make SPACK_SHELL detection more robust (#9712)

Spack shell detection in setup-env.sh was originally based on
examining the executable name of $$ (from "ps"). In some cases this
does not actually give the name of the shell used, for example when
setup-env.sh was invoked from a script using "#!". To make shell
detection more robust, this adds a preliminary check for shell
variables which indicate that the shell is bash or zsh; the
executable name of $$ is retained as a fallback if those variables
are not defined.

share/spack/setup-env.sh

@@ -236,7 +236,18 @@ export SPACK_ROOT=${_sp_prefix}
 # Determine which shell is being used
 #
 function _spack_determine_shell() {
-	PS_FORMAT= ps -p $$ | tail -n 1 | awk '{print $4}' | sed 's/^-//' | xargs basename
+    # This logic is derived from the cea-hpc/modules profile.sh example at
+    # https://github.com/cea-hpc/modules/blob/master/init/profile.sh.in
+    #
+    # The objective is to correctly detect the shell type even when setup-env
+    # is sourced within a script itself rather than a login terminal.
+    if [ -n "${BASH:-}" ]; then
+        echo ${BASH##*/}
+    elif [ -n "${ZSH_NAME:-}" ]; then
+        echo $ZSH_NAME
+    else
+        PS_FORMAT= ps -p $$ | tail -n 1 | awk '{print $4}' | sed 's/^-//' | xargs basename
+    fi
 }
 export SPACK_SHELL=$(_spack_determine_shell)