Unable using Runtime.exec() to execute shell command "echo" in Android Java code

Question

I can use Runtime.exec() to execute shell commands like "getprop" and "ls system" and they work fine.

However, when I use "echo $BOOTCLASSPATH", "echo \\$BOOTCLASSPATH" or "echo HelloWorld", it won't show it in stdout.

The logcat shows:

I/AndroidRuntime( 4453): VM exiting with result code -1.

Here's my code:

try {
    java.lang.Process proc = Runtime.getRuntime().exec("echo -e \\$BOOTCLASSPATH");
    String line = null;

    InputStream stderr = proc.getErrorStream();
    InputStreamReader esr = new InputStreamReader (stderr);
    BufferedReader ebr = new BufferedReader (esr);
    while ( (line = ebr.readLine()) != null )
        Log.e("FXN-BOOTCLASSPATH", line);

    InputStream stdout = proc.getInputStream();
    InputStreamReader osr = new InputStreamReader (stdout);
    BufferedReader obr = new BufferedReader (osr);
    while ( (line = obr.readLine()) != null )
        Log.i("FXN-BOOTCLASSPATH", line);

    int exitVal = proc.waitFor();
    Log.d("FXN-BOOTCLASSPATH", "getprop exitValue: " + exitVal);
} catch (Exception e) {
    e.printStackTrace();
}

It seems that you don't have a /bin/echo on that system; echo is also available as a shell command. Try to exec sh -c "echo -e \\$BOOTCLASSPATH" — laune
– laune, Commented Aug 8, 2014 at 8:52
I tried this: String[] cmdline = { "sh", "-c", "echo $BOOTCLASSPATH" }; Runtime.getRuntime().exec(cmdline); and it works. Thank you guys. — QY Lin
– QY Lin, Commented Aug 8, 2014 at 9:48
Thanks Buddy @QYLin , you saved my life. can you elaborate why does this happen? — Adi Tiwari
– Adi Tiwari, Commented Jan 19, 2016 at 7:00

Dr.jacky · Accepted Answer · 2017-05-15 08:51:16Z

27

@Adi Tiwari, I've found the cause. Runtime.getRuntime.exec() doesn't execute a shell command directly, it executes an executable with arguments. "echo" is a builtin shell command. It is actually a part of the argument of the executable sh with the option -c. Commands like ls are actual executables. You can use type echo and type ls command in adb shell to see the difference.
So final code is:

String[] cmdline = { "sh", "-c", "echo $BOOTCLASSPATH" }; 
Runtime.getRuntime().exec(cmdline);

edited May 15, 2017 at 8:51

Dr.jacky

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answered Feb 2, 2016 at 3:11

QY Lin

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3 Comments

Antonino Over a year ago

NOTE: I wanted to better understand this explanation - in particular the theoretical part behind - and I found this article absolutely useful for this purpose

Jagar Over a year ago

Sorry this is an old topic but what about multi commands in this case not only one single echo command?

wholerabbit Over a year ago

Kind of surprised that no one has mentioned that shell built-ins (including echo) often have executable correlates. On most normal linux systems, which echo will return /bin/echo but type echo will return echo is a shell builtin. This is not the case on Android, however.

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