First the problem. If you wanted to change every , into a ; in a string in GNU Make you'd probably head for the $(subst) function and do the following:
See the problem? The argument separator for functions in GNU Make is , and hence the first , (the search text) is considered to be separator. Hence the search text in the above is actually the empty string, the replacement text is also the empty string and the ;, is just preprended to whatever is in $(string).
A similar problem occurs with spaces. Suppose you want to replace all spaces with ; in a string. You get a similar problem with $(subst), this time because the leading space is stripped:
That extra space isn't an argument it's just extraneous whitespace and hence it is ignored. GNU Make just ends up appending ; to the $(string).
So, how do you solve this?
The answer is define variables that contain just a comma and just a space and use them. Because the argument splitting is done before variable expansion it's possible to have an argument that's a comma or a space.
For a comma you just do:
comma := ,
And everything works.
For a space you need to get a space into a string, I find the easiest way is like this:
That works because += always space separates the value of the variable with the appended text.
Now, GNU Make has really liberal variable naming rules. Pretty much anything goes, so it's possible to define a variable with the name , or even having the name consisting of a space character.
First, here's how to define them:
, := ,
The first line is clear, it does an immediate define of a , to the variable named ,. The second one is a little more complex. First, I define a variable called space which contains a space character and then I use it to define a variable whose name is that space character.
You can verify that these work using $(warning) (I like to wrap the variable being printed in square brackets for absolute certainty of the content):
$(warning [$( )])
Makefile:2: [ ]
Yes, that's pretty odd looking, but it gets stranger. Since GNU Make will interpret $ followed by a single character as a variable expansion you can drop the braces and write:
$(warning [$ ])
Now that starts to look like escaping. In the examples above you can use these variables to make things a little clearer:
$(subst $ ,;,$(string))
Note that you have to use the $(,) form because function argument splitting occurs before the expansion and GNU Make gets confused. In the second line the space is 'escaped' with the $ sign.
You might be wondering about other crazy variable names: here are a few that's possible with GNU Make:
# Defining the $= or $(=) variable which has the value =
equals := =
$(equals) := =
# Define the $# or $(#) variable which has the value #
hash := \#
$(hash) := \#
# Define the $: or $(:) variable which has the value :
colon := :
$(colon) := :
# Define the $($$) variable which has the value $
dollar := $$
$(dollar) := $$
; := ;
% := %
You probably don't need any of those, but you never know...