Advanced tests using [[

The use of the double brackets [[ condition ]] allows us to do more advanced condition testing but it is not compatible with the Bourne shell. The double brackets were first introduced as a defined keyword in the KornShell and are also available in bash and Zsh. Unlike the single bracket, this is not a command but a keyword. The use of the type command can confirm this:

$ type [[

White space

The fact that [[ is not a command is significant where white space is concerned. As a keyword, [[ parses its arguments before bash expands them. As such, a single parameter will always be represented as a single argument. Even though it goes against best practice, [[ can alleviate some of the issues associated with white space within parameter values. Reconsidering the condition we tested earlier, we can omit the quotes when using [[, as shown in the following example:

$ echo "The File Contents">"my file"
$ FILE="my file"
$ [[ -f $FILE && -r $FILE ]] && cat "$FILE...