What is the difference between using single brackets [] and double brackets [[]] in bash?

In bash, single brackets [] and double brackets [[]] are used for conditions, but there are important differences between them:

• [] is the standard POSIX-compatible test, which runs the test command. It is limited in syntax and requires careful escaping and quoting.

• [[]] is an extension of bash (and some other shells), which supports more powerful syntax:

  • Allows the use of logical operators &&, || inside the conditional expression.
  • Supports regular expressions with the operator =~.
  • Does not require strict variable escaping, fewer issues with spaces.

Example:

if [ "$var" = "value" ]; then
  echo "Regular test"
fi

if [[ $var == v* ]]; then
  echo "Using pattern with [[ ]]"
fi

In summary, [[]] is more flexible and convenient for writing complex conditions in bash, but it is not POSIX-compatible.