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adb android shell

How can I run Linux commands on an Android device?

On some Android devices, in the ADB shell, I can only run echo, cd, ls. When I run:

tar -cvf //mnt/sdcard/BackUp1669/apk/test.tar /mnt/sdcard/test.apk

Or the command cp, it returns:

sh: tar: not found

Why can I not run these commands? Some devices support these commands. My end goal is to copy a file from the /data/data folder to SD card. I got su and I got the following code:

int timeout = 1000;
String command = "tar -cvf /" + Environment.getExternalStorageDirectory() + "/cp/"
+ packageName + ".tar" + " " + path;
DataOutputStream os = new DataOutputStream(process.getOutputStream());
BufferedReader is = new BufferedReader(new InputStreamReader(new DataInputStream(
process.getInputStream())), 64);
String inLine;
try {
StringBuilder sbCommand = new StringBuilder();
sbCommand.append(command).append(" ");
sbCommand.append("\n");
os.writeBytes(command.toString());
if (is != null) {
for (int i = 0; i < timeout; i++) {
if (is.ready())
break;
try {
Thread.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
if (is.ready()) {
inLine = is.readLine();
} else {
}
}
} catch (IOException e) {
e.printStackTrace();
}

It always stops in is.ready(), and when I changed it to process.waitfor() it also stopped. Why?