After each shell command returns, make looks at its exit status.
If the command completed successfully, the next command line is executed
in a new shell; after the last command line is finished, the rule is
finished.
If there is an error (the exit status is nonzero), make gives up on
the current rule, and perhaps on all rules.
Sometimes the failure of a certain command does not indicate a problem.
For example, you may use the mkdir command to ensure that a
directory exists. If the directory already exists, mkdir will
report an error, but you probably want make to continue regardless.
To ignore errors in a command line, write a `-' at the beginning of the line's text (after the initial tab). The `-' is discarded before the command is passed to the shell for execution.
For example,
clean:
-rm -f *.o
This causes rm to continue even if it is unable to remove a file.
When you run make with the `-i' or `--ignore-errors'
flag, errors are ignored in all commands of all rules. A rule in the
makefile for the special target .IGNORE has the same effect, if
there are no dependencies. These ways of ignoring errors are obsolete
because `-' is more flexible.
When errors are to be ignored, because of either a `-' or the
`-i' flag, make treats an error return just like success,
except that it prints out a message that tells you the status code
the command exited with, and says that the error has been ignored.
When an error happens that make has not been told to ignore,
it implies that the current target cannot be correctly remade, and neither
can any other that depends on it either directly or indirectly. No further
commands will be executed for these targets, since their preconditions
have not been achieved.
Normally make gives up immediately in this circumstance, returning a
nonzero status. However, if the `-k' or `--keep-going'
flag is specified, make
continues to consider the other dependencies of the pending targets,
remaking them if necessary, before it gives up and returns nonzero status.
For example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already knows
that linking them will be impossible. See section Summary of Options.
The usual behavior assumes that your purpose is to get the specified
targets up to date; once make learns that this is impossible, it
might as well report the failure immediately. The `-k' option says
that the real purpose is to test as many of the changes made in the
program as possible, perhaps to find several independent problems so
that you can correct them all before the next attempt to compile. This
is why Emacs' compile command passes the `-k' flag by
default.
Usually when a command fails, if it has changed the target file at all,
the file is corrupted and cannot be used--or at least it is not
completely updated. Yet the file's timestamp says that it is now up to
date, so the next time make runs, it will not try to update that
file. The situation is just the same as when the command is killed by a
signal; see section Interrupting or Killing make. So generally the right thing to do is to
delete the target file if the command fails after beginning to change
the file. make will do this if .DELETE_ON_ERROR appears
as a target. This is almost always what you want make to do, but
it is not historical practice; so for compatibility, you must explicitly
request it.