How to understand "${PATH:+:${PATH}}" in bash?

user_178732 0
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I am seeing the following code in a bash script:

export PATH=/opt/rh/rh-python38/root/usr/local/bin:/opt/rh/rh-python38/root/usr/bin${PATH:+:${PATH}}

I do not understand the last part, where ${PATH:+:${PATH}} is manipulated.

parameter-expansion bash
1 Answers

It's to append : and $PATH, but only if $PATH is non-empty; otherwise, nothing gets added.

The syntax for the expansion is ${parameter:+word}, and it stands for

If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

In your example, word is :$PATH.

There is a subtlety around ${parameter:+word} vs. ${parameter+word}, where the former substitutes nothing if parameter is unset or null, and the latter substitutes nothing only if parameter is unset (but could be empty).

user_32668470
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