how to smart append LD_LIBRARY_PATH in shell when nounset

You could use this construct:

export LD_LIBRARY_PATH=/mypath${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}

Explanation:

• If LD_LIBRARY_PATH is not set, then ${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH} expands to nothing without evaluating $LD_LIBRARY_PATH, thus the result is equivalent to export LD_LIBRARY_PATH=/mypath and no error is raised.

• If LD_LIBRARY_PATH is already set, then ${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH} expands to :$LD_LIBRARY_PATH, thus the result is equivalent to export LD_LIBRARY_PATH=/mypath:$LD_LIBRARY_PATH.

See the Bash Reference Manual / 3.5.3 Shell Parameter Expansion for more information on these expansions.

This is an important security practice as two adjacent colons or a trailing/leading colon count as adding the current directory to $PATH or $LD_LIBRARY_PATH. See also:

• What corner cases must we consider when parsing path on linux
• bash: $PATH ending in colon puts working directory in PATH
• How Torch broke ls