A better text editor for your adb shell

Bad vibes

I often find myself needing to edit text on an emulated Android system and it’s always a pain. Rebuilding from source takes too long. Editing the file on my computer and then adb pushing it over is cumbersome.

So I often reach for the toybox implementation of vi to read and edit files, but it leaves a lot to be desired. The thing I miss the most is syntax highlighting. It’s absence especially painful given there is no file reader native to the emulator shell aside from cat and more.

And the prospect of cross compiling vim or emacs for Android seemed quite daunting - why spend a day or two in build system hell? I’d rather just adb push and adb pull instead.

Though recently I was able to cross compile a Rust project for Android with ease using cargo-ndk. It made me wonder if there was a modern text editor written in Rust that I could quickly cargo build, throw on my emulator, and make my text editing in the adb shell a lot easier.

Screenshot from the Pokemon Red in the cave where you choose your fossil. Text reads: “You want the Helix Fossil?”

Cross compiling Helix for Android using cargo ndk

Helix is terminal-based modal (vi-like) editor written in Rust. It has lots of built in functionality including language server support, tree-sitter integration, sane defaults, but most importantly, syntax highlighting. A perfect candidate for use in our adb shell.

To compile helix on a modern Linux machine, first install basic compilation tools (build-essential package on Debian-based distros) and install Rust with rustup.

Then install cargo-ndk and the various Android Rust targets with rustup:

cargo install cargo-ndk
rustup target add \
    aarch64-linux-android \
    armv7-linux-androideabi \
    x86_64-linux-android \
    i686-linux-android
rustup target install \
    aarch64-linux-android \
    armv7-linux-androideabi \
    x86_64-linux-android \
    i686-linux-android

Download the Android NDK and set your ANDROID_NDK_HOME environment variable:

wget https://dl.google.com/android/repository/android-ndk-r27c-linux.zip
unzip android-ndk-r27c-linux.zip
export ANDROID_NDK_HOME=$(pwd)/android-ndk-r27c

Finally get the Helix source code and build it using cargo ndk (for arm64-v8a or x86_64):

git clone https://github.com/helix-editor/helix
cd helix
cargo ndk -t arm64-v8a build --release

It’s that easy.

It’s not that easy

A helix binary built for android will be at target/aarch64-linux-android/release/hx and the various runtime files at runtime/. We can push them to the emulated device:

adb push target/aarch64-linux-android/release/hx /data/local/tmp
adb push runtime /data/local/tmp

But when we run it we get a logging error:

adb shell
emu64a:/ # /data/local/tmp/hx
Error: failed to initialize logging

Caused by:
    No such file or directory (os error 2)

The default log file for helix is ~/.cache/helix/helix.log but on android the default $HOME is /, which is read only:

emu64a:/ # env | grep HOME
HOME=/
emu64a:/ # mount | head -1
/dev/block/dm-0 on / type ext4 (ro,seclabel,relatime)

So we can set $HOME to the writable /data/local/tmp and move the runtime/ directory to where helix expects: ~/.config/helix/runtime/:

emu64a:/ # export HOME=/data/local/tmp
emu64a:/ # cd
emu64a:/data/local/tmp # mkdir -p .config/helix
emu64a:/data/local/tmp # mv runtime/ .config/helix/

But we still don’t get syntax highlighting – and the log tells us why:

emu64a:/data/local/tmp # ./hx test.c
emu64a:/data/local/tmp # cat .cache/helix/helix.log
2025-02-10T00:00:19.701 helix_core::syntax [ERROR] Failed to load tree-sitter parser for language "c": Error opening dynamic library "/data/local/tmp/.config/helix/runtime/grammars/c.so": dlopen failed: library "libc++_shared.so" not found: needed by /data/local/tmp/.config/helix/runtime/grammars/c.so in namespace (default)

Helix loads the relevant tree-sitter grammer as a dynamic library and it requires libc++_shared.so. libc++_shared.so is part of the Android NDK and is often bundled in apps but doesn’t exist natively on the Android system.

So let’s find the relevant libc++_shared.so on the host system, adb push it into our emulated Android, and set LD_LIBRARY_PATH to point the Android linker to load the library:

~/android-ndk-r27c$ find . -name libc++_shared.so
./toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/i686-linux-android/libc++_shared.so
./toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/riscv64-linux-android/libc++_shared.so
./toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android/libc++_shared.so
./toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/arm-linux-androideabi/libc++_shared.so
./toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/x86_64-linux-android/libc++_shared.so
~/android-ndk-r27c$ adb push ./toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android/libc++_shared.so /data/local/tmp
~/android-ndk-r27c$ adb shell
emu64a:/ # export HOME=/data/local/tmp
emu64a:/ # export LD_LIBRARY_PATH=/data/local/tmp
emu64a:/ # cd

And we can finally get helix running in all it’s syntax highlighting glory:

emu64a:/data/local/tmp # ./hx test.c

Final config notes

The above steps are enough to get the helix editor working using the default configuration of Android Studio’s emulator. The downside being, of course, that you will need to export HOME and LD_LIBRARY_PATH every time you open a new adb shell.

Normally you could add these exports to your .*shrc, but on Android your adb shell runs mksh, whose sole rc file is in the read-only partition /system/etc/mkshrc.

You might want to boot your emulator as a writable-system and disable verity in order to edit this file (and while you’re at it, add the hx binary location to your PATH).

And then you can use helix to edit any file on your Android filesystem :) Happy Hacking!