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How it works...
The prepend() function first confirms that 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.
If the variable is empty when we try to prepend, there will be a trailing : at the end. To fix this, modify the function to 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:
${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.
${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.