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arraysbashstring-interpolation

Bash idioms for conditional array append?

I am trying to figure out if there is a shorter way in bash of conditionally appending the value of a variable to an array only if the value is non-null.

I believe the following is correct, but kind of "wordy"

my_array=()

if [[ "${my_first_var:-}" ]]; then
    my_array+=( "${my_first_var}" )
fi
if [[ "${my_second_var:-}" ]]; then
    my_array+= ( "${my_second_var}" )
fi

I'm asking because I'm trying to clean up some code that does this instead:

my_array+=( ${my_first_var:-} ${my_second_var:-} )

which is a hack that "works" to conditionally append only the non-null values to my_array, and is nice because it uses array interpolation, but has the problem that if the string in my_var contains a space (or whatever IFS is set to) it does something very unintended (appends multiple elements into the array).

I was thinking of this maybe, but I'm not sure if my intent is sufficiently clear.

[[ "${my_first_var:-}" ]] && my_array+=( "${my_first_var}" )
[[ "${my_second_var:-}" ]] && my_array+=( "${my_second_var}" )

is there a way to conditionally append only non-null values that is terse but is idiomatic and clear?

Solution

  • One option is

    my_array+=( ${my_var+"$my_var"} )

    That will add the value of my_var to the array if it is defined, even if it is empty. If you want to add it only if it is defined and not empty, use

    my_array+=( ${my_var:+"$my_var"} )