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The makefile tells make how to tell whether a target is up to date,
and how to update each target. But updating the targets is not always
what you want. Certain options specify other activities for make.
“No-op”. The activity is to print what recipe would be used to make
the targets up to date, but not actually execute it. Some recipes are
still executed, even with this flag (see section How the MAKE Variable Works).
“Touch”. The activity is to mark the targets as up to date without
actually changing them. In other words, make pretends to compile
the targets but does not really change their contents.
“Question”. The activity is to find out silently whether the targets are up to date already; but execute no recipe in either case. In other words, neither compilation nor output will occur.
“What if”. Each ‘-W’ flag is followed by a file name. The given
files’ modification times are recorded by make as being the present
time, although the actual modification times remain the same.
You can use the ‘-W’ flag in conjunction with the ‘-n’ flag
to see what would happen if you were to modify specific files.
With the ‘-n’ flag, make prints the recipe that it would
normally execute but usually does not execute it.
With the ‘-t’ flag, make ignores the recipes in the rules
and uses (in effect) the command touch for each target that needs to
be remade. The touch command is also printed, unless ‘-s’ or
.SILENT is used. For speed, make does not actually invoke
the program touch. It does the work directly.
With the ‘-q’ flag, make prints nothing and executes no
recipes, but the exit status code it returns is zero if and only if the
targets to be considered are already up to date. If the exit status is
one, then some updating needs to be done. If make encounters an
error, the exit status is two, so you can distinguish an error from a
target that is not up to date.
It is an error to use more than one of these three flags in the same
invocation of make.
The ‘-n’, ‘-t’, and ‘-q’ options do not affect recipe
lines that begin with ‘+’ characters or contain the strings
‘$(MAKE)’ or ‘${MAKE}’. Note that only the line containing
the ‘+’ character or the strings ‘$(MAKE)’ or ‘${MAKE}’
is run regardless of these options. Other lines in the same rule are
not run unless they too begin with ‘+’ or contain ‘$(MAKE)’ or
‘${MAKE}’ (See section How the MAKE Variable Works.)
The ‘-t’ flag prevents phony targets (see section Phony Targets) from being updated, unless there are recipe lines beginning with ‘+’ or containing ‘$(MAKE)’ or ‘${MAKE}’.
The ‘-W’ flag provides two features:
make would do if you were to modify some files.
make is actually
executing recipes, the ‘-W’ flag can direct make to act as
if some files had been modified, without actually running the recipes
for those files.
Note that the options ‘-p’ and ‘-v’ allow you to obtain other
information about make or about the makefiles in use
(see section Summary of Options).
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