This chapter describes how to schedule routine or one-time system events by using the crontab and at commands. It also explains how to control access to these commands by using cron.deny, cron.allow, and at.deny files.
This is a list of the step-by-step instructions in this chapter.
You can schedule system events to execute repetitively, at regular intervals, by using the crontab command.You can schedule a single system event for execution at a specified time by using the at command. The following table summarizes crontab and at, as well as the files that enable you to control access to these commands.
Table 30-1 Command Summary: Scheduling System Events
Command |
What It Schedules |
Location of Files |
Files That Control Access |
---|---|---|---|
crontab |
Multiple system events at regular intervals |
/var/spool/cron/crontabs |
/etc/cron.d/cron.allow and /etc/cron.d/cron.deny |
at |
A single system event |
/var/spool/cron/atjobs |
/etc/cron.d/at.deny |
The following sections describe how to create, edit, display, and remove crontab files, as well as how to control access to them.
The cron daemon schedules system events according to commands found within each crontab file. A crontab file consists of commands, one per line, that will be executed at regular intervals. The beginning of each line contains date and time information that tells the cron daemon when to execute the command.
For example, a crontab file named root is supplied during SunOS software installation. Its contents include these command lines:
10 3 * * 0,4 /etc/cron.d/logchecker 10 3 * * 0 /usr/lib/newsyslog 15 3 * * 0 /usr/lib/fs/nfs/nfsfind 1 2 * * * [ -x /usr/sbin/rtc ] && /usr/sbin/rtc -c > /dev/null 2>&1 30 3 * * * [ -x /usr/lib/gss/gsscred_clean ] && /usr/lib/gss/gsscred_clean |
The first command line instructs the system to run logchecker at 3:10 on Sundays and Thursdays nights. The second command line schedules the system to run newsyslog at 3:10 every Sunday morning. The third command line orders the system to execute nfsfind Sundays at 3:15 in the morning. The fourth command line instructs the system to check daily for daylight savings time and make corrections if necessary. If there is no RTC time zone nor an /etc/rtc_config file, this entry will do nothing. The fifth command line instructs the system to check for and remove duplicate entries in the Generic Security Service table, /etc/gss/gsscred_db.
For more information about the syntax of lines within a crontab file, see "Syntax of crontab File Entries".
The crontab files are stored in /var/spool/cron/crontabs. Several crontab files besides root are provided during SunOS software installation (see the following table).
Table 30-2 Default crontab Files
crontab File |
Function |
---|---|
adm |
Accounting |
lp |
Printing |
root |
General system functions and file system cleanup |
sys |
Performance collection |
uucp |
General uucp cleanup |
Besides the default crontab file, users can create crontab files to schedule their own system events.
Other crontab files are named after the user accounts in which they are created, such as bob, mary, smith, or jones.
To access crontab files belonging to root or other users, superuser privileges are required.
Procedures explaining how to create, edit, display, and remove crontab files are described in "Commands for Scheduling System Events".
The cron daemon handles the automatic scheduling of crontab commands. Its function is to check the /var/spool/cron/crontab directory for the presence of crontab files, normally every 15 minutes. It checks for new crontab files or changes to existing ones, reads the execution times listed within the files, and submits the commands for execution at the proper times.
In much the same way, the cron daemon controls the scheduling of at files, which are stored in the /var/spool/cron/atjobs directory.
A crontab file consists of commands, one per line, that execute automatically at the time specified by the first five fields at the beginning of each command line. These first five fields, described in the following table, are separated by spaces. They indicate when the command will be executed.
Table 30-3 Values for crontab Time Fields
Time Field |
Values |
---|---|
Minute |
0-59 |
Hour |
0-23 |
Day of month |
1-31 |
Month |
1-12 |
Day of week |
0-6 (0 = Sunday) |
Follow these guidelines to use special characters in crontab time fields:
Use a space to separate each field.
Use a comma to separate multiple values.
Use a hyphen to designate a range of values.
Use an asterisk as a wildcard to include all possible values.
Use a comment mark (#) at the beginning of a line to indicate a comment or a blank line.
For example, the following sample crontab command entry displays a reminder in the user's console window at 4 p.m. on the first and fifteenth of every month.
0 16 1,15 * * echo Timesheets Due > /dev/console |
Each command within a crontab file must consist of one line, even if it is very long, because crontab does not recognize extra carriage returns. For more detailed information about crontab entries and command options, refer to crontab(1).
The simplest way to create a crontab file is to use the crontab -e command to invoke the text editor set up for your system environment, defined by the EDITOR environment variable. If this variable has not been set, crontab uses the default editor ed. Define your EDITOR environment to be an editor you are familiar with. The following example shows how to check to see whether an editor has been defined, and how to set up vi as the default.
$ which $EDITOR $ $ EDITOR=vi $ export EDITOR |
When you create a crontab file, it is automatically placed in the /var/spool/cron/crontabs directory and is given your user name. You can create or edit a crontab file for another user, or root, if you have superuser privileges.
Enter crontab command entries as described in "Syntax of crontab File Entries".
(Optional) Become superuser to create or edit a crontab file belonging to root or another user.
Create a new crontab file, or edit an existing one.
$ crontab -e [username] |
username |
Name of another user's account, requires root privileges to create or edit. |
If you accidentally enter the crontab command with no option, press the interrupt character for your editor. This allows you to quit without saving changes. Exiting the file and saving changes at this point would overwrite an existing crontab file with an empty file.
Add command lines to the file, following the syntax described in "Syntax of crontab File Entries".
The crontab file will be placed in /var/spool/cron/crontabs.
Verify the crontab file by using the crontab -l command.
# crontab -l [username] |
The following example shows how to create a crontab file for another user.
# crontab -e jones |
The following command entry added to a new crontab file will automatically remove any log files from the user's home directory at 1:00 am every Sunday morning. Because the command entry does not redirect output, redirect characters are added to the command line after *.log to make sure that the command executes properly.
# This command helps clean up user accounts. 1 0 * * 0 rm /home/jones/*.log > /dev/null 2>&1 |
To verify that a crontab file exists for a user, use the ls -l command in the /var/spool/cron/crontabs directory. For example, the following display shows that crontab files exist for users smith and jones.
$ ls -l /var/spool/cron/crontabs -rw-r--r-- 1 root sys 190 Feb 26 16:23 adm -rw------- 1 root staff 225 Mar 1 9:19 jones -rw-r--r-- 1 root root 1063 Feb 26 16:23 lp -rw-r--r-- 1 root sys 441 Feb 26 16:25 root -rw------- 1 root staff 60 Mar 1 9:15 smith -rw-r--r-- 1 root sys 308 Feb 26 16:23 sys |
Verify the contents of user's crontab file by using crontab -l as described in "How to Display a crontab File".
The crontab -l command displays the contents of your crontab file much the way the cat command displays the contents of other types of files. You do not have to change directories to /var/spool/cron/crontabs (where crontab files are located) to use this command.
By default, the crontab -l command displays your own crontab file. To display crontab files belonging to other users, you must be superuser.
(Optional) Become superuser to display a crontab file belonging to root or another user.
Display the crontab file.
$ crontab -l [username] |
username |
Name of another user's account, and requires superuser privileges to create or edit. |
If you accidentally enter the crontab command with no option, press the interrupt character for your editor. This allows you to quit without saving changes. Exiting the file and saving changes at this point would overwrite an existing crontab file with an empty file.
The following example shows how to use crontab -l to display the contents of the default user's crontab file, the default root crontab file, and the crontab file belonging to another user.
$ crontab -l 13 13 * * * chmod g+w /home1/documents/*.book > /dev/null 2>&1 $ su Password: # crontab -l #ident "@(#)root 1.19 98/07/06 SMI" /* SVr4.0 1.1.3.1 */ # # The root crontab should be used to perform accounting data collection. # # The rtc command is run to adjust the real time clock if and when # daylight savings time changes. # 10 3 * * 0,4 /etc/cron.d/logchecker 10 3 * * 0 /usr/lib/newsyslog 15 3 * * 0 /usr/lib/fs/nfs/nfsfind 1 2 * * * [ -x /usr/sbin/rtc ] && /usr/sbin/rtc -c > /dev/null 2>&1 30 3 * * * [ -x /usr/lib/gss/gsscred_clean ] && /usr/lib/gss/gsscred_clean # crontab -l jones 13 13 * * * cp /home/jones/work_files /usr/backup/. > /dev/null 2>&1 |
By default, crontab file protections are set up so that you cannot inadvertently delete a crontab file by using the rm command. Instead, use the crontab -r command to remove crontab files.
By default, crontab -r removes your own crontab file. You must be superuser to remove crontab files belonging to superuser or other users.
You do not have to change directories to /var/spool/cron/crontabs (where crontab files are located) to use this command.
(Optional) Become superuser to remove a crontab file belonging to root or another user.
Remove the crontab file.
$ crontab -r [username] |
username |
Name of another user's account, and requires superuser privileges to create or edit. |
If you accidentally enter the crontab command with no option, press the interrupt character for your editor. This allows you to quit without saving changes. Exiting the file and saving changes at this point would overwrite an existing crontab file with an empty file.
Verify the crontab file is removed.
# ls /var/spool/cron/crontabs |
The following example shows how user smith uses the crontab -r command to remove his crontab file.
$ ls /var/spool/cron/crontabs adm jones lp root smith sys uucp $ crontab -r $ ls /var/spool/cron/crontabs adm jones lp root sys uucp |
You can control access to crontab by using two files in the /etc/cron.d directory: cron.deny and cron.allow. These files permit only specified users to perform crontab tasks such as creating, editing, displaying, or removing their own crontab files.
The cron.deny and cron.allow files consist of a list of user names, one per line. These access control files work together like this:
If cron.allow exists, only the users listed in this file can create, edit, display, or remove crontab files.
If cron.allow doesn't exist, all users may submit crontab files, except for users listed in cron.deny.
If neither cron.allow nor cron.deny exists, superuser privileges are required to run crontab.
Superuser privileges are required to edit or create the cron.deny and cron.allow files.
During SunOS software installation, a default cron.deny file is provided:
$ cat /etc/cron.d/cron.deny daemon bin smtp nuucp listen nobody noaccess |
None of these user names can access crontab commands. You can edit this file to add other user names who will be denied access to the crontab command.
No default cron.allow file is supplied. This means that, after Solaris software installation, all users (except the ones listed in the default cron.deny file) can access crontab. If you create a cron.allow file, only these users can access crontab commands.
Become superuser.
Edit the /etc/cron.d/cron.deny file and add user names, one per line, who will be prevented from using crontab commands.
daemon bin smtp nuucp listen nobody noaccess username1 username2 username3 . . . |
Verify the /etc/cron.d/cron.deny file.
# cat /etc/cron.d/cron.deny |
Create the /etc/cron.d/cron.allow file.
Enter the user names, one per line, who will be allowed to use crontab commands.
root username1 username2 username3 . . . |
Be sure to add root to this list. If you do not, superuser access to crontab commands will be denied.
The following example shows a cron.deny file that prevents user names visitor, jones, and temp from accessing crontab.
$ cat /etc/cron.d/cron.deny daemon bin smtp nuucp listen nobody noaccess jones temp visitor |
The following example shows a cron.allow file. The users smith, jones, lp, and root are the only ones who may access crontab.
$ cat /etc/cron.d/cron.allow root jones lp smith |
To verify whether or not a specific user can access crontab, use the crontab -l command while logged into the user account.
$ crontab -l |
If the user can access crontab, and already has created a crontab file, it will be displayed. Otherwise, if the user can access crontab but no crontab file exists, a message like the following will be displayed:
crontab: can't open your crontab file |
This user either is listed in cron.allow (if it exists), or is not listed in cron.deny.
If the user cannot access crontab, the following message is displayed whether or not a previous crontab file exists:
crontab: you are not authorized to use cron. Sorry. |
This means either that the user is not listed in cron.allow (if it exists), or the user is listed in cron.deny.
The following sections describe how to use at(1) to schedule jobs (commands and scripts) for execution at a later time, how to display and remove these jobs, and how to control access to the at command.
By default, users can create, display, and remove their own at job files. To access at files belonging to root or other users, you must have superuser privileges.
When you submit an at job, it is assigned a job identification number along with the .a extension that becomes its file name.
Submitting an at job file includes:
Invoking the at utility, specifying a command execution time.
Entering a command or script to execute later.
If output from this command or script is important, be sure to direct it to a file for later examination.
For example, the following at job removes core files from the user account smith near midnight on the last day of July.
$ at 11:45pm July 31 at> rm /home/smith/*core* at> Press Control-d commands will be executed using /bin/csh job 933486300.a at Sat Jul 31 23:45:00 1999 |
You can set up a file to control access to the at command, permitting only specified users to create, remove, or display queue information about their at jobs. The file that controls access to at, /etc/cron.d/at.deny, consists of a list of user names, one per line. The users listed in this file cannot access at commands.
The at.deny file, created during SunOS software installation, contains the following user names:
daemon bin smtp nuucp listen nobody noaccess |
With superuser privileges, you can edit this file to add other user names whose at access you want to restrict.
Start the at utility, specifying the time you want your job executed, and press Return.
$ at [-m] time [date] |
-m |
Sends you mail after the job is completed. |
time |
Hour that you want to schedule the job. Add am or pm if you do not specify the hours according to a 24-hour clock. midnight, noon, and now are acceptable keywords. Minutes are optional. |
date |
First three or more letters of a month, a day of the week, or the keywords today or tomorrow. |
At the at prompt, enter the commands or scripts you want to execute, one per line. You may enter more than one command by pressing Return at the end of each line.
Exit the at utility and save the at job by pressing Control-d.
Your at job is assigned a queue number, which is also its file name. This number is displayed when you exit the at utility.
The following example shows the at job that user jones created to remove her backup files at 7:30 at night. She used the -m option so that she would receive a mail message after her job completed.
$ at -m 1930 at> rm /home/jones/*.backup at> Press Control-d job 897355800.a at Mon Jul 12 19:30:00 1999 |
She received a mail message which confirmed the execution of her at job.
Your "at" job "rm /home/jones/*.backup" completed. |
The following example shows how jones scheduled a large at job for 4:00 Saturday morning. The output of which was directed to big.file.
$ at 4 am Saturday at> sort -r /usr/dict/words > /export/home/jones/big.file |
To check your jobs that are waiting in the at queue, use the atq command. This command displays status information about the at jobs that you created.
$ atq |
To verify that you have created an at job, use the atq command. The atq command confirms that at jobs belonging to jones have been submitted to the queue.
$ atq Rank Execution Date Owner Job Queue Job Name 1st Jul 12, 1999 19:30 jones 897355800.a a stdin 2nd Jul 14, 1999 23:45 jones 897543900.a a stdin 3rd Jul 17, 1999 04:00 jones 897732000.a a stdin |
To display information about the execution times of your at jobs, use the at -l command.
$ at -l [job-id] |
-l job-id |
Identification number of the job whose status you want to examine. |
The following example shows output from the at -l command, used to get status information on all jobs submitted by a user.
$ at -l 897543900.a Wed Jul 14 23:45:00 1999 897355800.a Mon Jul 12 19:30:00 1999 897732000.a Sat Jul 17 04:00:00 1999 |
The following example shows output displayed when a single job is specified with the at -l command.
$ at -l 897732000.a 897732000.a Sat Jul 17 04:00:00 1999 |
(Optional) Become superuser to remove an at job belonging to root or another user.
Remove the at job from the queue before it is executed.
$ at -r [job-id] |
-r job-id |
Identification number of the job you want to remove. |
Verify the at job is removed by using the at -l (or the atq) command to display the jobs remaining in the at queue. The job whose identification number you specified should not appear.
$ at -l [job-id] |
In the following example, a user wants to remove an at job that was scheduled to execute at 4 am on July 17th. First, the user displays the at queue to locate the job identification number. Next, the user removes this job from the at queue. Finally, the user verifies that this job has been removed from the queue.
$ at -l 897543900.a Wed Jul 14 23:45:00 1999 897355800.a Mon Jul 12 19:30:00 1999 897732000.a Sat Jul 17 04:00:00 1999 $ at -r 897732000.a $ at -l 897732000.a at: 858142000.a: No such file or directory |
Users listed in the at.deny file cannot use at to schedule jobs or to check the at queue status.
The at.deny file is placed in the /etc/cron.d directory during Solaris software installation. At that time, the same users are listed in both this file and the default cron.deny file.
daemon bin smtp nuucp listen nobody noaccess |
Root permissions are required to edit this file.
Become superuser.
Edit the /etc/cron.d/at.deny file and add the names of users, one per line, who will be prevented from using at commands.
daemon bin smtp nuucp listen nobody noaccess username1 username2 username3 . . . |
The following example shows an at.deny file that has been edited so that the users smith and jones may not access the at command.
$ cat at.deny daemon bin smtp nuucp listen nobody noaccess jones smith |
To verify whether or not a user's name was added correctly to /etc/cron.d/at.deny, use the at -l command while logged in as the user. If the user cannot access at commands, the following message is displayed.
# su smith Password: $ at -l at: you are not authorized to use at. Sorry. |
Likewise, if the user tries to submit an at job, the following message is displayed:
$ at 2:30pm at: you are not authorized to use at. Sorry. |
This confirms that the user is listed in the at.deny file.
If at access is allowed, the at -l command returns nothing.