Question

I’m dynamically generating config.mk with a bash script which will be used by a Makefile. The file is constructed with:

cat > config.mk <<CFG
SOMEVAR := $value_from_bash1
ANOTHER := $value_from_bash2
CFG

How do I ensure that the generated file really contains the contents of $value_from_bash*, and not something expanded / interpreted? I probably need to escape $ to $$ and \ to \\, but are there other characters that needs to be escaped? Perhaps there is a special literal assignment I’ve not heard of?

Spaces seems to be troublesome too:

$ ls -1
a b
a
$ cat Makefile
f := a b
default_target:
    echo "$(firstword $(wildcard ${f}))"
$ make
a

If I use f := a\ b it works (using quotes like f := 'a b' did not work either, makefile just treats it as a regular character)

Answer 1

Okay, it turned out that Makefiles need little escaping for itself, but the commands which are executed by the shell interpreter need to be escaped.

Characters which have a special meaning in Makefile and that need to be escaped are:

• sharp (#, comment) becomes \#
• dollar ($, begin of variable) becomes $$

Newlines cannot be inserted in a variable, but to avoid breaking the rest of the Makefile, prepend it with a backslash so the line break will be ignored.

Too bad a backslash itself cannot be escaped (\\ will still be \\ and not \ as you might expect). This makes it not possible to put a literal slash on the end of a string as it will either eat the newline or the hash of a following comment. A space can be put on the end of the line, but that’ll also be put in the variable itself.

The recipe itself is interpreted as a shell command, without any fancy escaping, so you’ve to escape data yourself, just imagine that you’re writing a shellscript and inserting the variables from other files. The strategy here would be putting the variables between single quotes and escape only ' with '\'' (close the string, insert a literal ' and start a new string). Example: mornin' all becomes 'morning'\'' all' which is equivalent to "morning' all".

The firstword+wildcard issue is caused by the fact that filenames with spaces in them are treated as separate filenames by firstword. Furthermore, wildcard expands escapes using \ so x\ y is matches as one word, x y and not two words.