When adding a string that possibly contains whitespace or quotes to a list (example 1), it must be quoted using the :Q modifier.

When adding another list to a list (example 2), it must not be quoted, since its elements are already quoted.

STRING=         foo * bar `date`
LIST=           # empty
ANOTHER_LIST=   a=b c=d

LIST+=          ${STRING:Q}       # 1
LIST+=          ${ANOTHER_LIST}   # 2

Echoing a string containing special characters needs special work.

STRING=         foo bar <    > * `date` $$HOME ' "
EXAMPLE_ENV=    string=${STRING:Q} x=multiple\ quoted\ words

all:
        echo ${STRING}                  # 1
        echo ${STRING:Q}                # 2
        printf '%s\n' ${STRING:Q}''     # 3
        env ${EXAMPLE_ENV} sh -c 'echo "$$string"; echo "$$x"'   # 4

Example 1 leads to a syntax error in the shell, as the characters are just copied.

Example 2 quotes the string so that the shell interprets it correctly. But the echo command may additionally interpret strings with a leading dash or those containing backslashes.

Example 3 can handle arbitrary strings, since printf(1) only interprets the format string, but not the next argument. The trailing single quotes handle the case when the string is empty. In that case, the :Q modifier would result in an empty string too, which would then be skipped by the shell. For printf(1) this doesn't make a difference, but other programs may care.

In example 4, the EXAMPLE_ENV does not need to be quoted because the quoting has already been done when adding elements to the list.

When passing CFLAGS or similar variables to a GNU-style configure script (especially those that call other configure scripts), it must not have leading or trailing whitespace, since otherwise the configure script gets confused. To trim leading and trailing whitespace, use the :M modifier, as in the following example:

CPPFLAGS=               # empty
CPPFLAGS+=              -Wundef -DPREFIX=\"${PREFIX}\"
CPPFLAGS+=              ${MY_CPPFLAGS}

CONFIGURE_ARGS+=        CPPFLAGS=${CPPFLAGS:M*:Q}

all:
        echo x${CPPFLAGS:Q}x            # leading and trailing whitespace
        echo x${CONFIGURE_ARGS:Q}x      # properly trimmed

In this example, CPPFLAGS has both leading and trailing whitespace because the += operator always adds a space.

When a possibly empty variable is used in a shell program, it may lead to a syntax error.

EGFILES=        # empty

install-examples:   # produces a syntax error in the shell
        for egfile in ${EGFILES}; do            \
                echo "Installing $$egfile";     \
        done

The shell only sees the text for egfile in ; do, since ${EGFILES} is replaced with an empty string by make(1). To fix this syntax error, use one of the snippets below.

EMPTY=          # empty

install-examples:
        for egfile in ${EGFILES} ""; do         \
                [ -n "$$egfile" ] || continue;  \
                echo "Installing $$egfile";     \
        done

In this case, an empty string is appended to the iteration list (to prevent the syntax error) and filtered out later.

EGFILES=        # empty

install-examples:
.for egfile in ${EGFILES}
        echo "Installing ${egfile}"
.endfor

This variant only works when EGFILES does not contain filenames with spaces, since the .for loop splits on simple whitespace.

To have a shell command test whether a make variable is empty, use the following code: ${TEST} -z ${POSSIBLY_EMPTY:Q}"".