linuxcrontab的格式规范和例子

#service crond start
或者
#/etc/init.d/crond start

*　　*　　*　　*　　*　　command
分　时　日　月　周　命令 
第1列表示分钟1～59 （每分钟用*或者 */1表示，*/5 代表每5分钟）
第2列表示小时1～23（0表示0点）
第3列表示日期1～31
第4列表示月份1～12
第5列标识号星期0～6（0表示星期天）
第6列要运行的命令

MIN HOUR DOM MON DOW CMD

30 08 10 06 * /home/ramesh/full-backup

00 11,16 * * * /home/ramesh/bin/incremental-backup

00 09-18 * * * /home/ramesh/bin/check-db-status

00 09-18 * * 1-5 /home/ramesh/bin/check-db-status

 

ramesh@dev-db$ crontab -l
@yearly /home/ramesh/annual-maintenance
*/10 * * * * /home/ramesh/check-disk-space

[Note: This displays crontab of the current logged in user]

View Root Crontab entries

Login as root user (su – root) and do crontab -l as shown below.

root@dev-db# crontab -l
no crontab for root

Crontab HowTo: View Other Linux User’s Crontabs entries

To view crontab entries of other Linux users, login to root and use -u {username} -l as shown below.

root@dev-db# crontab -u sathiya -l
@monthly /home/sathiya/monthly-backup
00 09-18 * * * /home/sathiya/check-db-status

5. How to Edit Crontab Entries?

Edit Current Logged-In User’s Crontab entries

To edit a crontab entries, use crontab -e as shown below. By default this will edit the current logged-in users crontab.

ramesh@dev-db$ crontab -e
@yearly /home/ramesh/centos/bin/annual-maintenance
*/10 * * * * /home/ramesh/debian/bin/check-disk-space
~
"/tmp/crontab.XXXXyjWkHw" 2L, 83C

[Note: This will open the crontab file in Vim editor for editing.
Please note cron created a temporary /tmp/crontab.XX... ]

When you save the above temporary file with :wq, it will save the crontab and display the following message indicating the crontab is successfully modified.

~
"crontab.XXXXyjWkHw" 2L, 83C written
crontab: installing new crontab

Edit Root Crontab entries

Login as root user (su – root) and do crontab -e as shown below.

root@dev-db# crontab -e

Edit Other Linux User’s Crontab File entries

To edit crontab entries of other Linux users, login to root and use -u {username} -e as shown below.

root@dev-db# crontab -u sathiya -e
@monthly /home/sathiya/fedora/bin/monthly-backup
00 09-18 * * * /home/sathiya/ubuntu/bin/check-db-status
~
~
~
"/tmp/crontab.XXXXyjWkHw" 2L, 83C

6. Schedule a Job for Every Minute Using Cron.

Ideally you may not have a requirement to schedule a job every minute. But understanding this example will will help you understand the other examples mentioned below in this article.

* * * * * CMD

The * means all the possible unit — i.e every minute of every hour through out the year. More than using this * directly, you will find it very useful in the following cases.

• When you specify */5 in minute field means every 5 minutes.
• When you specify 0-10/2 in minute field mean every 2 minutes in the first 10 minute.
• Thus the above convention can be used for all the other 4 fields.

7. Schedule a Background Cron Job For Every 10 Minutes.

Use the following, if you want to check the disk space every 10 minutes.

*/10 * * * * /home/ramesh/check-disk-space

It executes the specified command check-disk-space every 10 minutes through out the year. But you may have a requirement of executing the command only during office hours or vice versa. The above examples shows how to do those things.

Instead of specifying values in the 5 fields, we can specify it using a single keyword as mentioned below.

There are special cases in which instead of the above 5 fields you can use @ followed by a keyword — such as reboot, midnight, yearly, hourly.

Table: Cron special keywords and its meaning

Keyword	Equivalent
@yearly	0 0 1 1 *
@daily	0 0 * * *
@hourly	0 * * * *
@reboot	Run at startup.

8. Schedule a Job For First Minute of Every Year using @yearly

If you want a job to be executed on the first minute of every year, then you can use the@yearly cron keyword as shown below.

This will execute the system annual maintenance using annual-maintenance shell script at 00:00 on Jan 1st for every year.

@yearly /home/ramesh/red-hat/bin/annual-maintenance

9. Schedule a Cron Job Beginning of Every Month using @monthly

It is as similar as the @yearly as above. But executes the command monthly once using@monthly cron keyword.

This will execute the shell script tape-backup at 00:00 on 1st of every month.

@monthly /home/ramesh/suse/bin/tape-backup

10. Schedule a Background Job Every Day using @daily

Using the @daily cron keyword, this will do a daily log file cleanup using cleanup-logs shell scriptat 00:00 on every day.

@daily /home/ramesh/arch-linux/bin/cleanup-logs "day started"

11. How to Execute a Linux Command After Every Reboot using @reboot?

Using the @reboot cron keyword, this will execute the specified command once after the machine got booted every time.

@reboot CMD

12. How to Disable/Redirect the Crontab Mail Output using MAIL keyword?

By default crontab sends the job output to the user who scheduled the job. If you want to redirect the output to a specific user, add or update the MAIL variable in the crontab as shown below.

ramesh@dev-db$ crontab -l
MAIL="ramesh"

@yearly /home/ramesh/annual-maintenance
*/10 * * * * /home/ramesh/check-disk-space

[Note: Crontab of the current logged in user with MAIL variable]

If you wanted the mail not to be sent to anywhere, i.e to stop the crontab output to be emailed, add or update the MAIL variable in the crontab as shown below.

MAIL=""

13. How to Execute a Linux Cron Jobs Every Second Using Crontab.

You cannot schedule a every-second cronjob. Because in cron the minimum unit you can specify is minute. In a typical scenario, there is no reason for most of us to run any job every second in the system.

14. Specify PATH Variable in the Crontab

All the above examples we specified absolute path of the Linux command or the shell-script that needs to be executed.

For example, instead of specifying /home/ramesh/tape-backup, if you want to just specify tape-backup, then add the path /home/ramesh to the PATH variable in the crontab as shown below.

ramesh@dev-db$ crontab -l

PATH=/bin:/sbin:/usr/bin:/usr/sbin:/home/ramesh

@yearly annual-maintenance
*/10 * * * * check-disk-space

[Note: Crontab of the current logged in user with PATH variable]

15. Installing Crontab From a Cron File

Instead of directly editing the crontab file, you can also add all the entries to a cron-file first. Once you have all thoese entries in the file, you can upload or install them to the cron as shown below.

ramesh@dev-db$ crontab -l
no crontab for ramesh

$ cat cron-file.txt
@yearly /home/ramesh/annual-maintenance
*/10 * * * * /home/ramesh/check-disk-space

ramesh@dev-db$ crontab cron-file.txt

ramesh@dev-db$ crontab -l
@yearly /home/ramesh/annual-maintenance
*/10 * * * * /home/ramesh/check-disk-space

Note: This will install the cron-file.txt to your crontab, which will also remove your old cron entries. So, please be careful while uploading cron entries from a cron-file.txt.

 

 

 

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http://www.pantz.org/software/cron/croninfo.html

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Cron

Cron is a daemon that executes scheduled commands. Cron is started automatically from /etc/init.d on entering multi-user runlevels. Cron searches its spool area (/var/spool/cron/crontabs) for crontab files (which are named after accounts in /etc/passwd); crontabs found are loaded into memory. Note that crontabs in this directory should not be accessed directly - the crontab command should be used to access and update them.

Cron also reads /etc/crontab, which is in a slightly different format. Additionally, cron reads the files in /etc/cron.d.

Cron then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists). The children copies of cron running these processes have their name coerced to uppercase, as will be seen in the syslog and ps output.

Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified. Note that the crontab(1) command updates the modtime of the spool directory whenever it changes a crontab.

Special considerations exist when the clock is changed by less than 3 hours, for example at the beginning and end of daylight savings time. If the time has moved forwards, those jobs which would have run in the time that was skipped will be run soon after the change. Conversely, if the time has moved backwards by less than 3 hours, those jobs that fall into the repeated time will not be re-run.

Only jobs that run at a particular time (not specified as @hourly, nor with '*' in the hour or minute specifier) are affected. Jobs which are specified with wild cards are run based on the new time immediately.

Clock changes of more than 3 hours are considered to be corrections to the clock, and the new time is used immediately.

In Debian and Redhat cron treats the files in /etc/cron.d as extensions to the /etc/crontab file (they follow the special format of that file, i.e. they include the user field). The intended purpose of this feature is to allow packages that require finer control of their scheduling than the /etc/cron.{daily,weekly,monthly} directories allow to add a crontab file to /etc/cron.d. Such files should be named after the package that supplies them. Files must conform to the same naming convention as used by run-parts: they must consist solely of upper- and lower-case letters, digits, underscores, and hyphens. Like /etc/crontab, the files in the /etc/cron.d directory are monitored for changes.

You should use absolute path names for commands like /bin/ls. This is to insure you call the correct command.

Crontab

Crontab is the program used to install, deinstall or list the tables used to drive the cron daemon in Vixie Cron. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.

Each user has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab. Uucp and News will usually have their own crontabs, eliminating the need for explicitly running su as part of a cron command.

Blank lines and leading spaces and tabs are ignored. Lines whose first non-space character is a hash-sign (#) are comments, and are ignored. Note that comments are not allowed on the same line as cron commands, since they will be taken to be part of the command. Similarly, comments are not allowed on the same line as environment variable settings.

An active line in a crontab will be either an environment setting or a cron command. An environ‐ ment setting is of the form: name = value where the spaces around the equal-sign (=) are optional, and any subsequent non-leading spaces in value will be part of the value assigned to name. The value string may be placed in quotes (single or double, but matching) to preserve leading or trailing blanks. The value string is not parsed for environmental substitutions, thus lines like: PATH = $HOME/bin:$PATH will not work as you might expect.

Several environment variables are set up automatically by the cron daemon. SHELL is set to /bin/sh, and LOGNAME and HOME are set from the /etc/passwd line of the crontab's owner. PATH is set to "/usr/bin:/bin". HOME, SHELL, and PATH may be overridden by settings in the crontab; LOGNAME is the user that the job is running from, and may not be changed. Another note: the LOGNAME variable is sometimes called USER on BSD systems... on these systems, USER will be set also.

In addition to LOGNAME, HOME, and SHELL, cron will look at MAILTO if it has any reason to send mail as a result of running commands in "this" crontab. If MAILTO is defined (and non-empty), mail is sent to the user so named. If MAILTO is defined but empty (MAILTO=""), no mail will be sent. Otherwise mail is sent to the owner of the crontab.

If the /etc/cron.allow file exists, then you must be listed therein in order to be allowed to use this command. If the /etc/cron.allow file does not exist but the /etc/cron.deny file does exist, then you must not be listed in the /etc/cron.deny file in order to use this command. If neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be allowed to use this command, or all users will be able to use this command. For standard Debian systems, all users may use this command.

If the -u option is given, it specifies the name of the user whose crontab is to be tweaked. If this option is not given, crontab examines "your" crontab, i.e., the crontab of the person executing the command. Note that su can confuse crontab and that if you are running inside of su you should always use the -u option for safety's sake.

The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename ``-'' is given.

The -l option causes the current crontab to be displayed on standard output.

The -r option causes the current crontab to be removed.

The -e option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables. The specified editor must edit the file in place; any editor that unlinks the file and recreates it cannot be used. After you exit from the editor, the modified crontab will be installed automatically.

On the Debian GNU/Linux system, cron supports the pam_env module, and loads the environment specified by /etc/security/pam_env.conf. However, the PAM setting do NOT override the settings described above nor any settings in the crontab file itself. Note in particular that if you want a PATH other than "/usr/bin:/bin", you will need to set it in the crontab file.

By default, cron will send mail using the mail "Content-Type:" header of "text/plain" with the "charset=" parameter set to the charmap / codeset of the locale in which crond is started up - ie. either the default system locale, if no LC_* environment variables are set, or the locale specified by the LC_* environment variables ( see locale(7)). You can use different character encodings for mailed cron job output by setting the CONTENT_TYPE and CONTENT_TRANSFER_ENCODING variables in crontabs, to the correct values of the mail headers of those names.

Crontab Format

Commands are executed by cron when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) match the current time.

A field may be an asterisk (*), which always stands for "first-last".

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an "hours" entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: "1,2,5,9", "0-4,8-12".

Step values can be used in conjunction with ranges. Following a range with "/" specifies skips of the number's value through the range. For example, "0-23/2" can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is "0,2,4,6,8,10,12,14,16,18,20,22"). Steps are also permitted after an asterisk, so if you want to say "every two hours", just use "*/2".

Names can also be used for the "month" and "day of week" fields. Use the first three letters of the particular day or month (case doesn't matter). Ranges or lists of names are not allowed.

The "sixth" field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the crontab file. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input. There is no way to split a single command line onto multiple lines, like the shell's trailing "\".

Note: The day of a command's execution can be specified by two fields - day of month, and day of week. If both fields are restricted (i.e., aren't *), the command will be run when either field matches the current time. For example, "30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.

Instead of the first five fields, one of eight special strings may appear:

string         meaning
------         -------
@reboot        Run once, at startup.
@yearly        Run once a year, "0 0 1 1 *".
@annually      (same as @yearly)
@monthly       Run once a month, "0 0 1 * *".
@weekly        Run once a week, "0 0 * * 0".
@daily         Run once a day, "0 0 * * *".
@midnight      (same as @daily)
@hourly        Run once an hour, "0 * * * *".

An example of crontab format with commented fields is as follows:

# Minute   Hour   Day of Month       Month          Day of Week        Command    
# (0-59)  (0-23)     (1-31)    (1-12 or Jan-Dec)  (0-6 or Sun-Sat)                
    0        2          12             *               0,6           /usr/bin/find

This line executes the "find" command at 2AM on the 12th of every month that a Sunday or Saturday falls on.

Examples

Here are some more examples of crontab lines. Use the command "crontab -e" to edit your crontab file.

This line executes the "ping" command every minute of every hour of every day of every month. The standard output is redirected to dev null so we will get no e-mail but will allow the standard error to be sent as a e-mail. If you want no e-mail ever change the command line to "/sbin/ping -c 1 192.168.0.1 > /dev/null 2>&1".

*       *       *       *       *       /sbin/ping -c 1 192.168.0.1 > /dev/null

This line executes the "ping" and the "ls" command every 12am and 12pm on the 1st day of every 2nd month. It also puts the output of the commands into the log file /var/log/cronrun.

0 0,12 1 */2 * /sbin/ping -c 192.168.0.1; ls -la >>/var/log/cronrun

This line executes the disk usage command to get the directory sizes every 2am on the 1st through the 10th of each month. E-mail is sent to the email addresses specified with the MAILTO line. The PATH is also set to something different.

PATH=/usr/local/sbin:/usr/local/bin:/home/user1/bin
MAILTO=user1@nowhere.org,user2@somewhere.org
0 2 1-10 * * du -h --max-depth=1 /

This line is and example of running a cron job every month, on Mondays whose dates are between 15-21. This means the third Monday only of the month at 4 a.m.

0 4 15-21 * 1 /command

Things to look out for! Gotchas!

1. When cron job is run from the users crontab it is executed as that user. It does not however source any files in the users home directory like their .cshrc or .bashrc or any other file. If you need cron to source (read) any file that your script will need you should do it from the script cron is calling. Setting paths, sourcing files, setting environment variables, etc.
2. If the users account has a crontab but no useable shell in /etc/passwd then the cronjob will not run. You will have to give the account a shell for the crontab to run.
3. If your cronjobs are not running check if the cron deamon is running. Then remember to check /etc/cron.allow and /etc/cron.deny files. If they exist then the user you want to be able to run jobs must be in /etc/cron.allow. You also might want to check if the /etc/security/access.conf file exists. You might need to add your user in there.
4. Crontab is not parsed for environmental substitutions. You can not use things like $PATH, $HOME, or ~/sbin. You can set things like MAILTO= or PATH= and other environment variables the /bin/sh shell uses.
5. Cron does not deal with seconds so you can't have cronjob's going off in any time period dealing with seconds. Like a cronjob going off every 30 seconds.
6. You can not use % in the command area. They will need to be escaped and if used with a command like the date command you can put it in backticks. Ex. `date +\%Y-\%m-\%d`