Cron on Linux

Scheduled command execution for your Linux server or workstation

This document explains how to configure the cron server to runprograms automatically for you every year, month, week, day, hour, minute, and many combinations of these using Red Hat Fedora Core 3.

Cron has many uses. I have used cron to create nightly backups, update server room temperature web pages at various intervals, generate new email '.signature' files every five minutes, and syncronize files nightly via SSH over the internet. I am sure there are many more.

Cron Documentation

Cron documentation is not bad (not bad at all), but I may have to know what to lookup and how. On aRed Hat-ishsystem like Fedora Core 3, I can use rpm to look up the available man page files. First I find the main cron package:

  $ rpm -qa | grep cron
  vixie-cron-4.1-33_FC3
  anacron-2.3-32
  crontabs-1.10-7

which turns out to be vixie-cron. To see the package descriptionand the list of files it installs, I use rpm again:

  $ rpm -qil vixie-cron
  Name        : vixie-cron                   Relocations: (not relocatable)
  Version     : 4.1                               Vendor: Red Hat, Inc.
  Release     : 33_FC3                        Build Date: Thu 14 Apr 2005 06:40:32 PM CDT
  Install Date: Mon 18 Apr 2005 04:26:48 AM CDT      Build Host: tweety.build.redhat.com
  Group       : System Environment/Base       Source RPM: vixie-cron-4.1-33_FC3.src.rpm
  Size        : 114302                           License: distributable
  Signature   : DSA/SHA1, Thu 14 Apr 2005 09:36:49 PM CDT, Key ID b44269d04f2a6fd2
  Packager    : Red Hat, Inc. 
  Summary     : The Vixie cron daemon for executing specified programs at set times.
  Description :
  The vixie-cron package contains the Vixie version of cron. Cron is a
  standard UNIX daemon that runs specified programs at scheduled times.
  Vixie cron adds better security and more powerful configuration
  options to the standard version of cron.
  /etc/cron.d
  /etc/pam.d/crond
  /etc/rc.d/init.d/crond
  /etc/sysconfig/crond
  /usr/bin/crontab
  /usr/sbin/crond
  /usr/share/man/man1/crontab.1.gz
  /usr/share/man/man5/crontab.5.gz
  /usr/share/man/man8/cron.8.gz
  /usr/share/man/man8/crond.8.gz
  /var/spool/cron

From this output I can see that there are a few man pages I can viewfor more insight into cron:

  $ man crontab
  $ man -s 5 crontab
  $ man cron
  $ man crond

Use the command 'man man' to see why the '-s 5' needed to be used toview the second crontab man page.

Using Cron

To use cron, it is easiest to start with a blank slate. The crontabcommand will do that for me, but by default I will need to know how to edit and save files with vi. If I am not comfortable with vi, and do not want to learn to use it, I can use another editor. To use nano (after making sure it is installed with 'which nano'), I could execute this command:

  $ export EDITOR=nano

before using the crontab command:

  $ crontab -e

which will start the editor (or vi) and load the current cron table file for this user, or a blank file if none exists.

If I don't want to set a MAILTO email address (after reading about it in the documentation above), the first line I want in my crontab is a comment. More specifically, this comment:

  #mh hd dm my dw command

(every line starting with a pound symbol (#) is a comment) because it reminds me that the space separated fields are:

• minute of the hour
• hour of the day
• day of the month
• month of the year
• day of the week
• command line

The command can contain spaces (and need not be quoted), but the other fields must not contain spaces.

Since I like the output and format of the top command, I'll research it a bit (using the 'man top' and 'top --help' commands)and put together a command line I can use in a cron job:

  /usr/bin/top -n 1 -b -S

Notice that I used the whole path to the top executable. Since cron jobs may not provide the same working environment (path, aliases, ...) I get at the command prompt, it is wise to use full paths to all commands and scripts I use. I test the command a couple times to make sure I get the output I thought I was going to get. The output of this command (by default) will be sent to my user via email whenever this cron job executes.

Crontab Examples

To use this command on cron job line using 'crontab -e', I need to fill in all of the fields:

  0 12 * * * /usr/bin/top -n 1 -b -S

This command will execute every day at noon. The first five fields can be values, lists, ranges, or ranges with step values.

The first and second fields in the example are values. They are just simple integers that exist in the range of the field. The '0' in thefirst field could be any integer between '0' and '59' (minutes of thehour). The '12' in the second field could be any integer between '0' and '23' (hours of the day).

A list is more than one value separated by commas (with no spaces).An example using lists would be:

  0 0,6,12,18 * * * /usr/bin/top -n 1 -b -S

which adds midnight (0), 6 am (6), and 6 pm (18) to the times this job will execute, in addition to the original time of noon (12).

A range is just a lower number (in the range of possible values) anda higher number (also in the range of possible values) separated by a dash ('-'). A cron job using a range might be:

  0 0,6,12,18 * * 1-5 /usr/bin/top -n 1 -b -S

making the 'top' cron job execute four times a day, but only on weekdays (Monday through Friday). A list may also include a rangein place of one (or more) of its values, like this:

  0 0,6,9-15,18 * * 1-5 /usr/bin/top -n 1 -b -S

which would allow this cron job to execute every hour from 9 am (9)until 3 pm (15), instead of just at noon (12).

The asterisk (*) fields in the example above are ranges, consisting of every value in the range from the first possible value through thelast possible value, which are different for each field. In theoriginal example, the third fields asterisk means '1-31' (daysof the month), the fourth fields asterisk means '1-12' (months of the year), and the fifth fields asterisk means '0-7' (days of the week, Sunday being both values zero (0) and seven(7)).

Ranges with step values express lists like this:

  0 0,6,12,18 * * * /usr/bin/top -n 1 -b -S

in this abbreiviated way:

  0 */6 * * * /usr/bin/top -n 1 -b -S

meaning all possible values ('*' or '0-23') that are evenly divisible by 6 (0, 6, 12, and 18). This way of expressing lists makes it much easierto run a cron job every 5 minutes:

  */5 * * * * /usr/bin/top -n 1 -b -S

or every other day:

  0 12 */2 * * /usr/bin/top -n 1 -b -S

in a way that is short, easy to read, and interpret.

Cron Job Output

Sometimes I never want to receive the output of a command I put in mycrontab. There are a couple of way to do this. The "all or nothing" way is to set MAILTO="" on the first line of my crontab, but then I will get no output (that I don't arrange to get somehow via the command) for any of my cron jobs. I typically do not do this.

I regularly use output redirection to funnel my command outputto a file (if I want a more permanent record of the cron job than an email),or to the 'electronic wastebin' (or 'bitbucket') which is /dev/null(if I want no record of the cron job).If I want to ping a list of hosts every 10 minutes, I can write a script called 'pinghosts.sh' that provides nicely formatted outputfrom the 'ping' command. The script could automatically put it's output to a file, but if I want to use 'pinghosts.sh' from the command line too, that might be a problem. Instead, I could do this:

  */10 * * * * /home/myuser/bin/pinghosts.sh > /home/myuser/pinghosts.log

and every ten minutes the output of 'pinghosts.sh' will replace the contents of the file 'pinghosts.log' ('>' causes the output of the 'pinghosts.sh' command to overwrite the contents of the 'pinghosts.log' file) in my home directory (if 'myuser' is my username).If I want 'pinghosts.log' to be more like a regular log file, trackingoutput over time, I need to use a different redirection symbol:

  */10 * * * * /home/myuser/bin/pinghosts.sh >> /home/myuser/pinghosts.log

('>>' cause the output of the 'pinghosts.sh' command to be appended to the 'pinghosts.log' file).If I plan to keep 'pinghosts.sh' output over time, I should make sure it doesn't produce more output than I have disk (or quota) space. I could just throw away the file when it gets to large, or maybe just every month or so (hmm...sounds like a job for cron, or maybe logrotate).

If 'pinghosts.sh' produces any error messages, 'pinghosts.log' will not contain them. Instead, an email message containing the error messages will be sent to my user. If that is what I want, I am set, but many times I want error messages to be recorded in my logs of output, so the above examples will not do. To put all output into the file I need to do this[1]:

  */10 * * * * /home/myuser/bin/pinghosts.sh >> /home/myuser/pinghosts.log 2>&1

This works for shells like 'bash' (sh, ksh, zsh) and redirects STDERR (error output) to STDOUT (normal output), just after STDOUT has been redirected to the file 'pinghosts.log'.

Sometimes I just want a cron job script to do what I want and never tell me about it. If I have a script that notifies me by email of "garbage day" every week, I don't want another email telling me it sent the first email successfully. To make this happen, instead of a log file I use /dev/null as the target of my output:

  0 19 * * 2 /home/myuser/bin/garbage-day-email.sh > /dev/null 2>&1

but this will never alert me if the email is unsuccessful, so I mayopt for this version:

  0 19 * * 2 /home/myuser/bin/garbage-day-email.sh > /dev/null

instead. This way, at 7 pm on Tuesday evenings I will get an email cheerfully reminding me of "garbage day", or an uglier error messagethat does the same (if email is working).

Cron Job Scripts

Sometimes what I want to do from a cron job is a whole lot more complicated that a single command. When that kind of situation occurs, it is time to make a script (usually a shell script) to run from my crontab.

A simple backup of my home directory could be one command[2]:

  0 3 * * * /bin/tar czvf /backup/myuser-`/bin/date +%Y%m%d`.tgz /home/myuser

which executes a backup of my home directory every day at 3 am(and uses a date code on the archive filename), but I may want to dosome more work in the cron job to report more information and setsome file attributes after the backup occurs. So, I would write a script to do these tasks:

  #!/bin/bash
  ARCHIVE=/backup/myuser-`/bin/date +%Y%m%d`.tgz
  /bin/tar czvf $ARCHIVE /home/myuser
  /bin/chown myuser:mygroup $ARCHIVE
  /bin/chmod 640 $ARCHIVE
  /bin/echo " "
  /bin/echo -n "Disk Space Used: "
  /bin/ls -sh $ARCHIVE
  /bin/echo "File Details:"
  /bin/ls -l $ARCHIVE

I could name it backup-myuser.sh and call it from my crontab:

  0 3 * * * /home/myuser/bin/backup-myuser.sh

and get my backups, control file ownership and permissions, and report on the resulting archive file. In addition to that, I can change the details of "what gets done" by editting a script and not messing around with my crontab unless I want to change thescript name or when it runs.

In my opinion there is something wrong with this backup script. Ituses only one shell variable ($ARCHIVE) to keep from refiguring the archive file name every time it is used, but it usesno others. When I am writing a cron script, I like to overuseshell variables. I like to put every executable path, every filepath, and any text used in more than one location into shellvariables. This can make the script file larger and it can make it smaller, depending on a few things, but I do not do it for saving some space in the script file. I do it for portabability,maintainability, and (if I choose representative[3]variable names)readability reasons. If I copy or move the script where files are in different locations, for whatever reason, I can change the script quickly and easily and adapt it to it's new environment.

An rewrite of the above backup example that follows the 'overuse shell variables' philosophy is here:

  #!/bin/bash
  # backup-myuser.sh

  # file paths
  TAR=/bin/tar
  CHOWN=/bin/chown
  CHMOD=/bin/chmod
  ECHO=/bin/echo
  LS=/bin/ls
  DATE=/bin/date

  # script variables
  DATECODE=`$DATE +%Y%m%d`
  ARCHIVE=/backup/myuser-$DATECODE.tgz
  BACKDIR=/home/myuser
  BUSER=myuser
  BGROUP=mygroup
  BMODE=640

  # do the work
  $TAR czvf $ARCHIVE $BACKDIR
  $CHOWN $BUSER:$BGROUP $ARCHIVE
  $CHMOD $BMODE $ARCHIVE

  # report the results
  $ECHO " "
  $ECHO -n "Disk Space Used: "
  $LS -sh $ARCHIVE
  $ECHO "File Details:"
  $LS -l $ARCHIVE

This example includes a few other "good things" (in my opinion) likewhitespace (the blank lines separating different parts of the script), comments (the lines with pound signs (#) followed by explanitory text),and some single use shell variables ($BUSER, $BGROUP, and $BMODE). The first two things are aimed at improving the ability of others (or me, six months after writing it) to quickly read and understand this script. The single use shell variable are an effort to provide a single place (the 'script variables' section)to make any forseen and simple changes to the script, hopefully reducing the probability of future editors (myself included) injecting bugs into the script.

Better Cron Job Scripts

I like to have up to date local mirrors of things like CPAN, ProjectGutenberg, and Fedora Core installation and update files. These areall potentially long running processes, so I will want to use somesort of locking mechanism. These "locks" will be used to tell newcron job processes that an older process is already on the job (it'sjust taking a while). The "locking" I use will check for a file in a directory (used only for cron job locks) and create it if it does not exist. Immediately afterward, I use shell trap commands to setup the removal of that lock file on exit (regular script end and various errors).

I will also want to run the same script on a few different hosts,hopefully without having to do a lot of editing and setup beforehand.To make this happen I will need to test for the existence ofsome directories and files I need, and create and set theirpermissions correctly if they do not exist. Some errors take a lot of effort to handle, and if you must handle them it may makesense to handle them in Perl or Python instead of acommand shell like bash. However, bash is more than capable of handling the "low hanging fruit" of directory and fileexistence and creation.

Here is the example:

  #!/bin/sh

  # rsync-fedora4-mirror

  # common source and destination
  CMNSRC=mirror.cs.wisc.edu::fedora-linux-core
  CMNDST=/var/ftp/pub/fedora
  # user
  CUSER=`/usr/bin/whoami`
  # directories
  CRONDIR=/home/$CUSER/cron
  LOCKDIR=$CRONDIR/lock
  EXFILE=$CRONDIR/fc4-excludes.txt
  # options
  GETSOURCES=0
  RSYNCOPTS="-rlHtS --delete --delete-excluded --exclude-from=$EXFILE"

  # files
  RSYNC=/usr/bin/rsync
  ECHO=/bin/echo
  RM=/bin/rm
  TOUCH=/bin/touch
  MKDIR=/bin/mkdir
  FIND=/usr/bin/find
  CHMOD=/bin/chmod
  # directory and file modes for cron and mirror files
  CDMODE=700
  CFMODE=600
  MDMODE=755
  MFMODE=644
  
  # testing for directories and files needed
  if [ ! -d $CRONDIR ]; then $MKDIR -p $CRONDIR; $CHMOD $CDMODE $CRONDIR; fi
  if [ ! -d $LOCKDIR ]; then $MKDIR -p $LOCKDIR; $CHMOD $CDMODE $LOCKDIR; fi
  if [ ! -f $EXFILE ]; then $TOUCH $EXFILE; $CHMOD $CFMODE $EXFILE; fi
  
  # lock file creation and removal
  LOCKFILE=$LOCKDIR/`basename $0`.lock
  [ -f $LOCKFILE ] && $ECHO $LOCKFILE exists && exit 0
  trap "{ $RM -f $LOCKFILE; exit 255; }" 2
  trap "{ $RM -f $LOCKFILE; exit 255; }" 9
  trap "{ $RM -f $LOCKFILE; exit 255; }" 15
  trap "{ $RM -f $LOCKFILE; exit 0; }" EXIT
  $TOUCH $LOCKFILE
 
  # now mirror and set permissions for each group of files
 
  # release
  RSYNCSRC=$CMNSRC/4/i386/os/
  RSYNCDST=$CMNDST/4/i386/os/
  if [ ! -d $RSYNCDST ]; then $MKDIR -p $RSYNCDST; fi
  cd $RSYNCDST
  $RSYNC $RSYNCOPTS $RSYNCSRC $RSYNCDST
  $FIND $RSYNCDST -type d -exec $CHMOD $MDMODE /{/} /;
  $FIND $RSYNCDST -type f -exec $CHMOD $MFMODE /{/} /;
  
  # updates
  RSYNCSRC=$CMNSRC/updates/4/i386/
  RSYNCDST=$CMNDST/updates/4/i386/
  if [ ! -d $RSYNCDST ]; then $MKDIR -p $RSYNCDST; fi
  cd $RSYNCDST
  $RSYNC $RSYNCOPTS $RSYNCSRC $RSYNCDST
  $FIND $RSYNCDST -type d -exec $CHMOD $MDMODE /{/} /;
  $FIND $RSYNCDST -type f -exec $CHMOD $MFMODE /{/} /;
  
  [ $GETSOURCES -lt 1 ] && exit 0
  
  # release sources
  RSYNCSRC=$CMNSRC/4/SRPMS/
  RSYNCDST=$CMNDST/4/SRPMS/
  if [ ! -d $RSYNCDST ]; then $MKDIR -p $RSYNCDST; fi
  cd $RSYNCDST
  $RSYNC $RSYNCOPTS $RSYNCSRC $RSYNCDST
  $FIND $RSYNCDST -type d -exec $CHMOD $MDMODE /{/} /;
  $FIND $RSYNCDST -type f -exec $CHMOD $MFMODE /{/} /;
  
  # updates sources
  RSYNCSRC=$CMNSRC/updates/4/SRPMS/
  RSYNCDST=$CMNDST/updates/4/SRPMS/
  if [ ! -d $RSYNCDST ]; then $MKDIR -p $RSYNCDST; fi
  cd $RSYNCDST
  $RSYNC $RSYNCOPTS $RSYNCSRC $RSYNCDST
  $FIND $RSYNCDST -type d -exec $CHMOD $MDMODE /{/} /;
  $FIND $RSYNCDST -type f -exec $CHMOD $MFMODE /{/} /;

This is a rather trusting example, as many problem could occur if the mirror you are using has "problems". Eliminate the "--delete" and "--delete-excluded" arguments from the RSYNCOPTS variable definition to be less trusting.

If you are not developing software based on Fedora Core RPMs,executing (as the user who will run the cron job):

  $ echo debug >> ~/cron/fc4-excludes.txt

will save space on the mirror, especially if you do not plan to use the 'debug' binaries and sources. However, if you do want to use many Fedora Core RPMs in your development plans, you will want to get the SRPMs, so change GETSOURCES to '1'instead of '0' and this script will get them for you.

I recommend using a mirror that is close to you (network wise), so changing the CMNSRC variable is a good thing to do. Unfortunately, you may have to change other parts fo the scriptto make it work because not all mirrors are arranged identically.

That's all for now, but I may add more later. Good luck!