diff --git a/lib/awful/completion.lua b/lib/awful/completion.lua
index 30f430ee9..ef819cb80 100644
--- a/lib/awful/completion.lua
+++ b/lib/awful/completion.lua
@@ -18,6 +18,8 @@ local print = print
 local pairs = pairs
 local string = string
 
+local gears_debug = require("gears.debug")
+
 local completion = {}
 
 -- mapping of command/completion function
@@ -54,6 +56,8 @@ local function bash_escape(str)
     return str
 end
 
+completion.default_shell = nil
+
 --- Use shell completion system to complete commands and filenames.
 -- @tparam string command The command line.
 -- @tparam number cur_pos The cursor position.
@@ -98,7 +102,21 @@ function completion.shell(command, cur_pos, ncomp, shell)
     end
 
     local shell_cmd
-    if shell == "zsh" or (not shell and os.getenv("SHELL"):match("zsh$")) then
+    if not shell then
+        if not completion.default_shell then
+            local env_shell = os.getenv('SHELL')
+            if not env_shell then
+                gears_debug.print_warning('SHELL not set in environment, falling back to bash.')
+                completion.default_shell = 'bash'
+            elseif env_shell:match('zsh$') then
+                completion.default_shell = 'zsh'
+            else
+                completion.default_shell = 'bash'
+            end
+        end
+        shell = completion.default_shell
+    end
+    if shell == 'zsh' then
         if comptype == "file" then
             -- NOTE: ${~:-"..."} turns on GLOB_SUBST, useful for expansion of
             -- "~/" ($HOME).  ${:-"foo"} is the string "foo" as var.
diff --git a/spec/awful/completion_spec.lua b/spec/awful/completion_spec.lua
index 1f6fbfea0..2c1507fc4 100644
--- a/spec/awful/completion_spec.lua
+++ b/spec/awful/completion_spec.lua
@@ -60,4 +60,55 @@ describe("awful.completion.shell", function()
     end)
 end)
 
+describe("awful.completion.shell handles $SHELL", function()
+    local orig_getenv = os.getenv
+    local gdebug = require("gears.debug")
+    local orig_print_warning
+    local print_warning_message
+    local os_getenv_shell
+
+    setup(function()
+        os.getenv = function(name)  -- luacheck: ignore
+            if name == 'SHELL' then return os_getenv_shell end
+            return orig_getenv(name)
+        end
+
+        orig_print_warning = gdebug.print_warning
+        gdebug.print_warning = function(message)
+            print_warning_message = message
+        end
+    end)
+
+    teardown(function()
+        os.getenv = orig_getenv  --luacheck: ignore
+        gdebug.print_warning = orig_print_warning
+    end)
+
+    before_each(function()
+        compl.default_shell = nil
+        print_warning_message = nil
+    end)
+
+    it("falls back to bash if unset", function()
+        assert.same(shell('true', 5, 1, nil), {'true', 5, {'true'}})
+        assert.same(print_warning_message,
+                    'SHELL not set in environment, falling back to bash.')
+        assert.same(compl.default_shell, "bash")
+    end)
+
+    it("uses zsh from path", function()
+        os_getenv_shell = '/opt/bin/zsh'
+        assert.same(shell('true', 5, 1, nil), {'true', 5, {'true'}})
+        assert.same(compl.default_shell, "zsh")
+        assert.is_nil(print_warning_message)
+    end)
+
+    it("uses bash for unknown", function()
+        os_getenv_shell = '/dev/null'
+        assert.same(shell('true', 5, 1, nil), {'true', 5, {'true'}})
+        assert.same(compl.default_shell, "bash")
+        assert.is_nil(print_warning_message)
+    end)
+end)
+
 -- vim: ft=lua:et:sw=4:ts=8:sts=4:tw=80