Shell Something Out | Linux Shell Scripting Cookbook, Second Edition

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:

PATH=/usr/bin;/bin
LD_LIBRARY_PATH=/usr/lib;/lib

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:

PATH=/opt/myapp/bin:/usr/bin:/bin
LD_LIBRARY_PATH=/opt/myapp/lib:/usr/lib;/lib

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 ; }

Note

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

${parameter:+expression}

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.

Tech Concepts

Programming languages

Tech Tools

Unlimited access to the largest independent learning library in tech of over 8,000 expert-authored tech books and videos.

Innovative learning tools, including AI book assistants, code context explainers, and text-to-speech.

50+ new titles added per month and exclusive early access to books as they are being written.

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Linux Shell Scripting Cookbook, Second Edition

Overview of this book

Function to prepend to environment variables

How to do it...

How it works...

Note

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Linux Shell Scripting Cookbook, Second Edition

Overview of this book

Function to prepend to environment variables

How to do it...

How it works...

Note

Confirmation

Buy this book with your credits?

Submit Your Feedback

Create a Free Account To Continue Reading

Sign in to activate your 7-day free access