Book Image

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Book Image

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Overview of this book

The shell remains one of the most powerful tools on a computer system — yet a large number of users are unaware of how much one can accomplish with it. Using a combination of simple commands, we will see how to solve complex problems in day to day computer usage.Linux Shell Scripting Cookbook, Second Edition will take you through useful real-world recipes designed to make your daily life easy when working with the shell. The book shows the reader how to effectively use the shell to accomplish complex tasks with ease.The book discusses basics of using the shell, general commands and proceeds to show the reader how to use them to perform complex tasks with ease.Starting with the basics of the shell, we will learn simple commands with their usages allowing us to perform operations on files of different kind. The book then proceeds to explain text processing, web interaction and concludes with backups, monitoring and other sysadmin tasks.Linux Shell Scripting Cookbook, Second Edition serves as an excellent guide to solving day to day problems using the shell and few powerful commands together to create solutions.
Table of Contents (16 chapters)
Linux Shell Scripting Cookbook
About the Authors
About the Reviewers

Function to prepend to environment variables

Environment variables are often used to store a list of paths of where to search for executables, libraries, and so on. Examples are $PATH, $LD_LIBRARY_PATH, which will typically look like this:


This essentially means that whenever the shell has to execute binaries, it will first look into /usr/bin followed by /bin.

A very common task that one has to do when building a program from source and installing to a custom path is to add its bin directory to the PATH environment variable. Let's say in this case we install myapp to /opt/myapp, which has binaries in a directory called bin and libraries in lib.

How to do it...

A way to do this is to say it as follows:

export PATH=/opt/myapp/bin:$PATH
export LD_LIBRARY_PATH=/opt/myapp/lib;$LD_LIBRARY_PATH

PATH and LD_LIBRARY_PATH should now look something like this:


However, we can make this easier by adding this function in .bashrc-:

prepend() { [ -d "$2" ] && eval $1=\"$2':'\$$1\" && export $1; }

This can be used in the following way:

prepend PATH /opt/myapp/bin
prepend LD_LIBRARY_PATH /opt/myapp/lib

How it works...

We define a function called prepend(), which first checks if the directory specified by the second parameter to the function exists. If it does, the eval expression sets the variable with the name in the first parameter equal to the second parameter string followed by : (the path separator) and then the original value for the variable.

However, there is one caveat, if the variable is empty when we try to prepend, there will be a trailing : at the end. To fix this, we can modify the function to look like this:

prepend() { [ -d "$2" ] && eval $1=\"$2\$\{$1:+':'\$$1\}\" && export $1 ; }


In this form of the function, we introduce a shell parameter expansion of the form:


This expands to expression if parameter is set and is not null.

With this change, we take care to try to append : and the old value if, and only if, the old value existed when trying to prepend.