diff --git a/pkgs/build-support/setup-hooks/autoreconf.sh b/pkgs/build-support/setup-hooks/autoreconf.sh
index 763ea649c1c4..bb168aad4c51 100644
--- a/pkgs/build-support/setup-hooks/autoreconf.sh
+++ b/pkgs/build-support/setup-hooks/autoreconf.sh
@@ -4,8 +4,11 @@ autoreconfPhase() {
     runHook preAutoreconf
 
     local flagsArray=()
-    : "${autoreconfFlags:=--install --force --verbose}"
-    concatTo flagsArray autoreconfFlags
+    if [[ -v autoreconfFlags ]]; then
+        concatTo flagsArray autoreconfFlags
+    else
+        flagsArray+=(--install --force --verbose)
+    fi
 
     autoreconf "${flagsArray[@]}"
     runHook postAutoreconf
diff --git a/pkgs/by-name/ni/ninja/setup-hook.sh b/pkgs/by-name/ni/ninja/setup-hook.sh
index 4f3bc5b5acfa..4573aceea3c6 100644
--- a/pkgs/by-name/ni/ninja/setup-hook.sh
+++ b/pkgs/by-name/ni/ninja/setup-hook.sh
@@ -65,8 +65,7 @@ ninjaInstallPhase() {
     local flagsArray=(
         "-j$buildCores"
     )
-    : "${installTargets:=install}"
-    concatTo flagsArray ninjaFlags ninjaFlagsArray installTargets
+    concatTo flagsArray ninjaFlags ninjaFlagsArray installTargets=install
 
     echoCmd 'install flags' "${flagsArray[@]}"
     TERM=dumb ninja "${flagsArray[@]}"
diff --git a/pkgs/stdenv/generic/setup.sh b/pkgs/stdenv/generic/setup.sh
index 1b4f7a89d358..3c197171e801 100644
--- a/pkgs/stdenv/generic/setup.sh
+++ b/pkgs/stdenv/generic/setup.sh
@@ -387,12 +387,16 @@ appendToVar() {
 # Accumulate flags from the named variables $2+ into the indexed array $1.
 #
 # Arrays are simply concatenated, strings are split on whitespace.
+# Default values can be passed via name=default.
 concatTo() {
     local -n targetref="$1"; shift
-    local name type
-    for name in "$@"; do
-        if type=$(declare -p "$name" 2> /dev/null); then
-            local -n nameref="$name"
+    local arg default name type
+    for arg in "$@"; do
+        IFS="=" read -r name default <<< "$arg"
+        local -n nameref="$name"
+        if [[ ! -n "${nameref[@]}" && -n "$default" ]]; then
+            targetref+=( "$default" )
+        elif type=$(declare -p "$name" 2> /dev/null); then
             case "${type#* }" in
                 -A*)
                     echo "concatTo(): ERROR: trying to use concatTo on an associative array." >&2
@@ -1340,8 +1344,7 @@ patchPhase() {
         esac
 
         local -a flagsArray
-        : "${patchFlags:=-p1}"
-        concatTo flagsArray patchFlags
+        concatTo flagsArray patchFlags=-p1
         # "2>&1" is a hack to make patch fail if the decompressor fails (nonexistent patch, etc.)
         # shellcheck disable=SC2086
         $uncompress < "$i" 2>&1 | patch "${flagsArray[@]}"
@@ -1493,8 +1496,7 @@ checkPhase() {
             SHELL="$SHELL"
         )
 
-        : "${checkFlags:=VERBOSE=y}"
-        concatTo flagsArray makeFlags makeFlagsArray checkFlags checkFlagsArray checkTarget
+        concatTo flagsArray makeFlags makeFlagsArray checkFlags=VERBOSE=y checkFlagsArray checkTarget
 
         echoCmd 'check flags' "${flagsArray[@]}"
         make ${makefile:+-f $makefile} "${flagsArray[@]}"
@@ -1528,8 +1530,7 @@ installPhase() {
         SHELL="$SHELL"
     )
 
-    : "${installTargets:=install}"
-    concatTo flagsArray makeFlags makeFlagsArray installFlags installFlagsArray installTargets
+    concatTo flagsArray makeFlags makeFlagsArray installFlags installFlagsArray installTargets=install
 
     echoCmd 'install flags' "${flagsArray[@]}"
     make ${makefile:+-f $makefile} "${flagsArray[@]}"
@@ -1612,9 +1613,8 @@ installCheckPhase() {
             SHELL="$SHELL"
         )
 
-        : "${installCheckTarget:=installcheck}"
         concatTo flagsArray makeFlags makeFlagsArray \
-          installCheckFlags installCheckFlagsArray installCheckTarget
+          installCheckFlags installCheckFlagsArray installCheckTarget=installcheck
 
         echoCmd 'installcheck flags' "${flagsArray[@]}"
         make ${makefile:+-f $makefile} "${flagsArray[@]}"
@@ -1629,8 +1629,7 @@ distPhase() {
     runHook preDist
 
     local flagsArray=()
-    : "${distTarget:=dist}"
-    concatTo flagsArray distFlags distFlagsArray distTarget
+    concatTo flagsArray distFlags distFlagsArray distTarget=dist
 
     echo 'dist flags: %q' "${flagsArray[@]}"
     make ${makefile:+-f $makefile} "${flagsArray[@]}"
diff --git a/pkgs/test/stdenv/default.nix b/pkgs/test/stdenv/default.nix
index a5b571b5a9f4..6ed430f5da3f 100644
--- a/pkgs/test/stdenv/default.nix
+++ b/pkgs/test/stdenv/default.nix
@@ -109,21 +109,30 @@ let
           declare -A associativeArray=(["X"]="Y")
           [[ $(concatTo nowhere associativeArray 2>&1) =~ "trying to use" ]] || (echo "concatTo did not throw concatenating associativeArray" && false)
 
+          empty_array=()
+          empty_string=""
+
           declare -a flagsArray
-          concatTo flagsArray string list
+          concatTo flagsArray string list notset=e=f empty_array=g empty_string=h
           declare -p flagsArray
           [[ "''${flagsArray[0]}" == "a" ]] || (echo "'\$flagsArray[0]' was not 'a'" && false)
           [[ "''${flagsArray[1]}" == "b" ]] || (echo "'\$flagsArray[1]' was not 'b'" && false)
           [[ "''${flagsArray[2]}" == "c" ]] || (echo "'\$flagsArray[2]' was not 'c'" && false)
           [[ "''${flagsArray[3]}" == "d" ]] || (echo "'\$flagsArray[3]' was not 'd'" && false)
+          [[ "''${flagsArray[4]}" == "e=f" ]] || (echo "'\$flagsArray[4]' was not 'e=f'" && false)
+          [[ "''${flagsArray[5]}" == "g" ]] || (echo "'\$flagsArray[5]' was not 'g'" && false)
+          [[ "''${flagsArray[6]}" == "h" ]] || (echo "'\$flagsArray[6]' was not 'h'" && false)
 
           # test concatenating to unset variable
-          concatTo nonExistant string list
+          concatTo nonExistant string list notset=e=f empty_array=g empty_string=h
           declare -p nonExistant
           [[ "''${nonExistant[0]}" == "a" ]] || (echo "'\$nonExistant[0]' was not 'a'" && false)
           [[ "''${nonExistant[1]}" == "b" ]] || (echo "'\$nonExistant[1]' was not 'b'" && false)
           [[ "''${nonExistant[2]}" == "c" ]] || (echo "'\$nonExistant[2]' was not 'c'" && false)
           [[ "''${nonExistant[3]}" == "d" ]] || (echo "'\$nonExistant[3]' was not 'd'" && false)
+          [[ "''${nonExistant[4]}" == "e=f" ]] || (echo "'\$nonExistant[4]' was not 'e=f'" && false)
+          [[ "''${nonExistant[5]}" == "g" ]] || (echo "'\$nonExistant[5]' was not 'g'" && false)
+          [[ "''${nonExistant[6]}" == "h" ]] || (echo "'\$nonExistant[6]' was not 'h'" && false)
 
           eval "$extraTest"