Databases, Systems & Networks

Source: thegeekstuff.com

Question: How do I execute certain shell script at a specific intervals in Linux using cron job? Provide examples using different time periods.

Answer: Crontab can be used to schedule a job that runs on certain internal. The example here show how to execute a backup.sh shell script using different intervals.

Also, don’t forget to read our previous crontab article that contains 15 practical examples, and also explains about @monthly, @daily, .. tags that you can use in your crontab.

1. Execute a cron job every 5 Minutes

The first field is for Minutes. If you specify * in this field, it runs every minutes. If you specify */5 in the 1st field, it runs every 5 minutes as shown below.

*/5 * * * * /home/ramesh/backup.sh

Note: In the same way, use */10 for every 10 minutes, */15 for every 15 minutes, */30 for every 30 minutes, etc.

2. Execute a cron job every 5 Hours

The second field is for hours. If you specify * in this field, it runs every hour. If you specify */5 in the 2nd field, it runs every 5 hours as shown below.

0 */5 * * * /home/ramesh/backup.sh

Note: In the same way, use */2 for every 2 hours, */3 for every 3 hours, */4 for every 4 hours, etc.

3. Execute a job every 5 Seconds

Cron job cannot be used to schedule a job in seconds interval. i.e You cannot schedule a cron job to run every 5 seconds. The alternative is to write a shell script that uses ‘sleep 5′ command in it.

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Introduction

Before we proceed, it would be best to cover some basic user administration topics that will be very useful in later chapters. Adding Users

One of the most important activities in administering a Linux box is the addition of users. Here you’ll find some simple examples to provide a foundation for future chapters. It is not intended to be comprehensive, but is a good memory refresher. You can use the command man useradd to get the help pages on adding users with the useradd command or the man usermod to become more familiar with modifying users with the usermod command.

Who Is the Super User?

The super user with unrestricted access to all system resources and files in Linux is the user named root. This user has a user ID, of 0 which is universally identified by Linux applications as belonging to a user with supreme privileges. You will need to log in as user root to add new users to your Linux server.

Debian Note: When installing Ubuntu Linux systems, you are prompted to create a primary user that is not root. A root user is created but no password is set, so you initially cannot log in as this user. The primary user can become the root user using the sudo su - command that will be discussed later.

How To Add Users

Adding users takes some planning; read through these steps below before starting:

1) Arrange your list of users into groups by function. In this example there are three groups “parents“, “children” and “soho“.

Parents    Children     Soho
Paul       Alice        Accounts
Jane       Derek        Sales

2) Add the Linux groups to your server:

[root@bigboy tmp]# groupadd parents
[root@bigboy tmp]# groupadd children
[root@bigboy tmp]# groupadd soho

3) Add the Linux users and assign them to their respective groups

[root@bigboy tmp]# useradd -g parents paul
[root@bigboy tmp]# useradd -g parents jane
[root@bigboy tmp]# useradd -g children derek
[root@bigboy tmp]# useradd -g children alice
[root@bigboy tmp]# useradd -g soho accounts
[root@bigboy tmp]# useradd -g soho sales

If you don’t specify the group with the -g, RedHat/Fedora Linux creates a group with the same name as the user you just created; this is also known as the User Private Group Scheme. When each new user first logs in, they are prompted for their new permanent password.

4) Each user’s personal directory is placed in the /home directory. The directory name will be the same as their user name.

[root@bigboy tmp]# ll /home
drwxr-xr-x 2 root root 12288 Jul 24 20:04 lost found
drwx------ 2 accounts soho 1024 Jul 24 20:33 accounts
drwx------ 2 alice children 1024 Jul 24 20:33 alice
drwx------ 2 derek children 1024 Jul 24 20:33 derek
drwx------ 2 jane parents 1024 Jul 24 20:33 jane
drwx------ 2 paul parents 1024 Jul 24 20:33 paul
drwx------ 2 sales soho 1024 Jul 24 20:33 sales
[root@bigboy tmp]#

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screen and tmux

A comparison of the features (or more-so just a table of notes for accessing some of those features) for GNU screen and BSD-licensed tmux.

Source: dayid.org

Tmux, à l’instar de Screen, est un multiplexeur de terminaux, outil permettant d’exploiter plusieurs terminaux au sein d’un seul et même affichage.

Installation

sudo apt-get install tmux

Utilisation de tmux

tmux

Les principaux raccourcis

Raccourcis de base

• <Ctrl> <b> suivi de <c> : Créer un nouveau terminal dans la session tmux active
• <Ctrl> <b> suivi de <n> : Switcher entre les différents terminaux de la session
• <Ctrl> <b> suivi de <X> : Choisir un terminal spécifique (ou X est le numéro du terminal)
• <Ctrl> <b> suivi de <d> : Se détacher de la session tmux (lancer ‘tmux a’ pour s’y rattacher)
• <Ctrl> <b> suivi de <,> : Permet de renommer un terminal
• <Ctrl> <b> suivi de <w> : Affiche la liste des terminaux disponibles
• <Ctrl> <b> suivi de <t> : Afficher l’heure dans un terminal

Commandes dans un Split

• <Ctrl> <b> suivi de <« > : Split vertical du terminal courant en deux ouverture d’un terminal dans le nouveau panel
• <Ctrl> <b> suivi de <%> : Split horizontal du terminal courant en deux ouverture d’un terminal dans le nouveau panel
• <Ctrl> <b> suivi de <o> : Switcher entre les terminaux splittés
• <Ctrl> <b> suivi de <espace> : Changer l’organisation visuelle des terminaux splittés
• <Ctrl> <b> suivi de <Alt> (flèches directionnelles) : Reduire, agrandir fenêtre du split
• <Ctrl> <b> suivi de <!> : Convertir un split en terminal seul
• <Ctrl> <b> suivi de <q> : Afficher les numéros des terminaux splittés
• <Ctrl> <b> puis saisissez :join : permet de joindre un terminal seul dans un split

:join -v -s 3.0 -p 50

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Many system administrators seem to have problems with the concepts of pipes and redirection in a shell. A coworker recently asked me how to deal with log files. How to find the information he was looking for. This article tries to shed some light on it.

Input / Output of shell commands

Many of the basic Linux/UNIX shell commands work in a similar way. Every command that you start from the shell gets three channels assigned:

• STDIN (channel 0):
  Where your command draws the input from. If you don’t specify anything special this will be your keyboard input.
• STDOUT (channel 1):
  Where your command’s output is sent to. If you don’t specify anything special the output is displayed in your shell.
• STDERR (channel 2):
  If anything wrong happens the command will send error message here. By default the output is also displayed in your shell.

Try it yourself. The most basic command that just passes everything through from STDIN to STDOUT is the ‘cat’ command. Just open a shell and type ‘cat’ and press Enter. Nothing seems to happen. But actually ‘cat’ is waiting for input. Type something like “hello world”. Every time you press ‘Enter’ after a line ‘cat’ will output your input. So you will get an echo of everything you type. To let ‘cat’ know that you are done with the input send it an ‘end-of-file’ (EOF) signal by pressing Ctrl-D on an empty line.

The pipe(line)

A more interesting application of the STDIN/STDOUT is to chain commands together. The output of the first command becomes the input of the second command. Imagine the following chain:

The contents of the file /var/log/syslog are sent (as input) to the grep command. grep will filter the stream for lines containing the word ‘postfix’ and output that. Now the next grep picks up what was filtered and filter it further for the word ‘removed’. So now we have only lines containing both ‘postfix’ and ‘removed’. And finally these lines are sent to ‘wc -l’ which is a shell command counting the lines of some input. In my case it found 27 of such lines and printed that number to my shell. In shell syntax this reads:

cat /var/log/syslog | grep 'postfix' | grep 'removed' | wc -l

The ‘|’ character is called pipe. A sequence of such commands joined together with pipes are called pipeline.

Useless use of ‘cat’

Actually ‘cat’ is supposed to be used for concatenating files. Like “cat file1 file2”. But some administrators abuse the command to put something into a pipeline. That’s bad style and the reason why Randal L. Schwartz (a seasoned programmer) used to hand out virtual “Useless use of cat” awards. Shell commands usually can take a filename as the last argument as an input. So this would be right:

grep something /var/log/syslog | wc -l

While this works but is considered bad style:

cat /var/log/syslog | grep something | wc

Or if you knew that grep even has a “-c” option to count lines the whole task could be done with just grep:

grep -c something /var/log/syslog

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