A script to get Arch Linux ARM rootfs running in proot on Android (Termux)

get_shell.c

/*
 * clang -static -O2 get_shell.c get_shell
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pwd.h>

int main(int argc, char *argv[]) {
    if(argc < 2) {
        printf("Usage: prog <username or uid:gid pair>\n");
        return 1;
    }

    // uid & gid
    int uid = -1,
        gid = -1;

    char* username = argv[1];
    size_t size = strlen(username);

    // Open passd file and struct
    struct passwd* passwd;
    FILE* passwd_file = fopen("/data/data/com.termux/files/home/.alarm/etc/passwd", "r");

    // Check for ':'
    // TODO: this is hackish
    char* separator = strstr(username, ":") == NULL ? NULL : strsep(&username, ":");
    if(separator != NULL) {
        uid = atoi(separator);
        gid = atoi(username);
    }

    // Iterate over password entries
    while((passwd = fgetpwent(passwd_file)) != NULL) {
        if(((uid != -1 && gid != -1) && (uid == passwd->pw_uid && gid == passwd->pw_gid)) || strncmp(passwd->pw_name, username, size) == 0) {
            printf("%s\n", passwd->pw_shell);
            return 0;
        }
    }

    return 1;
}

launch-arch.sh

#!/bin/bash -e

if [ -z "${ANDROID_ROOT}" ]; then
    echo "Don't :)"
    exit 1
fi

# This breaks proot, see https://wiki.termux.com/wiki/PRoot
unset LD_PRELOAD

# Arch Linux ARM unpacked root location
ROOT_LOCATION="$(realpath ~/.alarm)"

# Note: user `alarm` is 1000:1000, so use `WHO=1000:1000 launch-arch.sh`
WHO=${WHO:-0:0}

# Program to launch
PROG=${@:-$(get_shell ${WHO}) -l}

# Launch proot
proot \
	--link2symlink \
	--kill-on-exit \
	-i "${WHO}" \
	-r "${ROOT_LOCATION}" \
	-b /dev \
	-b /proc \
	-b /sys \
	-b /sdcard \
	-b /data/data/com.termux/files \
	/usr/bin/env -i -C "${HOME}" \
		PATH=/usr/bin:/usr/local/bin \
		TERM="${TERM}" \
		HOME="${HOME}" \
	${PROG}