Test command recap

As we have seen in some of our previous examples, we used the shell built-in test to perform some checks on variables and files along with the conditional if...then so that we could make our script react to conditions: if the test is successful it returns 0, if it is not, 1, and these are the values that triggered our reactions so far.

We can use a couple of different notations to a test and we already saw them:

[expression]

or

[[expression]]

We already spoke about the differences between the two, but let us quickly recap them before proceeding:

• The single bracket implements the standard POSIX compliant test command and it is available in all POSIX shells. [ is actually a command whose argument is ], and this prevents the single brackets from receiving further arguments.
  • Some Linux versions still have a /bin/[ command, but the built-in version has the precedence in execution.
• The double brackets is only available in Bash, zsh, and korn shells.
• The double brackets is a keyword...