Environment variables are often used to store a list of paths of where to search for executables, libraries, and so on. Examples are $PATH, $LD_LIBRARY_PATH, which will typically look like this:
PATH=/usr/bin;/bin LD_LIBRARY_PATH=/usr/lib;/lib
This essentially means that whenever the shell has to execute binaries, it will first look into /usr/bin followed by /bin.
A very common task that one has to do when building a program from source and installing to a custom path is to add its bin directory to the PATH environment variable. Let's say in this case we install myapp to /opt/myapp, which has binaries in a directory called bin and libraries in lib.
A way to do this is to say it as follows:
export PATH=/opt/myapp/bin:$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
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.