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Planificació de tasques a Linux

Sinopsi

Aquest article tracta d'algunes eines utilitzades a un sistema Linux per a planificar tasques i que s'executen automàticament a intervals de temps especificats o en un moment determinat en el futur. Aquest curt article no cobrirà aquestes ordres en profunditat; només és una breu introducció a la utilització d'estes ordres. Mire els COM individuals de cada ordre per a veure en profunditat totes les opcions rellevants i configuracions.

Alguns dels daemons de planificació de tasques utilitzats a Linux/UNIX són:

  • at – panifica tasques per a que s'executen una vegada en el futur
  • cron – el planificador periòdic més comunament utilitzat
  • anacron – cron anacrònic; una planificador periòdic que no requereix deixar el sistema encés 24×7

Utilitzant at

La ordre at permet a un usuari executar ordres o guions a (en anglés at) una determinada hora (obligatori) i data (opcional). Les ordres poden introduir-se via la entrada estàndard, redirecció o fitxer.

darkstar:~% at

Interactive at

Using the command at with standard input (keyboard) is a little more complicated than typing one line in at the prompt. The command uses an internal “sub-shell” to gather the required information. Once the command information entry is complete, Ctrl+D (EOT) will signify entry completion. The -m flag specifies a mail message will be sent to the user when the job is finished, regardless if any output was created.

darkstar:~% at 12:01 -m
warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
at> ./my_script.sh
at> <EOT>
job 4 at 2015-06-22 12:01 
darkstar:~% 

File-driven at

Commands can also be contained within a file and run by at:

darkstar:~% at 12:32 -m -f /usr/local/bin/my_script.sh
warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
job 8 at 2015-06-22 12:10

The -m flag will email the user after completion of the command; the -f flag specifies the command will read the job from a file, not from standard input. After the command is typed in (and the appropriate warning is displayed), the at job number1) is displayed.

at Internal Scheduling

The job numbers provided after a command is typed in, or when a file is read, allow the user to know which internal job will be run in sequential order. If a user wants to delete a specific task, all that needs to be known is this internal job number. To remove the job, the command atrm (at remove) is used:

darkstar:~%  at -l
7      2015-06-22 12:10 p tux
8      2015-06-22 12:15 p root

The command atq (at queue) is the same as at -l:

darkstar:~%  atq
7      2015-06-22 12:10 p tux
8      2015-06-22 12:15 p root

To remove the user job, use atrm with the job number:

darkstar:~%  atrm 7

Using cron

cron is a daemon that runs tasks in the background at specific times. For example, if you want to automate downloads of patches on a specific day (Monday), date (2 July), or time (1300), cron will allow you to set this up in a variety of ways. The flexibility inherent in cron can allow administrators and power users to automate repetitive tasks, such as creating backups and system maintenance.

cron is usually configured using a crontab file. The following command will open your user account crontab file:

darkstar:~% crontab -e

To edit the system-level crontab, first log into the root account:

darkstar:~# crontab -e

If your system has sudo installed, type in:

darkstar:~% sudo crontab -e 

The crontab file syntax is:

 # * * * * *  command to execute
 # │ │ │ │ │
 # │ │ │ │ │
 # │ │ │ │ └───── day of week (0 - 6) (Sun(0) /Mon (1)/Tue (2)/Wed (3)/Thu (4)/Fri (5)/Sat (6))
 # │ │ │ └────────── month (1 - 12)
 # │ │ └─────────────── day of month (1 - 31)
 # │ └──────────────────── hour (0 - 23)
 # └───────────────────────── min (0 - 59)

Using an asterisk in any placeholder location, will match any value. For example, the following will run example_script.sh at noon (1200) everyday during the first three months of the year:

#For more information see the manual pages of crontab(5) and cron(8)
#
# min hr day month weekday command
#
#
 
0 11 * 1-3 * /home/user/example_script.sh

Using anacron

anacron is not installed in Slackware by default.2)

anacron is unique from cron in the respect that it does not expect the operating system to be running continuously like a 24×7 server. If the time of execution passes while the system is turned off, anacron executes the command automatically when the machine is turned back on. The reverse is not true for cron - if the computer is turned off during the time of scheduled execution, cron will not execute the job. Another key difference between anacron and cron is the minimum chronological “granularity” - anacron can only execute jobs by day, versus the ability of cron to execute by the minute. Finally, anacron can only be used by root, while cron can be used by root and normal users.

Sources

1)
As distinct from a process ID (PID) known to the operating system
2)
See Slackbuilds.org for more information on anacron on Slackware

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