ish-app/ish

iSH is a project that runs a full Linux shell environment on an iPhone or iPad. Normally, iOS locks down the operating system so you cannot run a terminal or command-line tools. iSH gets around this by running a software emulator of an x86 (32-bit) processor entirely in user space, meaning it simulates CPU instructions and translates Linux system calls (the low-level requests a program makes to the operating system) into equivalent iOS operations. The result is a working Alpine Linux environment inside an iOS app, complete with a package manager, so you can install tools like Python, Git, or SSH clients directly on your device. The core technical trick is an interpreter written largely in assembly language. Rather than compiling Linux programs to run natively on Apple's ARM chip, iSH interprets x86 instructions at runtime using a technique borrowed from Forth interpreters called threaded code, where a sequence of small functions are chained together via tail calls. The authors note this gives a 3, 5x speed improvement over simpler emulation approaches. Someone would use iSH when they need command-line access on an iOS device, for example, connecting to a remote server via SSH, running scripts, or experimenting with Linux tools during travel without a laptop. It is available on the App Store and as a TestFlight beta. The project is written in C and built with Clang and the Meson build system.