, it is /usr/local/runfake
that you would invoke from cron, not "fakecommand".
You probably know that you do not edit the crontab files directly. Some people
use "crontab -e", but a more safe procedure is:
crontab -l > tmp/mycrontab
vi /tmp/mycrontab
(make your changes)
crontab /tmp/this
Or, if for another user:
crontab -u john -l > tmp/mycrontab
vi /tmp/mycrontab
(make your changes)
crontab -u john /tmp/this
Be careful with crontabs set to other users. Remember that cron cd's to the
user's directory when it starts up your job. If the user's directory doesn't exist
cron fails and sends mail to that user. That doesn't sound too horrible, does
it? Well, on the older 3.2v4.2 release, there was some bug somewhere that sometimes
caused /usr/sys to be removed. The "sys" user still existed, and still had its
crontab file, but when cron tried to run, it couldn't cd to the non-existent home
directory. This caused it to start sending mail to "sys" complaining. That wouldn't
have been too bad, but who reads "the "sys" user's mail? Usually nobody, so the
mail file would build up larger and larger. Eventually it would start to affect
the performance of mmdf, and mmdf would get backed up- it couldn't clear out its
own spool directories quite as fast as they were growing, which meant that the
size of the directories in /usr/spool/mmdf started growing. The larger a directory
is, the more time it takes to search it, so this made mmdf run even more slowly,
which caused it to get more behind, which, of course, caused the directories to
grow larger yet... and to add insult to injury, mmdf would itself start generating
messages about mail it couldn't deliver (now that's dumb!), and those only added
to the problem. Eventually this would get bad enough to affect performance, because
mmdf was spending every spare cpu cycle available trying to deliver mail. So sar
would show 0 idle time, the disk would be thrashing as mmdf tried to catch up,
and performanced nose dived. What a mess, and all because of a missing directory.
The "at" and "batch" programs are much less complex. Batch takes no arguments;
it just runs your program. The details of these are covered in the man pages.
--------------------------------------------------------------------------------
About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com
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