Detection and return code of unixshell learning

When unixshell detects the returned code and runs any command, linux returns the response related to the execution of the previous command, called the return code (returncode). how can this code be returned? Simple. use $? You can view the results. you can view the results. if the command successfully runs the unix shell learning check and returns the code. when you run any command, linux returns the response of the previous command execution, it is called return code. how can I get back to the code? Simple. use $? You can view it. you can view it. if the command is successfully executed, the return code is 0. if the command is not successfully executed, a value other than 0 is returned. We can use a small example in the book to perform a test. the following code writes three different if statements to better learn shell, but their functions are the same. #! /Bin/bashtest-d/usr/local/binif ["$? "-Eq 0] then echo"/usr/local/bin exist! "Else echo"/usr/local/bin does not exist! "Fi # --------------------------------------- if test-d/usr/local/binthen echo"/usr/local/bin exist! "Else echo"/usr/local/bin does not exist! "Fi # ------------------------------------- if [-d/usr/local/bin] then echo"/usr/local/bin exist! "Else echo"/usr/local/bin does not exist! "Fi