Use $PREFIX/bin/sh for script file without shebang

Also try to handle #!(/usr)/bin/foo shebangs.

app/src/main/java/com/termux/app/BackgroundJob.java

@@ -4,12 +4,15 @@ import android.util.Log;
 
 import java.io.BufferedReader;
 import java.io.File;
+import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.lang.reflect.Field;
 import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.List;
 
 /**
  * A background job launched by Termux.
@@ -24,16 +27,7 @@ public final class BackgroundJob {
         String[] env = buildEnvironment(false, cwd);
         if (cwd == null) cwd = TermuxService.HOME_PATH;
 
-        String[] modifiedArgs;
-        if (args == null) {
-            modifiedArgs = new String[]{fileToExecute};
-        } else {
-            modifiedArgs = new String[args.length + 1];
-            modifiedArgs[0] = fileToExecute;
-            System.arraycopy(args, 0, modifiedArgs, 1, args.length);
-        }
-        final String[] progArray = modifiedArgs;
-
+        final String[] progArray = setupProcessArgs(fileToExecute, args);
         final String processDescription = Arrays.toString(progArray);
 
         Process process;
@@ -45,6 +39,7 @@ public final class BackgroundJob {
             Log.e(LOG_TAG, "Failed running background job: " + processDescription, e);
             return;
         }
+
         mProcess = process;
         final int pid = getPid(mProcess);
 
@@ -126,17 +121,74 @@ public final class BackgroundJob {
     }
 
     public static int getPid(Process p) {
-        int pid = -1;
-
         try {
             Field f = p.getClass().getDeclaredField("pid");
             f.setAccessible(true);
-            pid = f.getInt(p);
+            try {
+                return f.getInt(p);
+            } finally {
                 f.setAccessible(false);
-        } catch (Throwable e) {
-            pid = -1;
             }
-        return pid;
+        } catch (Throwable e) {
+            return -1;
+        }
+    }
+
+    static String[] setupProcessArgs(String fileToExecute, String[] args) {
+        // The file to execute may either be:
+        // - An elf file, in which we execute it directly.
+        // - A script file without shebang, which we execute with our standard shell $PREFIX/bin/sh instead of the
+        //   system /system/bin/sh. The system shell may vary and may not work at all due to LD_LIBRARY_PATH.
+        // - A file with shebang, which we try to handle with e.g. /bin/foo -> $PREFIX/bin/foo.
+        String interpreter = null;
+        try {
+            File file = new File(fileToExecute);
+            try (FileInputStream in = new FileInputStream(file)) {
+                byte[] buffer = new byte[256];
+                int bytesRead = in.read(buffer);
+                if (bytesRead > 4) {
+                    if (buffer[0] == 0x7F && buffer[1] == 'E' && buffer[2] == 'L' && buffer[3] == 'F') {
+                        // Elf file, do nothing.
+                    } else if (buffer[0] == '#' && buffer[1] == '!') {
+                        // Try to parse shebang.
+                        StringBuilder builder = new StringBuilder();
+                        for (int i = 2; i < bytesRead; i++) {
+                            char c = (char) buffer[i];
+                            if (c == ' ' || c == '\n') {
+                                if (builder.length() == 0) {
+                                    // Skip whitespace after shebang.
+                                    continue;
+                                } else {
+                                    // End of shebang.
+                                    String executable = builder.toString();
+                                    if (executable.startsWith("/usr") || executable.startsWith("/bin")) {
+                                        String[] parts = executable.split("/");
+                                        String binary = parts[parts.length - 1];
+                                        interpreter = TermuxService.PREFIX_PATH + "/bin/" + binary;
+                                    }
+                                    break;
+                                }
+                            } else {
+                                builder.append(c);
+                            }
+                        }
+                    } else {
+                        // No shebang and no ELF, use standard shell.
+                        interpreter = TermuxService.PREFIX_PATH + "/bin/sh";
+                    }
+                }
+            }
+        } catch (IOException e) {
+            // Ignore.
+        }
+
+        List<String> result = new ArrayList<>();
+        if (interpreter != null) result.add(interpreter);
+        result.add(fileToExecute);
+        if (args != null) {
+            for (String arg : args) result.add(arg);
+        }
+        return result.toArray(new String[result.size()]);
     }
 
 }