Shell scripting

SShell script wrappers can make the *nix command more transparent to the user. The most common shell scripts are simple wrappers around the third party or system binaries. A wrapper is nothing but a shell script or a shell function or an alias that includes a system command or utility.

Linux and a Unix-like operating system can run both 32bit and 64bit specific versions of applications. You can write a wrapper script that can select and execute correct version on a 32bit or 64bit hardware platform. In clustered environment and High-Performance computing environment you may find 100s of wrapper scripts written in Perl, Shell, and Python to get cluster usage, setting up shared storage, submitting and managing jobs, backups, troubleshooting, invokes commands with specified arguments, redirecting stdout/stderr and much more.

In this post, I will explain how to create a shell wrapper to enhance the primary troubleshooting tool such as ping and host.
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An bash shell alias is nothing but the shortcut to commands. The alias command allows the user to launch any command or group of commands (including options and filenames) by entering a single word. Use alias command to display a list of all defined aliases. You can add user-defined aliases to ~/.bashrc file. You can cut down typing time with these aliases, work smartly, and increase productivity at the command prompt.
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The $ character is used for parameter expansion, arithmetic expansion and command substitution. You can use it for manipulating and expanding variables on demands without using external commands such as perl, python, sed or awk. This guide shows you how to use parameter expansion modifiers to transform Bash shell variables for your scripting needs.
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There are some misconceptions that shell scripts are only for a CLI environment. You can efficiently use various tools to write GUI and network (socket) scripts under KDE or Gnome desktops. Shell scripts can make use of some of the GUI widget (menus, warning boxes, progress bars, etc.). You can always control the final output, cursor position on the screen, various output effects, and more. With the following tools, you can build powerful, interactive, user-friendly UNIX / Linux bash shell scripts.
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The Blue Screen of Death (BSoD) is used by Microsoft Windows, after encountering a critical system error. Linux / UNIX like operating system may get a kernel panic. It is just like BSoD. The BSoD and a kernel panic generated using a Machine Check Exception (MCE). MCE is nothing but feature of AMD / Intel 64 bit systems which is used to detect an unrecoverable hardware problem. MCE can detect:

  • Communication error between CPU and motherboard.
  • Memory error – ECC problems.
  • CPU cache errors and so on.

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Like most sys admin, I’m lazy. I try to automate almost all things in order to save time. Inexperienced sys admin and help desk staff working under me finds all these tools useful. It saves their time and avoids security issues. Automation allows help desk staff to do things that they don’t have enough direct system knowledge to do themselves. However, selecting correct tool and applying correct methodology is very important.
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This may come handy while writing cross-platform scripts.

If you don’t want to commit to the idiosyncrasies of a specific shell running on a particular platform, try the Squirrel Shell. The Squirrel Shell provides an advanced, object-oriented scripting language that works equally well on UNIX, Linux, Mac OS X, and Windows systems. Write a script once, and run it anywhere.

Squirrel is a high level imperative/OO programming language, designed to be a light-weight scripting language that fits in the size, memory bandwidth, and real-time requirements of applications like video games.

=> Speaking UNIX: The Squirrel portable shell and scripting language

WordPress version 2.7 has been released and can be upgraded easily using UNIX / Linux shell prompt. A few assumptions:

  1. WordPress database name: wptheosblog
  2. WordPress directory name: /var/www/html/theos.in
  3. WordPress domain name: theos.in
  4. Operating system: Linux
WARNING! If you are not comfortable with UNIX shell prompt (IF YOU ARE new to a UNIX/Linux os), please follow traditional way of upgrading wordpress and DO NOT use the three Steps described here.

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I’ve already written a small tutorial about finding out if a file exists or not under Linux / UNIX bash shell. However, couple of our regular readers like to know more about a directory checking using if and test shell command.

General syntax to see if a directory exists or not

[ -d directory ]
OR
test directory
See if a directory exists or not with NOT operator:
[ ! -d directory ]
OR
! test directory

Find out if /tmp directory exists or not

Type the following command:
$ [ ! -d /tmp ] && echo 'Directory /tmp not found'
OR
$ [ -d /tmp ] && echo 'Directory found' || echo 'Directory /tmp not found'

Sample Shell Script to gives message if directory exists

Here is a sample shell script:

#!/bin/bash
DIR="$1"
 
if [ $# -ne 1 ]
then
	echo "Usage: $0 {dir-name}"
	exit 1
fi
 
if [ -d "$DIR" ]
then
	echo "$DIR directory  exists!"
else
	echo "$DIR directory not found!"
fi