Cron, Batch and At
These three commands are used to run commands at some other
time. They differ in their usage, their environment, and their
default actions, so are sometimes a source of confusion.
If you are having trouble with cron, you might want to read
Cron is not working first.
If you don't know how to create the script that cron or at will
call, see New To Unix
The "batch" command really just calls "at" with special flags
at -q b -m now
but "cron" is truly a totally separate command.
One of the common problems posted to the Unix newsgroups goes
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
Note: this article covers both SCO and Linux cron. I keep
the SCO stuff here for those unfortunate folks who haven't been able
to move to Linux yet. There's lots here to help you
do that: SCO/Linux Transition Guide is a start.
Very often, it's just the PATH that is different. For example,
root's environment usually has
but cron (in SCO OSR5, see below for Linux which has a much smarter cron system) sets its environment
Note: the man page for SCO 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)
IFS= <TAB><LF> (not actually set to this; edited for viewing)
LOGNAME=root (or whomever's job is running)
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). But don't make the mistake of
exporting them. This doesn't work in crontab:
That would end up with your scripts seeing FOO as "xyz;exort FOO"
rather than the "xyz" you wanted. You don't need to worry about export anyway;
cron will do that automatically.
It also allows you to control
who mail is sent to when the commands generate output, or to have
no mail sent at all (MAILTO=""). That saves you from having to
redirect output if you don't care about anything the script has to say.
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. Also, some of the
files have an extra argument: files put in /etc/cron.d specify a
user before the command to be run. Don't forget that, or they won't work.
And of course files in /etc/cron.daily (hourly, etc.) are just scripts
that are called by "run-part"entries in /etc/crontab. Note that
/etc/crontab also requires that extra "user" field. If you
run "crontab -l", you'll be listing files from /var/spool/cron,
and these are familiar Unix format.
If your command script requires anything variables not
in cron's normal environment and not otherwise set (in the script or in
a Linux crontab file), it will fail. If
your script required Korn shell features, 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. On Linux, it uses Bash).
A simple way to fix the environment issue is to use "at" to set up your
environment. You can, for example, type:
at now + 1 hour <ENTER>
"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"
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/bin/runfake that you would
invoke from cron, not "fakecommand".
Recently someone asked me why their crontab wasn't working. They
understood that they needed to set their environment, but what they
did was something like this:
17 5 * * * ./setmyenv.cmd;domystuff.cmd
That'll never work, and wouldn't work from the shell either. You
would need to add a ". ./setmyenv.cmd" inside "domystuff" (that's
dot space dot slash setnyenv.cmd).
You probably know that you do not edit the crontab files
directly (on Linux that's fine, but not on most Unixes). Some people use "crontab -e", but a more safe procedure
crontab -l > /tmp/mycrontab
(make your changes)
Or, if for another user:
crontab -u john -l > /tmp/mycrontab
(make your changes)
crontab -u john /tmp/mycrontab
Two reasons that is safer: first, if your EDITOR variable is accidentally set to something you don't know (as it may very well be on an unfamiliar system), you may accidentally wipe out the crontab getting out of it. Another reason for avoiding -e is the proximity of e and r on your keyboard - if you type -r, that crontab is gone instantly.
By the way, some old systems wouldn't work if you did (for example) "export EDITOR=vi" rather than "export EDITOR=/usr/bin/vi". This was simply a matter of whether they used execlp instead of execl (execlp searches $PATH).
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
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.
Note for some SCO 5.0.6 users
Some versions of 5.0.6 cron had a bug where a cron job could be run twice. The
fix is to get the proper version of cron or (if that's not possible) be sure your
script can handle that possibility (set some flag that says it has run or is running and exit
if the other instance sees that).
Got something to add? Send me email.
More Articles by Tony Lawrence
Find me on Google+
© 2014-12-09 Tony Lawrence