9.3 Instead of Executing the Commands

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.

‘-n’

‘--just-print’

‘--dry-run’

‘--recon’

“No-op”. The activity is to print what commands would be used to make the targets up to date, but not actually execute them.

‘-t’

‘--touch’

“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.

‘-q’

‘--question’

“Question”. The activity is to find out silently whether the targets are up to date already; but execute no commands in either case. In other words, neither compilation nor output will occur.

‘-W file’

‘--what-if=file’

‘--assume-new=file’

‘--new-file=file’

“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 commands that it would normally execute but does not execute them.

With the ‘-t’ flag, make ignores the commands 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 commands, 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 command 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 How the MAKE Variable Works.)

The ‘-W’ flag provides two features:

• If you also use the ‘-n’ or ‘-q’ flag, you can see what make would do if you were to modify some files.
• Without the ‘-n’ or ‘-q’ flag, when make is actually executing commands, the ‘-W’ flag can direct make to act as if some files had been modified, without actually modifying the files.

Note that the options ‘-p’ and ‘-v’ allow you to obtain other information about make or about the makefiles in use (see Summary of Options).