Why do conditional statements in autoconf scripts prefix their variables with "x"? For example, the macro provided by GNU to test for Boost has conditionals such as
if test "x$want_boost" = "xyes"; then
Why is this not defined as:
if test "$want_boost" = "yes"; then
In some early shells, testing for an empty string variable wasn't as easy as it is now, so the best alternative was to see if "x$variable" was equal to just "x". Also, since that's apparently using test, that's simpler than trying to properly quote/escape sequences like '$x != "y"' without losing sanity and/or portability.
来源：https://stackoverflow.com/questions/11265847/why-do-conditionals-in-autoconf-scripts-prefix-variables-with-x