Allow keeping Cygwin out of PATH

In some circumstances, it might be useful to have Cygwin installed, but
not have its executables take precedence over pre-installed packages.
Add an "add-to-path" option that, if set to "false", will skip the step
of adding Cygwin's /bin directory to PATH.

Also update documentation to mention this option, and update the test
scripts to check it works as expected; the default behaviour is tested
as part of the "install-dir-test", and the new behaviour as part of the
main test.

.github/workflows/test.yml

@@ -31,6 +31,7 @@ jobs:
       with:
         platform: ${{ matrix.platform }}
         packages: ${{ matrix.packages }}
+        add-to-path: false
 
     - name: Explicitly running shell
       run: C:\cygwin\bin\bash -lc "echo 'Explicitly run shell'"
@@ -52,6 +53,21 @@ jobs:
       run: C:\cygwin\bin\sponge.exe tmp
       if: contains(matrix.packages, 'moreutils')
 
+    - name: Check PATH hasn't changed
+      run: |
+        if [[ "${OSTYPE}" = "cygwin" ]]; then
+          echo "unexpectedly using Cygwin Bash"
+          exit 1
+        fi
+
+        if [[ "$(cygpath -aw "$(which cygpath)")" = 'C:\cygwin\'* ]]; then
+          echo "unexpectedly using Cygwin cygpath"
+          exit 1
+        fi
+      shell: bash  # If Cygwin is not in PATH -- as expected -- this will be Git Bash
+      env:
+        SHELLOPTS: igncr
+
   complex-test:
     runs-on: windows-latest
     name: 'Complex Test'

README.md

@@ -25,6 +25,7 @@ Parameters
 | install-dir | C:\cygwin                                    | Installation directory
 | site        | http://mirrors.kernel.org/sourceware/cygwin/ | Mirror site to install from
 | check-sig   | true                                         | Whether to check the setup.ini signature
+| add-to-path | true                                         | Whether to add Cygwin's `/bin` directory to the system `PATH`
 
 Line endings
 ------------
@@ -52,7 +53,7 @@ Alternatively, you can also use:
 PATH
 ----
 
-This action prepends Cygwin's /usr/bin directory to the PATH.
+By default, this action prepends Cygwin's /usr/bin directory to the PATH.
 
 However, if you want to ensure that PATH only contains Cygwin executables,
 and other stuff installed in the VM image isn't going to get picked up:
@@ -69,6 +70,8 @@ or,
   * prevent the profile script from changing directory by putting
     `CHERE_INVOKING` into the environment
 
+If you want the opposite – the system PATH to remain unchanged by this action – add `add-to-path: false` to the action settings.
+
 Symlinks
 --------
 

action.yml

@@ -22,6 +22,10 @@ inputs:
   site:
     description: Download site URL
     required: false
+  add-to-path:
+    description: Should Cygwin's bin directory be added to the system PATH?
+    required: false
+    default: true
 
 runs:
   using: "composite"
@@ -78,8 +82,11 @@ runs:
         & C:\setup.exe $args | Out-Default
       shell: powershell
 
+    - if: ${{ inputs.add-to-path  == 'true' }}
+      run: echo "${{ inputs.install-dir }}\bin" >> $env:GITHUB_PATH
+      shell: powershell
+
     - run: |
-        echo "${{ inputs.install-dir }}\bin" >> $env:GITHUB_PATH
         # run login shell to copy skeleton profile files
         ${{ inputs.install-dir }}\bin\bash.exe --login
       shell: powershell