update shell cycling logic to match documentation

and I updated the documentation already because this new way seems
easier to think about

tests/Cross-shell testing/.test-TEST_SHELL-passed-through/$TEST_SHELL should be .special-shell.

@@ -8,4 +8,4 @@
 
 echo "\$TEST_SHELL: $TEST_SHELL"
 
-[ "$(basename "$TEST_SHELL")" = '.special-shell' ]
+basename "$TEST_SHELL" | grep '.special-shell'

tests/Cross-shell testing/.test-run-by-specified-shell/With -s, a sh test script should be invoked with the specified shell.

@@ -1,9 +1,10 @@
 #!/bin/sh
 
-# Assuming that urchin was invoked with `-s bash`, this script should be being run with bash.
+# This script should run with /bin/sh
+# regardless of whether -s or -n is passed.
 
 this_shell=$(ps -o pid,comm | sed -n "s/^ *$$//p" | cut -d\  -f2)
 
 echo "Running shell: $this_shell"
 
-basename "$this_shell" | grep .special-shell
+test $(basename "$this_shell") = sh

tests/Cross-shell testing/.test-run-by-specified-shell/With -s, a shebang-less test script should be invoked with the specified shell.

@@ -3,7 +3,7 @@ set -e
 
 # Assuming that urchin was invoked with `-s bash`, this script should be being run with bash.
 
-this_shell=$(ps -o pid,comm | sed -n "s/^ *$$//p" | cut -d\  -f2)
+this_shell="$(ps -o pid,comm | sed -n "s/^ *$$//p" | cut -d\  -f2)"
 
 echo "Running shell: $this_shell"
 

tests/Cross-shell testing/.test-run-by-specified-shell/With -s, the TEST_SHELL environment variable should be set to the specified shell too.

@@ -7,4 +7,4 @@ echo "Running shell: $this_shell"
 
 echo "\$TEST_SHELL: $TEST_SHELL"
 
-basename "$this_shell" | grep .special-shell
+echo "$TEST_SHELL" | grep .special-shell

tests/Cross-shell testing/Environment variable TEST_SHELL should be passed through.

@@ -4,5 +4,5 @@
 # (for test scripts that want to invoke shell scripts with a specified shell).
 
 # Test if $TEST_SHELL, when placed in urchin's environment, is passed through to the test scripts.
-TEST_SHELL="$PWD/.special-shell" $TEST_SHELL \
+TEST_SHELL=.special-shell $TEST_SHELL \
   ../../urchin --disable-cycling ./.test-TEST_SHELL-passed-through

tests/Cross-shell testing/Environment variable TEST_SHELL should default to sh if empty.

@@ -5,5 +5,5 @@
 
 # Test if $TEST_SHELL - if *defined, but empty* - is exported with value '/bin/sh' by urchin
 # and thus has that value inside the scripts.
-TEST_SHELL="$PWD/.special-shell" $TEST_SHELL \
+TEST_SHELL= $TEST_SHELL \
   ../../urchin ./.test-TEST_SHELL-undefined_or_empty

tests/Cross-shell testing/The -s option should invoke tests with the specified shell.

@@ -1,4 +1,5 @@
 #!/bin/sh
 
-# Tests the `-s <shell> option, which invokes shebang-less and sh-shebang-line test scripts with the specified shell (for testing *sourced* shell code).
-../../urchin -s "$PWD/.special-shell" ./.test-run-by-specified-shell
+# Tests the `-s <shell> option, which invokes shebang-less test scripts with
+# the specified shell (for testing *sourced* shell code).
+../../urchin -s .special-shell ./.test-run-by-specified-shell

tests/Cross-shell testing/setup

@@ -0,0 +1 @@
+export PATH="$PWD:$PATH"

urchin

@@ -192,9 +192,13 @@ recurse() {
           start=$(date +%s)
           set +e
           { 
-            if $cycle_shell && ! has_shebang_line "$potential_test"; then
-              TEST_SHELL="$the_test_shell" $TIMEOUT \
-                "$the_test_shell" "$potential_test"
+            if $cycle_shell; then
+              if has_shebang_line "$potential_test"; then
+                TEST_SHELL="$the_test_shell" $TIMEOUT "$potential_test"
+              else
+                TEST_SHELL="$the_test_shell" $TIMEOUT \
+                  "$the_test_shell" "$potential_test"
+              fi
             else
               # Shell cycling is disabled with -n; use the present value of
               # TEST_SHELL or default to /bin/sh