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crontab(1)Name | Synopsis | Description | Options | Examples | Environment Variables | Exit Status | Files | Attributes | See Also | Notes Name
Synopsis/usr/bin/crontab [filename] /usr/bin/crontab -e [username] /usr/bin/crontab -l [username] /usr/bin/crontab -r [username] /usr/xpg4/bin/crontab [filename] /usr/xpg4/bin/crontab -e [username] /usr/xpg4/bin/crontab -l [username] /usr/xpg4/bin/crontab -r [username] /usr/xpg6/bin/crontab [filename] /usr/xpg6/bin/crontab -e [username] /usr/xpg6/bin/crontab -l [username] /usr/xpg6/bin/crontab -r [username] Description
The crontab utility manages a user's access with cron (see cron(1M)) by copying, creating, listing, and removing crontab files. If invoked without options, crontab copies the specified file, or the standard input if no file is specified, into a directory that holds all users' crontabs. If crontab is invoked with filename, this overwrites an existing crontab entry for the user that invokes it. crontab Access ControlUsers: Access to crontab is allowed: Users: Access to crontab is denied: The rules for allow and deny apply to root only if the allow/deny files exist. The allow/deny files consist of one user name per line. crontab Entry FormatA crontab file consists of lines of six fields each. The fields are separated by spaces or tabs. The first five are integer patterns that specify the following:
Each of these patterns can be either an asterisk (meaning all legal values) or a list of elements separated by commas. An element is either a number or two numbers separated by a minus sign (meaning an inclusive range). Time specified here is interpreted in the currently active timezone. At the top of the crontab file this is the timezone which is set system-wide in /etc/default/init. A user can add a line such as:
...and all subsequent entries will be interpreted using that timezone, until a new TZ=timezone line is encountered. The specification of days can be made by two fields (day of the month and day of the week). Both are adhered to if specified as a list of elements. See EXAMPLES. The sixth field of a line in a crontab file is a string that is executed by the shell at the specified times. A percent character in this field (unless escaped by \ ) is translated to a NEWLINE character. Only the first line (up to a ` % ' or end of line) of the command field is executed by the shell. Other lines are made available to the command as standard input. Any blank line or line beginning with a ` # ' is a comment and is ignored. The shell is invoked from your $HOME directory. As with $TZ, both $SHELL and $HOME can be set by having a line such as:
...or:
...which will take precedence for all the remaining entries in the crontab or until there is another HOME or SHELL entry. It is invoked with an arg0 of the basename of the $SHELL that is currently in effect. A user who wants to have his .profile or equivalent file executed must explicitly do so in the crontab file. cron supplies a default environment for every shell, defining HOME, LOGNAME, SHELL, TZ, and PATH. The default PATH for user cron jobs is /usr/bin; while root cron jobs default to /usr/sbin:/usr/bin. The default PATH can be set in /etc/default/cron (see cron(1M)). The TZ, HOME, and SHELL environment variables are set to match those that are in effect in the crontab file at the time. If you do not redirect the standard output and standard error of your commands, any generated output or errors are mailed to you. crontab Environment VariablesThe following variables are supported: Each of these variables affects all of the lines that follow it in the crontab file, until it is reset by a subsequent line resetting that variable. Hence, it is possible to have multiple timezones supported within a single crontab file. The lines that are not setting these environment variables are the same as crontab entries that conform to the UNIX standard and are described elsewhere in this man page. Setting cron Jobs Across TimezonesThe default timezone of the cron daemon sets the system-wide timezone for cron entries. This, in turn, is by set by default system-wide using /etc/default/init. If some form of daylight savings or summer/winter time is in effect, then jobs scheduled during the switchover period could be executed once, twice, or not at all. Options
The following options are supported: ExamplesExample 1 Cleaning up Core FilesThis example cleans up core files every weekday morning at 3:15 am:
Example 2 Mailing a Birthday GreetingThis example mails a birthday greeting:
Example 3 Specifying Days of the Month and WeekThis example runs a command on the first and fifteenth of each month, as well as on every Monday:
To specify days by only one field, the other field should be set to *. For example:
would run a command only on Mondays. Example 4 Using Environment VariablesThe following entries take advantage of crontab support for certain environment variables.
The preceding entries allow two jobs to run. The first one would run at midnight in the GMT timezone and the second would run at midnight in the PST timezone. Both would be run in the directory /local/home/user using the Korn shell. The file concludes with TZ, HOME, and SHELL entries that return those variable to their default values. Environment Variables
See environ(5) for descriptions of the following environment variables that affect the execution of crontab: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and NLSPATH. /usr/bin/crontab/usr/xpg4/bin/crontab/usr/xpg6/bin/crontabExit StatusFiles
AttributesSee attributes(5) for descriptions of the following attributes: /usr/bin/crontab/usr/xpg4/bin/crontab/usr/xpg6/bin/crontabSee Alsoatq(1), atrm(1), auths(1), ed(1), sh(1), vi(1), cron(1M), su(1M), auth_attr(4), attributes(5), environ(5), standards(5) NotesIf you inadvertently enter the crontab command with no arguments, do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c. When updating cron, check first for existing crontab entries that can be scheduled close to the time of the update. Such entries can be lost if the update process completes after the scheduled event. This can happen because, when cron is notified by crontab to update the internal view of a user's crontab file, it first removes the user's existing internal crontab and any internal scheduled events. Then it reads the new crontab file and rebuilds the internal crontab and events. This last step takes time, especially with a large crontab file, and can complete after an existing crontab entry is scheduled to run if it is scheduled too close to the update. To be safe, start a new job at least 60 seconds after the current date and time. If an authorized user other than root modifies another user's crontab file, the resulting behavior can be unpredictable. Instead, the authorized user should first use su(1M) to become superuser to the other user's login before making any changes to the crontab file. Care should be taken when adding TZ, SHELL and HOME variables to the crontab file when the crontab file could be shared with applications that do not expect those variables to be changed from the default. Resetting the values to their defaults at the bottom of the file will minimize the risk of problems. Name | Synopsis | Description | Options | Examples | Environment Variables | Exit Status | Files | Attributes | See Also | Notes |
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