diff --git a/.gitignore b/.gitignore
index 712f02bb5f..68f83ea38d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -210,6 +210,9 @@ tramp
 /eshell/history
 /eshell/lastdir
 
+# zsh byte-compiled files
+*.zwc
+
 # elpa packages
 /elpa/
 
diff --git a/share/spack/bash/spack-completion.in b/share/spack/bash/spack-completion.in
index ae9cbc555c..77cd16d076 100755
--- a/share/spack/bash/spack-completion.in
+++ b/share/spack/bash/spack-completion.in
@@ -39,10 +39,14 @@
 
 if test -n "${ZSH_VERSION:-}" ; then
   if [[ "$(emulate)" = zsh ]] ; then
-    # ensure base completion support is enabled, ignore insecure directories
-    autoload -U +X compinit && compinit -i
-    # ensure bash compatible completion support is enabled
-    autoload -U +X bashcompinit && bashcompinit
+    if ! typeset -f compdef >& /dev/null ; then
+        # ensure base completion support is enabled, ignore insecure directories
+        autoload -U +X compinit && compinit -i
+    fi
+    if ! typeset -f complete >& /dev/null ; then
+        # ensure bash compatible completion support is enabled
+        autoload -U +X bashcompinit && bashcompinit
+    fi
     emulate sh -c "source '$0:A'"
     return # stop interpreting file
   fi
diff --git a/share/spack/setup-env.sh b/share/spack/setup-env.sh
index 8936f6ca0d..21bd696386 100755
--- a/share/spack/setup-env.sh
+++ b/share/spack/setup-env.sh
@@ -276,8 +276,13 @@ fi
 #
 # We send cd output to /dev/null to avoid because a lot of users set up
 # their shell so that cd prints things out to the tty.
-_sp_share_dir="$(cd "$(dirname $_sp_source_file)" > /dev/null && pwd)"
-_sp_prefix="$(cd "$(dirname $(dirname $_sp_share_dir))" > /dev/null && pwd)"
+if [ "$_sp_shell" = zsh ]; then
+    _sp_share_dir="${_sp_source_file:A:h}"
+    _sp_prefix="${_sp_share_dir:h:h}"
+else
+    _sp_share_dir="$(cd "$(dirname $_sp_source_file)" > /dev/null && pwd)"
+    _sp_prefix="$(cd "$(dirname $(dirname $_sp_share_dir))" > /dev/null && pwd)"
+fi
 if [ -x "$_sp_prefix/bin/spack" ]; then
     export SPACK_ROOT="${_sp_prefix}"
 else
diff --git a/share/spack/spack-completion.bash b/share/spack/spack-completion.bash
index 72fa08a33a..1c971cb6c4 100755
--- a/share/spack/spack-completion.bash
+++ b/share/spack/spack-completion.bash
@@ -39,10 +39,14 @@
 
 if test -n "${ZSH_VERSION:-}" ; then
   if [[ "$(emulate)" = zsh ]] ; then
-    # ensure base completion support is enabled, ignore insecure directories
-    autoload -U +X compinit && compinit -i
-    # ensure bash compatible completion support is enabled
-    autoload -U +X bashcompinit && bashcompinit
+    if ! typeset -f compdef >& /dev/null ; then
+        # ensure base completion support is enabled, ignore insecure directories
+        autoload -U +X compinit && compinit -i
+    fi
+    if ! typeset -f complete >& /dev/null ; then
+        # ensure bash compatible completion support is enabled
+        autoload -U +X bashcompinit && bashcompinit
+    fi
     emulate sh -c "source '$0:A'"
     return # stop interpreting file
   fi