How to Build an Executable for Android Shell

How to build an executable for Android shell

The answer provides a sequence of steps for building an executable for Android shell through both Eclipse (outdated) and Android Studio (4.1+ by the time of this writing). The last includes ndk-build and CMake.

I. PREPARE SOURCE CODE

As an example consider mycommand.c:

#include <stdio.h>

int main()
{
    printf("My Command!\n");
    return 0;
}

II. BUILD EXECUTABLE

Eclipse (might be outdated)

In assumption that NDK location is set in Eclipse, create a new Android Application Project and do the following steps.

Add native support. Right click on the project in Project Explorer > Android Tools > Add Native Support > Finish
Add source code, i.e. put mycommand.c under project_root/jni folder.

Edit Android.mk under project_root/jni as follows:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE     := mycommand
LOCAL_SRC_FILES  := mycommand.c

include $(BUILD_EXECUTABLE)

Create Application.mk * under the project_root/jni folder:
```
APP_ABI := all
```
Build executable and find it under project_root/libs/<abi>/mycommand.

*Binaries for all supported CPU architectures are generated here. Use adb shell cat /proc/cpuinfo to find out the CPU architecture and set APP_ABI as per Supported ABIs.

Android Studio and ndk-build

The steps are as follows.

Add mycommand.c, Android.mk (same as in the Eclipse section above) to the /app/src/main/cpp folder.

Edit build.gradle:

android {
    ...
    defaultConfig {
        ...
        externalNativeBuild {
            ndkBuild {
                targets "mycommand"
                // use a specific ABI filter if needed
                // abiFilters "armeabi-v7a"
            }
        }
    }
    externalNativeBuild {
        ndkBuild {
            path "src/main/cpp/Android.mk"
        }
    }
}

Build project and find the executable under /app/.externalNativeBuild/ndkBuild/debug/obj/local/<abi>/mycommand

Android Studio and CMake

Create a project using the Native C++ template.

Add mycommand.c to the /app/src/main/cpp folder and edit CMakeLists.txt:

cmake_minimum_required(VERSION x.x.x)

add_executable(mycommand
               mycommand.c )

Edit build.gradle:

android {
    ...
    defaultConfig {
        ...
        externalNativeBuild {
            cmake {
                targets "mycommand"
                // use a specific ABI filter if needed
                // abiFilters "armeabi-v7a"
            }
        }
    }
    ...
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
        }
    }
}

Build project and find the executable under /app/build/intermediates/cmake/debug/obj/<abi>/mycommand

III. PUSH BINARY INTO DEVICE

Push mycommand binary from where it is located into your device. Keep in mind that files on SD card aren't executable by default, so the binary should be pushed into the device's internal storage. Depending of whether device is rooted or not you have the following options:

On non-rooted device you can push the binary to /data/local/tmp:
```
 adb push mycommand /data/local/tmp
```
On rooted device you can push the binary to SD card and then copy it to /system/bin (after remounting the partition in read-write mode) along with the other executable files:
```
 adb push mycommand /path/to/sdcard
 adb shell
 su
 mount -o rw,remount /system
 cp /path/to/sdcard/mycommand /system/bin
```

IV. SET EXECUTABLE PERMISSION (optional)

Set the permission of the binary to be executable (this might not be needed in case of /data/local/tmp). Below chmod 555(r-xr-xr-x) is used:

adb shell chmod 555 /path/to/mycommand

V. RUN COMMAND

Now you can shell into your device (with adb shell) and execute the command.

On non-rooted device use the absolute path to the command:
```
 $ /data/local/tmp/mycommand
 My Command!
```
On rooted device, in case the binary has been copied to /system/bin, you can call it by the file name:
```
 $ mycommand
 My Command!
```

Running executable from Android shell

If running the executable with a full path to it doesn't work, the problem might be with the executable itself, meaning it either

wasn't successfully compiled, or
was compiled for another CPU, not the one your device had.

In this case the options you have are as follows:

if you have access to the source code, compile it for your device's ABI,
if it is a 3rd party executable, ask for an executable compiled for your device's ABI (or, if possible, find it yourself).

How to create an Android executable for launch from the shell?

For me, I didn't need eclipse nor any java code in my application. My application that was being ported from Linux, already had a set of makefiles already in place. I was able to use my existing makefile system by way of the standalone tool chain portion of the NDK.

This answer got me going with the standalone tool chain. In my case, here is how I invoked it where I am cross compiling from my Macintosh, having already installed the NDK at /Users/me/Development/android-ndk-r10d:

ANDROID_NDK=/Users/me/Development/android-ndk-r10d
SYSROOT=$(ANDROID_NDK)/platforms/android-19/arch-arm
${ANDROID_NDK}/build/tools/make-standalone-toolchain.sh --platform=android-19 --install-dir=${ANDROID_NDK}/myapp-android-toolchain --ndk-dir=/Users/me/Development/android-ndk-r10d --toolchain=arm-linux-androideabi-4.8 --system=darwin-x86_64

With this setup, I launched my build like this:

cd ~/base/of/myapp/src/
ANDROID_NDK=/Users/me/Development/android-ndk-r10d PATH=${ANDROID_NDK}/myapp-android-toolchain/bin:$PATH make

After I fixed some issues with config for my cross-compilation in some third-party libraries I'm using, I had a working build. Next, I wanted to install it on my phone:

# Push to a location with write permission (sdcard):
cd ~/base/of/myapp/bin/ # My build puts the executable here.
adb push myapp /sdcard/
# Go into phone and complete the install manually:
adb shell
su
mount -oremount,rw rootfs /system  # /vendor/bin is in /system mount.
# One time creation of /vendor/bin (that is in the PATH by default):
mkdir /vendor/bin
# Copy the myapp executable over
cp /sdcard/myapp /vendor/bin/myapp

Now, I can invoke myapp from the shell on my phone, like a normal command line interface Linux application. The argc and argv from my original Linux code work as written in the Android build by this process.

How do I build a native (command line) executable to run on Android?

As of NDK r8d, this can be solved in a much simpler way.

Create a project with the following directory hierarchy:

project/
    jni/
        Android.mk
        Application.mk
        *.c, *.cpp, *.h, etc.

Fill in Android.mk with the following content. The most important thing is the last line. Check the NDK doc for the meaning of the other variables.

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := name-of-your-executable
LOCAL_SRC_FILES := a.cpp b.cpp c.cpp etc.cpp
LOCAL_CPPFLAGS := -std=gnu++0x -Wall -fPIE         # whatever g++ flags you like
LOCAL_LDLIBS := -L$(SYSROOT)/usr/lib -llog -fPIE -pie   # whatever ld flags you like

include $(BUILD_EXECUTABLE)    # <-- Use this to build an executable.

Go to the project/ directory, and simply type
```
ndk-build
```
The result will be placed in project/libs/<arch>/name-of-your-executable.

Android shell don't find the file to execute

Your executable is missing a library. The most likely reason is that you just built a Linux ARM executable (not an Android one) which is linked against some libc version other than Android Bionic.

You could either learn how to build proper Android executables or a statically linked binary might just work for your purpose.

Compile and run C code for Android

Let's start from the bottom up.

Going C (or C++) does not grant your code extra rights under Android.

There are different options of doing all your development without Java. There are frameworks for pure C++ (like NativeActivity), or for C# (MonoDroid), or Scala, or Ruby, or whatnot.

Android NDK also allows to simply build an executable - one that can be run from command line, and has no GUI. Your Android app can launch such executable via standard Java Runtime.exec() and her siblings. Or you can launch this executable from a terminal emulator or adb shell.

If you have root access on your device (or if you flash your own mod), you can launch an executable through the standard Linux init.rc or similar.

Note that in the latter case, you can easily grant your executable root or other permissions.

How to Build an Executable for Android Shell