Makefile and shell scripts

1. in Makefile, Shell scripts can only be called in target, but cannot be output elsewhere. For example, the following code has no output: VAR = & quot; Hello & quot; echo & quot; $ VAR & quot; all: the above code is not output at any time, if the above code is not in the target, it should be changed to the following: 1. in Makefile, Shell scripts can only be called in the target, but cannot be output elsewhere. For example, the following code shows no output: VAR = "Hello" echo "$ VAR" all :..... the above code is not output at any time and is not in the target. if the above code is changed to the following: VAR = "Hello" all: echo "$ VAR "..... the above code will execute the echo command when making all. 2. execute the shell command in Makefile and create a process in one line for execution. This is why many makefiles have many rows ending with "; \" to ensure that the code is a line rather than multiple lines. in this way, Makefile can be executed in a process, for example: SUBDIR = src exampleall: @ for subdir in $ (SUBDIR); \ do \ echo "building"; \ done the above shows that each row in the for loop ends. 3. all words starting with $ in Makefile are interpreted as variables in Makefile. If you need to call the variables in the shell (or the variable $ in the regular expression), you need to add two $ symbols ($ ). Example: PATH = "/data/" all: echo $ {PATH} echo $ PATH in the example, the first $ {PATH} references variables in Makefile, instead of the PATH environment variable in shell, the latter references the PATH environment variable in Shell. The above three points should be noted when using Makefile to call shell, so be sure to write Makefile.