Shell script: How to prepend env variable with prefix only if environment variable is defined?

Question

Given command that requires flag --flag. We want to insert this flag only when the value of the flag is provided as environment variable VALUE.

I tried the following:

echo "command ${X+--flag ${VALUE}}"

VALUE=1
echo "command ${X+--flag ${VALUE}}"'

References:

• https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02

Answer 1

command ${VALUE:+--flag "$VALUE"}

This runs command with the option --flag and its option-argument "$VALUE" if the variable VALUE is defined and non-empty. If the variable is empty or undefined, it runs command without arguments.

In the zsh shell, which does not perform word-splitting on unquoted expansions, you will have to set the SH_WORD_SPLIT shell option (setopt SH_WORD_SPLIT), run the shell in sh compatibility mode (zsh --emulate sh), or use

command ${=VALUE:+--flag "$VALUE"}

... to ensure that --flag and "$VALUE" are passed as two separate arguments when VALUE is set to a non-empty value.

Remove the : in the parameter substitution if you want the --flag option added when VALUE is defined but empty.