feat: enable shell for --oci mode

Enable `singularity shell --oci ...` with behavior matching native
runtime, i.e.

* Run shell set with SINGULARITY_SHELL or --shell
* If not set, try /bin/bash --norc
* If not available, use /bin/sh

Closes sylabs/singularity#1025

Signed-off-by: Edita Kizinevic <edita.kizinevic@cern.ch>

cmd/internal/cli/actions.go

@@ -243,10 +243,19 @@ var ShellCmd = &cobra.Command{
 		image := args[0]
 		containerCmd := "/.singularity.d/actions/shell"
 		containerArgs := []string{}
-		// OCI runtime does not use an action script
+		// OCI runtime does not use an action script, but must match behavior.
+		// See - internal/pkg/util/fs/files/action_scripts.go (case shell).
 		if ociRuntime {
-			// TODO - needs to have bash -> sh fallback logic implemented somewhere.
-			containerCmd = "/bin/sh"
+			// APPTAINER_SHELL or --shell has priority
+			if shellPath != "" {
+				containerCmd = shellPath
+				// Clear the shellPath - not handled internally by the OCI runtime, as we exec directly without an action script.
+				shellPath = ""
+			} else {
+				// Otherwise try to exec /bin/bash --norc, falling back to /bin/sh
+				containerCmd = "/bin/sh"
+				containerArgs = []string{"-c", "test -x /bin/bash && PS1='Apptainer> ' exec /bin/bash --norc || PS1='Apptainer> ' exec /bin/sh"}
+			}
 		}
 		setVM(cmd)
 		if vm {

e2e/actions/actions.go

@@ -2869,7 +2869,8 @@ func E2ETests(env e2e.TestEnv) testhelper.Tests {
 		//
 		// OCI Runtime Mode
 		//
-		"ociRun":  c.actionOciRun,  // apptainer run --oci
-		"ociExec": c.actionOciExec, // apptainer exec --oci
+		"ociRun":   c.actionOciRun,   // apptainer run --oci
+		"ociExec":  c.actionOciExec,  // apptainer exec --oci
+		"ociShell": c.actionOciShell, // apptainer shell --oci
 	}
 }

e2e/actions/oci.go

@@ -146,3 +146,53 @@ func (c actionTests) actionOciExec(t *testing.T) {
 		)
 	}
 }
+
+// Shell interaction tests
+func (c actionTests) actionOciShell(t *testing.T) {
+	e2e.EnsureOCIImage(t, c.env)
+
+	tests := []struct {
+		name       string
+		argv       []string
+		consoleOps []e2e.ApptainerConsoleOp
+		exit       int
+	}{
+		{
+			name: "ShellExit",
+			argv: []string{"oci-archive:" + c.env.OCIImagePath},
+			consoleOps: []e2e.ApptainerConsoleOp{
+				// "cd /" to work around issue where a long
+				// working directory name causes the test
+				// to fail because the "Apptainer" that
+				// we are looking for is chopped from the
+				// front.
+				// TODO(mem): This test was added back in 491a71716013654acb2276e4b37c2e015d2dfe09
+				e2e.ConsoleSendLine("cd /"),
+				e2e.ConsoleExpect("Apptainer"),
+				e2e.ConsoleSendLine("exit"),
+			},
+			exit: 0,
+		},
+		{
+			name: "ShellBadCommand",
+			argv: []string{"oci-archive:" + c.env.OCIImagePath},
+			consoleOps: []e2e.ApptainerConsoleOp{
+				e2e.ConsoleSendLine("_a_fake_command"),
+				e2e.ConsoleSendLine("exit"),
+			},
+			exit: 127,
+		},
+	}
+
+	for _, tt := range tests {
+		c.env.RunApptainer(
+			t,
+			e2e.AsSubtest(tt.name),
+			e2e.WithProfile(e2e.OCIUserProfile),
+			e2e.WithCommand("shell"),
+			e2e.WithArgs(tt.argv...),
+			e2e.ConsoleRun(tt.consoleOps...),
+			e2e.ExpectExit(tt.exit),
+		)
+	}
+}

internal/pkg/runtime/launcher/launcher.go

@@ -27,7 +27,7 @@ import "context"
 type Launcher interface {
 	// Exec will execute the container image 'image', starting 'process', and
 	// passing arguments 'args'. If instanceName is specified, the container
-	// must be launched as a background instance, otherwist it must run
+	// must be launched as a background instance, otherwise it must run
 	// interactively, attached to the console.
 	Exec(ctx context.Context, image string, process string, args []string, instanceName string) error
 }