Testing if a variable is empty in a shell script

Some historical shells implemented a very simple parser that could get confused by things like [ -n = "" ] where the first operand to = looks like an operator, and would parse this as [ -n = ] or cause a syntax error. In [ "x$1" = x"" ], the x prefix ensures that x"$1" cannot possibly look like an operator, and so the only way the shell can parse this test is by treating = as a binary operator.

All modern shells, and even most older shells still in operation, follow the POSIX rules which mandate that all test expressions of up to 4 words be parsed correctly. So [ -z "$1" ] is a proper way of testing if $1 is empty, and [ "$x" = "$y" ] is a proper way to test the equality of two variables.

Even some current shells can get confused with longer expressions, and a few expressions are actually ambiguous, so avoid using the -a and -o operators to construct longer boolean tests, and instead use separate calls to [ and the shell's own && and || boolean operators.

According to http://www.mpi-inf.mpg.de/~uwe/lehre/unixffb/quoting-guide.html, the -z test is unsafe in some implementations, presumably when "interesting" strings like "-o a=a" are tested.

The above tests will also cause an error if you run with "set -u" or "set -o nounset"

A more stable way to check for an empty variable would be to use parameter expansion:

MYVAR=${MYVAR:-"Bad Value"}

This method works for the traditional bourne shell, as well as ksh, and bash.