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= INVESTIGATION 1: MONITORING HARD DISK SPACE =

=== Part 1: Hard Disk Space Utilities===

Another essential duty of a Linux system administrator is to anticipate problems and take preventative measures to avoid computer system problems <u>before</u> that occur.

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|{{Admon/tip|Bash Shell Scripting Tips:|<br>'''<u>Using awk to Manipulate Text</u>'''<br><br><ul><li>Very useful command for report generation, text file repair, or text and floating-point decimal manipulation. The command mimics a C program, with braces '''{ }''' that surround the action to perform based on records from a database file matching either test conditions, regular expressions, etc. Fields appear as numbers with $.<br><br></li><li>Examples:<br><br>''awk '{print}' data-file.txt''<br>''awk -F";" '{print $5,$3}' data-file.txt''<br>''awk -F"," '$4 &gt;&#61; 10000 {print $1, $2}' salary.txt''<br><br></li></ul>'''<u>Crontab (Chronograph Tables)</u>'''<br><br><ul><li>Used to automatically run (as opposed to manually run) scripts, programs, or commands. There are many tables (files), but the main one is: '''/etc/cron'''. The '''crontab''' command can be used to ''list'', ''create'', ''modify'' or ''remove'' scheduled jobs in the file.<br><br></li><li>Examples:<br><br>''crontab -e -u user'' # create/modify<br>''crontab -r -u user-name'' # remove specific user's crontab<br>''crontab -l -u username'' #List current schedules</li></ul>}}

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This emphasis on this section focuses on how to run useful shell scripts or shell script that we have created at specific dates/times. It would be silly to expect a system administrator to stay up late (eg. 2 a.m.) to manually run a shell script to terminate processes or to re-boot Linux servers. Database files (tables) are used to provide instructions on how frequent shell scripts or commands can be run.

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[[Image:add_virtual_disk.png|thumb|200px|right|You can add virtual hard disks for a VM by changing to the '''Details''' section for the VM (as opposed to ''console''), click '''Add Hardware''', fill information in the '''Add New Virtual Hardware''' dialog box and clicking '''Finish'''.]]

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Let's say that you have run out of disk space on your '''centos3''' VM, and you need more space on the root file-system in order to host more web-pages, or to support a larger database. What are your options? Getting a replacement hard-drive would probably require re-installation of the operating system and/or backing up and restoring data on the old hard disk.