Setting up cron jobs in Unix and Solaris

cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.  Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

This document covers following aspects of Unix cron jobs
1. Crontab Restrictions
2. Crontab Commands
3. Crontab file – syntax
4. Crontab Example
5. Crontab Environment
6. Disable Email
7. Generate log file for crontab activity

1. Crontab Restrictions
You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file /usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.

2. Crontab Commands

export EDITOR=vi ;to specify a editor to open crontab file.

crontab -e    Edit your crontab file, or create one if it doesn’t already exist.
crontab -l      Display your crontab file.
crontab -r      Remove your crontab file.
crontab -v      Display the last time you edited your crontab file. (This option is only available on a few systems.)

3. Crontab file
Crontab syntax :
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.

*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of        month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).
Notes
A. ) Repeat pattern like /2 for every 2 minutes or /10 for every 10 minutes is not supported by all operating systems. If you try to use it and crontab complains it is probably not supported.

B.) The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed .

4. Crontab Example
A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30     18     *     *     *         rm /home/someuser/tmp/*

Changing the parameter values as below will cause this command to run at different time schedule below :

Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.

5. Crontab Environment
cron invokes the command from the user’s HOME directory with the shell, (/usr/bin/sh).
cron supplies a default environment for every shell, defining:
HOME=user’s-home-directory
LOGNAME=user’s-login-id
PATH=/usr/bin:/usr/sbin:.
SHELL=/usr/bin/sh

Users who desire to have their .profile executed must explicitly do so in the crontab entry or in a script called by the entry.

6. Disable Email
By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

>/dev/null 2>&1

7. Generate log file
To collect the cron execution execution log in a file :

30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log

 Crontab Examples

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  #!/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

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

  #!/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

  #!/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 \{\} \;

 Here's a crontab example that shows how to run a command from the cron daemon once every day

0 1 * * * /var/www/backup.sh

crontab: invalid option -- -

crontab: usage error: unrecognized option

usage:  crontab [-u user] file

        crontab [-u user] [ -e | -l | -r ]

                (default operation is replace, per 1003.2)

        -e      (edit user's crontab)

        -l      (list user's crontab)

        -r      (delete user's crontab)

        -i      (prompt before deleting user's crontab)

 Like most Linux users, you may find it necessary to schedule repetitive tasks to be run at a certain time. Such tasks can occur as frequently as once a minute, to as infrequently as once a year. This scheduling can be done by using the ``cron'' facilities.

The cron facilities as implemented in Linux are fairly similar to those available in other Unix implementations. However, Red Hat has adopted a slightly different way of scheduling tasks than is usually done in other distributions of Linux. Just as in other distributions, scheduling information is placed in the system ``crontab'' file (locating in the ``/etc/'' directory), using the following format:

minute hour day month year command

You can specify each time component as an integer number (eg. 1 through 12 for the months January through December), or specify one or more components as ``*'' characters which will be treated as wildcards (eg. * in the month component means the command will run at the given day and time in every month. Here are some examples:

# Mail the system logs at 4:30pm every June 15th.
30 16 15 06 * for x in /var/log/*; do cat ${x} | mail postmaster; done

# Inform the administrator, at midnight, of the changing seasons.
00 00 20 04 * echo 'Woohoo, spring is here!'
00 00 20 06 * echo 'Yeah, summer has arrived, time to hit the beach!'
00 00 20 10 * echo 'Fall has arrived.  Get those jackets out.  :-('
00 00 20 12 * echo 'Time for 5 months of misery.  ;-('

Note that commands which produce output to standard out (ie. a terminal) such as the examples above using ``echo'' will have their output mailed to the ``root'' account. If you want to avoid this, simply pipe the output to the null device as follows:

00 06 * * * echo 'I bug the system administrator daily at 6:00am!' >/dev/null

In addition to the standard ``crontab'' entries, Red Hat adds several directories:

/etc/cron.hourly/
/etc/cron.daily/
/etc/cron.weekly/

As their names suggest, executable files can be placed in any of these directories, and will be executed on an hourly, daily, or weekly basis. This saves a bit of time when setting up frequent tasks; just place the executable script or program (or a symbolic link to one stores elsewhere) in the appropriate directory and forget about it.

From :http://tldp.org/LDP/lame/LAME/linux-admin-made-easy/using-cron.html

  Examples of crontabs

# record the memory usage of the system every monday  # at 3:30AM in the file /tmp/meminfo 30 3 * * mon cat /proc/meminfo >> /tmp/meminfo # run custom script the first day of every month at 4:10AM 10 4 1 * * /root/scripts/backup.sh

As you can see from the /etc/crontab file, it uses the run-parts script to execute the scripts in the /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and/etc/cron.monthly files on an hourly, daily, weekly, or monthly basis respectively. The files in these directory should be shell scripts.

If a cron tasks needs to be executed on a schedule other than hourly, daily, weekly, or monthly, it can be added to the /etc/cron.d directory. All files in this directory use the same syntax as /etc/crontab.

The cron daemon checks the etc/crontab file, the etc/cron.d/ directory, and the /var/spool/cron directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed.

Users other than root can configure cron tasks by using the crontab utility. All user-defined crontabs are stored in the /var/spool/cron directory and are executed using the usernames of the users that created them. To create a crontab as a user, login as that user and type the command crontab -e to edit the user's crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab. When the changes to the crontab are saved, the crontab is stored according to username and written to the file /var/spool/cron/username.

*/1 * * * * php /var/www/html/poller.php > /dev/null 2>&1
0 1 * * * nice -n 15 /var/www/backup.sh
*/1 * * * * mysql -h localhost -u root -p123456

 The main configuration file for cron, /etc/crontab, contains the following lines:

SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/  # run-parts 01 * * * * root run-parts /etc/cron.hourly 02 4 * * * root run-parts /etc/cron.daily 22 4 * * 0 root run-parts /etc/cron.weekly 42 4 1 * * root run-parts /etc/cron.monthly

The first four lines are variables used to configure the environment in which the cron tasks are run. The value of the SHELL variable tells the system which shell environment to use (in this example the bash shell), and the PATH variable defines the path used to execute commands. The output of the cron tasks are emailed to the username defined with the MAILTOvariable. If the MAILTO variable is defined as an empty string (MAILTO=""), email will not be sent. The HOME variable can be used to set the home directory to use when executing commands or scripts.

Each line in the /etc/crontab file has the format:

minute   hour   day   month   dayofweek   command

• minute — any integer from 0 to 59

• hour — any integer from 0 to 23

• day — any integer from 1 to 31 (must be a valid day if a month is specified)

• month — any integer from 1 to 12 (or the short name of the month such as jan, feb, and so on)

• dayofweek — any integer from 0 to 7 where 0 or 7 represents Sunday (or the short name of the week such as sun, mon, and so on)

• command — the command to execute. The command can either be a command such as ls /proc >> /tmp/proc or the command to execute a custom script that you wrote.

For any of the above values, an asterisk (*) can be used to specify all valid values. For example, an asterisk for the month value means execute the command every month within the constraints of the other values.

A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.

A list of values separated by commas (,) specifies a list. For example, 3, 4, 6, 8 indicates those four specific integers.

The forward slash (/) can be used to specify step values. The value of an integer can be skipped within a range by following the range with /<integer>. For example, 0-59/2 can be used to define every other minute in the minute field. Step values can also be used with an asterisk. For instance, the value */3 can be used in the month field to run the task every third month.

Any lines that begin with a hash mark (#) are comments and are not processed.

Linux  Command: crontab

 usage:  crontab [-u user] file

        crontab [-u user] [ -e | -l | -r ]

                (default operation is replace, per 1003.2)

        -e      (edit user's crontab)

        -l      (list user's crontab)

        -r      (delete user's crontab)

        -i      (prompt before deleting user's crontab)

Lines that can be in the crontab file.

minute (0-59),

hour (0-23),

day of the month (1-31),

month of the year (1-12),

day of the week (0-6 with 0=Sunday).

Examples

crontab -e = edits the crontab file to be used.

0 12 14 2 * mailx john%Happy Birthday!%Time for lunch.

#crontab -e

 */1 * * * * php /var/www/html/poller.php > /dev/null 2>&1

 0 1 * * * nice -n 15 /var/www/backup.sh

 I want to cat a file and view the crontab entries and not use
crontab -e
or
crontab -l

/var/spool/cron/  

there should be a file for every user that is allowed to and has a crontab