How to conditional set up a Makefile variable by testing if a file exists

Lazy Set

VARIABLE = value

Normal setting of a variable, but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

VARIABLE := value

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

Lazy Set If Absent

VARIABLE ?= value

Setting of a variable only if it doesn't have a value. value is always evaluated when VARIABLE is accessed. It is equivalent to

ifeq ($(origin VARIABLE), undefined)
  VARIABLE = value
endif

See the documentation for more details.

Append

VARIABLE += value

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)

It is actually executing the command, changing the directory to some_directory, however, this is performed in a sub-process shell, and affects neither make nor the shell you're working from.

If you're looking to perform more tasks within some_directory, you need to add a semi-colon and append the other commands as well. Note that you cannot use newlines as they are interpreted by make as the end of the rule, so any newlines you use for clarity needs to be escaped by a backslash.

For example:

all:
        cd some_dir; echo "I'm in some_dir"; \
          gcc -Wall -o myTest myTest.c

Note also that the semicolon is necessary between every command even though you add a backslash and a newline. This is due to the fact that the entire string is parsed as a single line by the shell. As noted in the comments, you should use '&&' to join commands, which mean they only get executed if the preceding command was successful.

all:
        cd some_dir && echo "I'm in some_dir" && \
          gcc -Wall -o myTest myTest.c

This is especially crucial when doing destructive work, such as clean-up, as you'll otherwise destroy the wrong stuff, should the cd fail for whatever reason.

A common usage though is to call make in the sub directory, which you might want to look into. There's a command line option for this so you don't have to call cd yourself, so your rule would look like this

all:
        $(MAKE) -C some_dir all

which will change into some_dir and execute the Makefile in there with the target "all". As a best practice, use $(MAKE) instead of calling make directly, as it'll take care to call the right make instance (if you, for example, use a special make version for your build environment), as well as provide slightly different behavior when running using certain switches, such as -t.

For the record, make always echos the command it executes (unless explicitly suppressed), even if it has no output, which is what you're seeing.