cron is a task scheduler and is available with most Linux distributions.For information on using cron, the following
man pages may be of help:
· man cron
· man crontab
· man 5 crontab
A user's crontab is a text file containing the commands to be run by cron for that user. To edit or create a crontab, open a terminal window and execute the following command:
crontab -e
This command will open a vi (vim) editor with your user's crontab (or an empty crontab file if you do not already have one).
In the vi editor, press the colon (:) key, then the i key, and press the Enter key to switch from command mode to edit mode so you can enter text into your crontab file. (:i switches from command mode to edit mode).
It is usually a good idea to include a comment with each command line. Comment lines begin with a pound (#) character, so add a comment such as:
# Run trial.sh each hour
Press the Enter key to move the cursor to a new line.
Then add the cron command:
0 * * * * ~/bin trial.sh
Again, press the Enter key to move the cursor to a new line.
Press the Escape (Esc) key.
Enter the following command:
:wq
Press the Enter key to write your new crontab file and quit the editor.
Note: A crontab command line contains six fields
separated by a space chaacter. The fields are: 1. minute
2. hour
3. day-of-month
4. month
5. day-of-week
6. command
Note2:
: preceeds commands in vi
so
i = switch to edit mode
wq = write changes to file and quit vi
w = write changes to file but do not quit vi
q! = quit but do not write changes to file
You enter these commands in vi as:
:i (switch to edit mode)
:wq (write changes to file and quit vi)
:w (write changes to file but do not quit vi)
:q! (quit vi but do not write changes to file)
From man 5 crontab:
The time and date fields are:
field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names)
day of week 0-7 (0 or 7 is Sun or use names)
A field may be an asterisk (*), which always stands for "first-last" (all).
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 "<number>" 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 cronfile. 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.
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 (ie, 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.
HTH,
Ernie Registered Linux User 247790
ICQ 41060744
