diff --git a/install.sh b/install.sh
index e53172df..af662c26 100755
--- a/install.sh
+++ b/install.sh
@@ -109,21 +109,77 @@ get_latest_version() {
     curl -s "https://api.github.com/repos/$REPO/releases/latest" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/'
 }
 
+# Resolve completion script destination for each shell
+completion_path() {
+    local os="$1"
+    local shell_name="$2"
+    local command_name="$3"
+
+    local data_home="${XDG_DATA_HOME:-$HOME/.local/share}"
+
+    case "$shell_name" in
+        bash)
+            if [ "$os" = "darwin" ]; then
+                echo "/usr/local/etc/bash_completion.d/$command_name"
+            else
+                echo "$data_home/bash-completion/completions/$command_name"
+            fi
+            ;;
+        zsh)
+            if [ "$os" = "darwin" ]; then
+                echo "/usr/local/share/zsh/site-functions/_$command_name"
+            else
+                echo "$data_home/zsh/site-functions/_$command_name"
+            fi
+            ;;
+        fish)
+            echo "${XDG_CONFIG_HOME:-$HOME/.config}/fish/completions/$command_name.fish"
+            ;;
+        *)
+            echo ""
+            ;;
+    esac
+}
+
+completion_dir() {
+    local os="$1"
+    local shell_name="$2"
+
+    case "$shell_name" in
+        bash | zsh)
+            dirname "$(completion_path "$os" "$shell_name" container-use)"
+            ;;
+        fish)
+            echo "${XDG_CONFIG_HOME:-$HOME/.config}/fish/completions"
+            ;;
+        *)
+            echo ""
+            ;;
+    esac
+}
+
 # Show shell completion setup instructions
 show_completion_instructions() {
     local binary="$1"
+    local os="$(detect_os)"
 
     log_info "To enable shell completions:"
     echo ""
     echo "  # For container-use command:"
-    echo "  $binary completion bash > /usr/local/etc/bash_completion.d/container-use"
-    echo "  $binary completion zsh > /usr/local/share/zsh/site-functions/_container-use"
-    echo "  $binary completion fish > ~/.config/fish/completions/container-use.fish"
+    echo "  mkdir -p $(completion_dir "$os" bash)"
+    echo "  $binary completion bash > $(completion_path "$os" bash container-use)"
+    echo "  mkdir -p $(completion_dir "$os" zsh)"
+    echo "  $binary completion zsh > $(completion_path "$os" zsh container-use)"
+    echo "  mkdir -p $(completion_dir "$os" fish)"
+    echo "  $binary completion fish > $(completion_path "$os" fish container-use)"
     echo ""
     echo "  # For cu command:"
-    echo "  $binary completion --command-name=cu bash > /usr/local/etc/bash_completion.d/cu"
-    echo "  $binary completion --command-name=cu zsh > /usr/local/share/zsh/site-functions/_cu"
-    echo "  $binary completion --command-name=cu fish > ~/.config/fish/completions/cu.fish"
+    echo "  mkdir -p $(completion_dir "$os" bash)"
+    echo "  $binary completion --command-name=cu bash > $(completion_path "$os" bash cu)"
+    echo "  mkdir -p $(completion_dir "$os" zsh)"
+    echo "  $binary completion --command-name=cu zsh > $(completion_path "$os" zsh cu)"
+    echo "  mkdir -p $(completion_dir "$os" fish)"
+    echo "  $binary completion --command-name=cu fish > $(completion_path "$os" fish cu)"
 }
 
 # Verify checksum of downloaded file
@@ -299,4 +355,6 @@ main() {
     fi
 }
 
-main "$@"
+if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+    main "$@"
+fi
diff --git a/scripts/test-install-completion-paths.sh b/scripts/test-install-completion-paths.sh
new file mode 100755
index 00000000..fa32ffa6
--- /dev/null
+++ b/scripts/test-install-completion-paths.sh
@@ -0,0 +1,103 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+
+source "$REPO_ROOT/install.sh"
+
+assert_equals() {
+    local actual="$1"
+    local expected="$2"
+    local label="$3"
+
+    if [ "$actual" != "$expected" ]; then
+        echo "❌ $label"
+        echo "  expected: $expected"
+        echo "  actual:   $actual"
+        exit 1
+    fi
+}
+
+assert_contains() {
+    local haystack="$1"
+    local needle="$2"
+    local label="$3"
+
+    case "$haystack" in
+        *"$needle"*)
+            ;;
+        *)
+            echo "❌ $label"
+            echo "  expected to find: $needle"
+            exit 1
+            ;;
+    esac
+}
+
+DATA_HOME="$HOME/.local/share"
+CONFIG_HOME="$HOME/.config"
+
+assert_equals "$(completion_path linux bash container-use)" "$DATA_HOME/bash-completion/completions/container-use" "linux bash container-use"
+assert_equals "$(completion_path linux zsh container-use)" "$DATA_HOME/zsh/site-functions/_container-use" "linux zsh container-use"
+assert_equals "$(completion_path linux fish container-use)" "$CONFIG_HOME/fish/completions/container-use.fish" "linux fish container-use"
+assert_equals "$(completion_path darwin bash container-use)" "/usr/local/etc/bash_completion.d/container-use" "darwin bash container-use"
+assert_equals "$(completion_path darwin zsh container-use)" "/usr/local/share/zsh/site-functions/_container-use" "darwin zsh container-use"
+assert_equals "$(completion_dir linux bash)" "$DATA_HOME/bash-completion/completions" "linux bash completion_dir"
+assert_equals "$(completion_dir linux zsh)" "$DATA_HOME/zsh/site-functions" "linux zsh completion_dir"
+assert_equals "$(completion_dir linux fish)" "$CONFIG_HOME/fish/completions" "linux fish completion_dir"
+assert_equals "$(completion_dir darwin bash)" "/usr/local/etc/bash_completion.d" "darwin bash completion_dir"
+assert_equals "$(completion_dir darwin zsh)" "/usr/local/share/zsh/site-functions" "darwin zsh completion_dir"
+assert_equals "$(completion_dir darwin fish)" "$CONFIG_HOME/fish/completions" "darwin fish completion_dir"
+
+CUSTOM_DATA_HOME="/tmp/container-use-test-xdg-data"
+CUSTOM_CONFIG_HOME="/tmp/container-use-test-xdg-config"
+mkdir -p "$CUSTOM_DATA_HOME" "$CUSTOM_CONFIG_HOME"
+XDG_DATA_HOME_BACKUP="${XDG_DATA_HOME:-}"
+XDG_CONFIG_HOME_BACKUP="${XDG_CONFIG_HOME:-}"
+export XDG_DATA_HOME="$CUSTOM_DATA_HOME"
+export XDG_CONFIG_HOME="$CUSTOM_CONFIG_HOME"
+
+assert_equals "$(completion_path linux bash container-use)" "$CUSTOM_DATA_HOME/bash-completion/completions/container-use" "linux bash container-use with XDG_DATA_HOME"
+assert_equals "$(completion_path linux zsh container-use)" "$CUSTOM_DATA_HOME/zsh/site-functions/_container-use" "linux zsh container-use with XDG_DATA_HOME"
+assert_equals "$(completion_path linux fish container-use)" "$CUSTOM_CONFIG_HOME/fish/completions/container-use.fish" "linux fish container-use with XDG_CONFIG_HOME"
+assert_equals "$(completion_dir linux bash)" "$CUSTOM_DATA_HOME/bash-completion/completions" "linux bash completion_dir with XDG_DATA_HOME"
+assert_equals "$(completion_dir linux zsh)" "$CUSTOM_DATA_HOME/zsh/site-functions" "linux zsh completion_dir with XDG_DATA_HOME"
+assert_equals "$(completion_dir linux fish)" "$CUSTOM_CONFIG_HOME/fish/completions" "linux fish completion_dir with XDG_CONFIG_HOME"
+
+if [ -n "$XDG_DATA_HOME_BACKUP" ]; then
+    export XDG_DATA_HOME="$XDG_DATA_HOME_BACKUP"
+else
+    unset XDG_DATA_HOME
+fi
+if [ -n "$XDG_CONFIG_HOME_BACKUP" ]; then
+    export XDG_CONFIG_HOME="$XDG_CONFIG_HOME_BACKUP"
+else
+    unset XDG_CONFIG_HOME
+fi
+
+detect_os() {
+    echo "linux"
+}
+
+linux_instructions="$(show_completion_instructions "container-use")"
+assert_contains "$linux_instructions" "mkdir -p $DATA_HOME/bash-completion/completions" "linux bash mkdirp instruction"
+assert_contains "$linux_instructions" "mkdir -p $DATA_HOME/zsh/site-functions" "linux zsh mkdirp instruction"
+assert_contains "$linux_instructions" "mkdir -p $CONFIG_HOME/fish/completions" "linux fish mkdirp instruction"
+assert_contains "$linux_instructions" "container-use completion bash > $DATA_HOME/bash-completion/completions/container-use" "linux container-use bash redirection"
+assert_contains "$linux_instructions" "container-use completion --command-name=cu fish > $CONFIG_HOME/fish/completions/cu.fish" "linux cu fish redirection"
+unset -f detect_os
+
+detect_os() {
+    echo "darwin"
+}
+
+darwin_instructions="$(show_completion_instructions "container-use")"
+assert_contains "$darwin_instructions" "mkdir -p /usr/local/etc/bash_completion.d" "darwin bash mkdirp instruction"
+assert_contains "$darwin_instructions" "mkdir -p /usr/local/share/zsh/site-functions" "darwin zsh mkdirp instruction"
+assert_contains "$darwin_instructions" "mkdir -p $HOME/.config/fish/completions" "darwin fish mkdirp instruction"
+assert_contains "$darwin_instructions" "container-use completion bash > /usr/local/etc/bash_completion.d/container-use" "darwin container-use bash redirection"
+unset -f detect_os
+
+echo "✅ install.sh completion paths behave as expected"