How to learn, use the good shell

Shell in the large data domain is an essential tool, how to learn how to use a good shell, start with a simple command

If [-Z "$DIRNAME"] what does this-Z mean?

File operations like this are useful in shell programming: I've summed up some of the things that are often used
-D: Determine whether the set is a directory
-Z: Determine if a variable exists with a value
-F: Judge whether a file is made
-L: Determine if a symbolic link is made
-R: Determining whether the made is readable
-S: Determine if the existing object length is 0
-W: Judge whether the formulated can be written
-X: Determines whether an existing object can be executed
。 : Negative sign of test condition
These file operations are often convenient for scripting, especially if they are used in an IF condition statement