12.11 Comments in Make Rules

Never put comments in a rule.

Some make treat anything starting with a tab as a command for the current rule, even if the tab is immediately followed by a #. The make from Tru64 Unix V5.1 is one of them. The following makefile runs # foo through the shell.

all:
        # foo

As a workaround, you can use the : no-op command with a string argument that gets ignored:

all:
        : "foo"

Conversely, if you want to use the ‘#’ character in some command, you can only do so by expanding it inside a rule (see Comments in Make Macros). So for example, if ‘COMMENT_CHAR’ is substituted by config.status as ‘#’, then the following substitutes ‘@COMMENT_CHAR@’ in a generated header:

foo.h: foo.h.in
        sed -e 's|@''COMMENT_CHAR''@|@COMMENT_CHAR@|g' \
            $(srcdir)/foo.h.in > $@

The funny shell quoting avoids a substitution at config.status run time of the left-hand side of the sed ‘s’ command.