|^^|
source: http://www.brunolinux.com/02-The_Terminal/The_at_Command.html
THE "AT" COMMAND
Today we have something simple that actually is a bit complicated to explain . . . . . it is about the command "at". This command "at" lets you execute a command or script at a later time, you can set the time in many different ways and even have the result mailed to you after the command has been executed.
Now, using "at" is not only just typing the command and hitting the enter-key . . . there is a little more to it, so let me give an example. In the example we want to have the command "play /usr/share/sounds/KDE_Startup.wav" executed at 12:49 . . . . have a look what I see in the terminal:
|^^|
source: http://serverfault.com/questions/204265/how-to-configure-cron-job-to-run-every-2-days-at-11pm
CRONTAB COMMAND
crontab -e
0 23 */2 * * insert_your_script_here.sh
crontab -e -u root
This will put you in VI editing root's crontab entry. Then as ewwhite says, enter:
0 23 */2 * * insert_your_script_here.sh
and then [^ESC] ZZ to save the changes.
This is a good first attempt, but this is not quite every alternate day, as it will run on the 30th of the month and then next run on the 2nd of the month. If you really do need it to be every 2nd day, then run the script EVERY night.
0 23 * * * insert_your_script_here.sh
and in the start of the script use
#/bin/sh if -f /tmp/altday.txt rm /tmp/altday.txt exit fi touch /tmp/altday.txt
This uses a text file to force the script to exit every alternate invocation.
QUOTE
$ at 12:49 -m
warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
at> play /usr/share/sounds/KDE_Startup.wav
at> <EOT>
job 7 at 2006-05-01 12:49
- On the 1st line you see the command "at 12:49 -m", meaning execute at 12:49 and the "-m" means mail me when the job is finished.
- On the 2nd line it will print some info about the at command and jump to line 3
- On the 3rd line you will see the "at>" prompt, at the "at>" prompt you type the command you want to have executed and you hit the enter-key again.
- On the 4th line you get the "at>" prompt appearing again, this time you pressCtrl+D and it will print <EOT>
- The 5th it will then automatically print the job number and the time the given command will be executed.
APPLICATION USAGE
The format of the at command line shown here is guaranteed only for the
POSIX locale. Other cultures may be supported with substantially dif‐
ferent interfaces, although implementations are encouraged to provide
comparable levels of functionality.
Since the commands run in a separate shell invocation, running in a
separate process group with no controlling terminal, open file descrip‐
tors, traps, and priority inherited from the invoking environment are
lost.
Some implementations do not allow substitution of different shells
using SHELL. System V systems, for example, have used the login shell
value for the user in /etc/passwd. To select reliably another command
interpreter, the user must include it as part of the script, such as:
$ at 1800
myshell myscript
EOT
job ... at ...
$
EXAMPLES
1. This sequence can be used at a terminal:
at −m 0730 tomorrow
sort < file >outfile
EOT
2. This sequence, which demonstrates redirecting standard error to a
pipe, is useful in a command procedure (the sequence of output re‐
direction specifications is significant):
at now + 1 hour <<!
diff file1 file2 2>&1 >outfile | mailx mygroup
!
3. To have a job reschedule itself, at can be invoked from within the
at-job. For example, this daily processing script named my.daily
runs every day (although crontab is a more appropriate vehicle for
such work):
# my.daily runs every day
daily processing
at now tomorrow < my.daily
4. The spacing of the three portions of the POSIX locale timespec is
quite flexible as long as there are no ambiguities. Examples of
various times and operand presentation include:
at 0815am Jan 24
at 8 :15amjan24
at now "+ 1day"
at 5 pm FRIday
at '17
utc+
30minutes'
eof