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linux下的定时运行代码！
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Commands Reference, Volume 1
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crontab Command
Purpose
Submits, edits, lists, or removes cron jobs.
Syntax
crontab [ -e | -l | -r | -v | File ]
Description
The crontab command submits, edits, lists, or removes cron jobs. A cron job is a
command run by the cron daemon at regularly scheduled intervals. To submit a
cron job, specify the crontab command with the -e flag. The crontab command
invokes an editing session that allows you to create a crontab file. You create
entries for each cron job in this file. Each entry must be in a form acceptable
to the cron daemon. For information on creating entries, see "The crontab File
Entry Format" .
When you finish creating entries and exit the file, the crontab command copies
it into the /var/spool/cron/crontabs directory and places it in a file named for
your current user name. If a file with your name already exists in the crontabs
directory, the crontab command overwrites it.
Alternatively, you can create a crontab file by specifying the File parameter.
If the file exists, it must be in the format the cron daemon expects. If the
file doesn't exist, the crontab command invokes the editor. If the EDITOR
environment variable exists, the command invokes the editor it specifies.
Otherwise, the crontab command uses the vi editor.
To list the contents of your crontab file, specify the crontab command with the
-l command. To remove an existing file, use the -r flag.
The cron Daemon
The cron daemon runs commands according to the crontab file entries. Unless you
redirect the output of a cron job to standard output or error, the cron daemon
mails you any command output or errors. If you specify a cron job incorrectly in
your crontab file, the cron daemon does not run the job.
The cron daemon examines crontab files only when the cron daemon is initialized.
When you make changes to your crontab file using the crontab command, a message
indicating the change is sent to the cron daemon. This eliminates the overhead
of checking for new or changed files at regularly scheduled intervals.
Controls on Using the crontab Command
The /var/adm/cron/cron.allow and /var/adm/cron/cron.deny files control which
users can use the crontab command. A root user can create, edit, or delete these
files. Entries in these files are user login names with one name to a line. If
your login ID is associated with more than one login name, the crontab command
uses the first login name that is in the /etc/passwd file, regardless of which
login name you might actually be using.
The following is an example of an cron.allow file:
root
nick
dee
sarah
If the cron.allow file exists, only users whose login names appear in it can use
the crontab command. The root user's log name must appear in the cron.allow file
if the file exists. A system administrator can explicitly stop a user from using
the crontab command by listing the user's login name in the cron.deny file. If
only the cron.deny file exists, any user whose name does not appear in the file
can use the crontab command.
A user cannot use the crontab command if one of the following is true:
o The cron.allow file and the cron.deny file do not exist (allows root user
only).
o The cron.allow file exists but the user's login name is not listed in it.
o The cron.deny file exists and the user's login name is listed in it.
If neither the cron.allow nor the cron.deny file exists, only someone with root
user authority can submit a job with the crontab command.
The crontab File Entry Format
A crontab file contains entries for each cron job. Entries are separated by
newline characters. Each crontab file entry contains six fields separated by
spaces or tabs in the following form:
minute hour day_of_month month weekday command
These fields accept the following values:
minute 0 through 59
hour 0 through 23
day_of_month 1 through 31
month 1 through 12
weekday 0 through 6 for Sunday through Saturday
command a shell command
You must specify a value for each field. Except for the command field, these
fields can contain the following:
o A number in the specified range. To run a command in May, specify 5 in the
month field.
o Two numbers separated by a dash to indicate an inclusive range. To run a
cron job on Tuesday through Friday, place 2-5 in the weekday field.
o A list of numbers separated by commas. To run a command on the first and
last day of January, you would specify 1,31 in the day_of_month field.
o An * (asterisk), meaning all allowed values. To run a job every hour,
specify an asterisk in the hour field.
Note: Any character preceeded by a backslash (including the %) causes that
character to be treated literally. The specification of days may be made by
two fields (day of the month and day of the week). If you specify both as a
list of elements, both are adhered to. For example, the following entry:
0 0 1,15 * 1 command
would run command on the first and fifteenth days of each month, as well as
every Monday. To specify days by only one field, the other field should
contain an * .
Specifying Commands
The cron daemon runs the command named in the sixth field at the selected date
and time. If you include a % (percent sign) in the sixth field, the cron daemon
treats everything that precedes it as the command invocation and makes all that
follows it available to standard input, unless you escape the percent sign (\%).
Blank lines and lines whose first non-blank character is the number sign (#)
will be ignored.
Note: The shell runs only the first line of the command field. All other
lines are made available to the command as standard input.
The cron daemon starts a subshell from your HOME directory. If you schedule a
command to run when you are not logged in and you want commands in your .profile
file to run, the command must explictly read your .profile file.
The cron daemon supplies a default environment for every shell, defining HOME,
LOGNAME, SHELL (=/usr/bin/sh), and PATH (=/usr/bin).
Flags
-e Edits a copy of your crontab file or starts an editing session if you don't
already have a crontab file. When editing is complete, the entry is installed as
your crontab file. The editing session is started using the editor specified by
the EDITOR environment variable. The default editor is vi.
-l Lists your crontab file.
-r Removes your crontab file from the crontab directory.
-v Lists the status of your cron jobs.
Security
Auditing Events: If the auditing subsystem has been properly configured and is
enabled, the crontab command generates the following audit record (event) every
time the command is run:
Event Information
CRON_JobRemove Lists which users removed a cron job and when.
CRON_JobAdd Lists which users added a cron job and when.
See "Setting up Auditing" in AIX Version 4.3 System Management Guide: Operating
System and Devices for more details about how to properly select and group audit
events, and how to configure audit event data collection.
Exit Status
This command returns the following exit values:
0 Successful completion.
>0 An error occurred.
Examples
1. To copy a file called mycronjobs into the /var/admn/cron/crontabs
directory, enter the following:
crontab mycronjobs
2. To write the time to the console every hour on the hour, enter:
0 * * * * echo The hour is `date` .
>/dev/console
3. To run the calendar command at 6:30 a.m. every Monday, Wednesday, and
Friday, enter:
30 6 * * 1,3,5 /usr/bin/calendar
4. To run the calendar command every day of the year at 6:30, enter the
following:
30 6 * * * /usr/bin/calendar
5. To run a script called maintenance every day at midnight in August, enter
the following:
0 0 * 8 * /u/harry/bin/maintenance
6. To define text for the standard input to a command, enter:
0 16 * 12 5 /usr/sbin/wall%HAPPY HOLIDAY!%Remember to
turn in your time card.
The text following the % (percent sign) defines the standard input to the
wall command as:
HAPPY HOLIDAY!
Remember to turn in your time card.
Files
/var/adm/cron/FIFO A named pipe that sends messages to the cron daemon when new
jobs are submitted with the crontab or at command.
/var/spool/cron/crontabs Specifies the crontab spool area.
/var/adm/cron/cron.allow Specifies a list of users allowed access to the crontab
command.
/var/adm/cron/cron.deny Specifies a list of users denied access to the crontab
command.
Related Information
The auditpr command, sh command, the wall command.
The cron daemon.
The Auditing Overview in AIX Version 4.3 System Management Concepts: Operating
System and Devices explains more about audits and audit events.
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发表于 2003-06-27 15:09
发表于 2003-06-27 15:27
