Bash create a path with variables divided by _ and check if exist - bash

Question

In Bash, I am trying to create a path with two variables within:
/path/to/my/file/${variable1_-}${variable2}/Still/some/path
My variable2 is always set, but the variable1 might be empty and in that case I don't want to print the "_"
I have tried the line above, but doesn't seem to be correct.
Can someone help in getting the right path printed?
Thanks in advance for your suggestions!

Answer 1

You have a simple typo (the underscore should be after the separator, not part of the variable name) and you want to include the underscore if variable1 is set, not it it's unset (so plus instead of minus in the parameter expansion; and add a colon to also cover the set but empty case). Presumably you also want to include the actual value of variable1 when it's set.
/path/to/my/file/${variable1}${variable1:+_}${variable2}/Still/some/path
or equivalently, nested
/path/to/my/file/${variable1:+${variable1}_}${variable2}/Still/some/path
where the braces before the underscore are necessary to separate the variable name from the literal text.

Answer 2

You can use this.
https://linux.die.net/man/1/bash
${parameter:+word}
set also variable1
variable1=VAR1
variable2=VAR2
variable3=${variable1:+_}
echo /path/to/my/file/${variable1}${variable3}${variable2}/Still/some/path
set only variable2
variable1=
variable2=VAR2
variable3=${variable1:+_}
echo /path/to/my/file/${variable1}${variable3}${variable2}/Still/some/path

Answer 3

With a few more lines of code this could work:
run () {
prefix="" # empty
if [ -n "$variable1" ]; then
prefix="${variable1}_"
fi
echo "/path/to/my/file/${prefix}${variable2}/Still/some/path"
}
# set only variable2
variable2=var2
run
# set also variable1
variable1=var1
run
output:
/path/to/my/file/var2/Still/some/path
/path/to/my/file/var1_var2/Still/some/path
description:
-n tests if the string is not empty, in that case I fill prefix with variable1 and the underscore