andy753421 1185363094 learn [[ $a ]] tests if $a is not null, [ $a ] tests if $a contains non whitespace characters, [ ${a+1} ] tests if $a has been defined.
Riviera 1309388080 forget
Riviera 1309388087 learn !faq defined
Riviera 1309388108 forget
Riviera 1309388123 learn http://mywiki.wooledge.org/BashFAQ/083 -- How do I determine whether a variable is already defined? Or a function?