Unix: Delayed execution
Introduction
In addition to interactive use of the computer, Gnu/Linux offers several tools that let the user set up programs (and even a sequence of programs, known as “scripts”) to be run at some point in the future without manual intervention. Three of these tools are:
- cron – time based scheduling to manage periodic execution of programs.
- at – tool to scehedulea set of programs to run once at a future date.
- sleep – command to pause a script before continuing execution.
Cron
Cron is the name of time based scheduling framework that lets you schedule a single program or a script to be executed at a specified periodic time/date.
Most Gnu/Linux systems comes with Vixie cron (a version of the cron framework created by Paul Vixie). This is the version that will be described below. Other versions may require a different syntax, so please read up on the documentation that exists on your particular system.
To use the cron framework, you create a crontab, which is a list of entries. Each entry is a single line of text, and consists of a set of time fields indicator followed by a commnd.
There are several ways to create the crontab entries.
I prefer to create an ordinary text file with all the entries I want
in my crontab in a file named crontab.txt. I
then can create an active crontab (or replace the current
crontab with the contents of that file with the following
shell command):
$ crontab crontab.txt
To list the content of your current active crontab, you log on to the host where your crontab is active, and type the following shell command:
$ crontab -l
The time fields for crontab entry are the following
five fields, separated by spaces:
| Field | Values | Comments |
|---|---|---|
| minute | 0-59 | |
| hour | 0-23 | 0 is midnight |
| day of month | 1-31 | |
| month | 1-12 | 1 is Jan, 12 is Dec |
| day of week | 0-6 | 0 is Sun, 6 is Sat (on most systems) |
These fields can contain a single number, a pair of numbers
separated by a dash (i.e. a range of numbers), a comma-separated list
of numbers and ranges, or an asterisk (* that represents
all valid values for that field). A slash is used used in conjunction
with ranges to specify step values.
Some versions may also accept strings of letters: for
instance, Vixie cron accepts month and day names instead of
numbers.
For instance: The first of the following four crontab entries will run command1 at 13:00 (1 p.m.) every 9th of February.
The second will run command2 at 15:05 (five minutes past 3 p.m.) every Monday.
The third will run command3 at 05:00 (5 a.m.) every Tuesday plus the 1st through the 7th of every month,
The fourth will run command4 every ten minutes.
The fifth will, at 03:00 (3 a.m. in the morning) every night, run the
find command to search for
and remove all files below the file system root ending with
.bak that have not been accessed in 7 days.
0 13 9 2 * command1
5 15 * * 1 command2
0 5 1-7 * 2 command3
*/10 * * * * command4
0 3 * * * find / -name "*. bak" -type f -atime +7 -exec rm {} \;
If the first character in a crontab entry is a hash mark
(#), cron will treat the entry as a comment and ignore
it. This is a quick way to disable an entry without deleting it.
At
Sometimes you may need to a program to run just once at one point in the future, rather than periodically. For this you use the at command.
[TBA]
Sleep
[TBA]
Summary
| Command | Meaning |
|---|---|
crontab |
maintain crontab files for individual users |
at |
- |
sleep |
- |
