This enables the environment variables controlling run.sh behavior to be
permanently set in a working environment (e.g. to automatically force
colored output without using "-c" in each runall.sh invocation).
Relevant runall.sh command line arguments still have a higher priority.
(cherry picked from commit
d989d20fe526716c774357ff372b38995e1e5065)
usage="Usage: ./runall.sh [-c] [-n] [numprocesses]"
-SYSTEMTEST_FORCE_COLOR=0
-SYSTEMTEST_NO_CLEAN=0
+# Preserve values of environment variables which are already set.
+
+SYSTEMTEST_FORCE_COLOR=${SYSTEMTEST_FORCE_COLOR:-0}
+SYSTEMTEST_NO_CLEAN=${SYSTEMTEST_NO_CLEAN:-0}
# Handle command line switches if present.