unix - Why do shell script comparisons often use x$VAR = xyes?

Question

I see this often in the build scripts of projects that use autotools (autoconf, automake). When somebody wants to check the value of a shell variable, they frequently use this idiom:

if test "x$SHELL_VAR" = "xyes"; then
...

What is the advantage to this over simply checking the value like this:

if test $SHELL_VAR = "yes"; then
...

I figure there must be some reason that I see this so often, but I can't figure out what it is.

Answer 1

If you're using a shell that does simple substitution and the SHELL_VAR variable does not exist (or is blank), then you need to watch out for the edge cases. The following translations will happen:

if test $SHELL_VAR = yes; then        -->  if test = yes; then
if test x$SHELL_VAR = xyes; then      -->  if test x = xyes; then

The first of these will generate an error since the fist argument to test has gone missing. The second does not have that problem.

Your case translates as follows:

if test "x$SHELL_VAR" = "xyes"; then  -->  if test "x" = "xyes"; then

The x, at least for POSIX-compliant shells, is actually redundant since the quotes ensue that both an empty argument and one containing spaces are interpreted as a single object.