The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION. If the value of the expression is
non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the
primaries described below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are not performed on the words
between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process sub‐
stitution, and quote removal are performed. Conditional operators such as -f must be unquoted to be recognized as primaries.
(( )) to perform mathematic and bitwise comparison, and
[[ ]] to perform more abstract comparisons (to
test file attributes and perform string and arithmetic comparisons), for example
if [ -e test ]; then
echo test exists
echo test does not exist