Disallow var syntax in command interpolations

This is special cased for compatibility. A more general fix would be to
make cmd interpolation syntax exactly the same as string interpolation.

Author: c42f

base/shell.jl

@@ -70,7 +70,14 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
             isempty(st) && error("\$ right before end of command")
             stpos, c = popfirst!(st)
             isspace(c) && error("space not allowed right after \$")
-            ex, j = Meta.parse(s,stpos,greedy=false)
+            if SubString(s, stpos:min(stpos+2, ncodeunits(s))) == "var"
+                # Disallow var"#" syntax in cmd interpolations.
+                # TODO: Allow only identifiers after the $ for consistency with
+                # string interpolation syntax (see #3150)
+                ex, j = :var, stpos+3
+            else
+                ex, j = Meta.parse(s,stpos,greedy=false)
+            end
             last_parse = (stpos:prevind(s, j)) .+ s.offset
             update_arg(ex);
             s = SubString(s, j)

test/spawn.jl

@@ -461,6 +461,13 @@ let c = setenv(`x`, "A"=>true)
     @test_throws ArgumentError `"$c "`
 end
 
+# Interaction of cmd parsing with var syntax (#32408)
+let var = "x"
+    @test `ls $var` == Cmd(["ls", "x"])
+    @test `ls $var"y"` == Cmd(["ls", "xy"])
+    @test `ls $var "y"` == Cmd(["ls", "x", "y"])
+end
+
 # equality tests for AndCmds
 @test Base.AndCmds(`$echocmd abc`, `$echocmd def`) == Base.AndCmds(`$echocmd abc`, `$echocmd def`)
 @test Base.AndCmds(`$echocmd abc`, `$echocmd def`) != Base.AndCmds(`$echocmd abc`, `$echocmd xyz`)