Added: Write termux shell environment to /data/data/com.termux/files/usr/etc/termux/termux.env on app startup and package changes

The `termux.env` can be sourced by shells to set termux environment normally exported. This can be useful for users starting termux shells with `adb` `run-as` or `root`. The file will not contain `SHELL_CMD__` variables since those are shell command specific.

The items in the `termux.env` file have the format `export name="value"`.
The `"`\$` characters will be escaped with `a backslash `\`, like `\"` if characters are for literal value. Note that if `$` is escaped and if its part of variable, then variable expansion will not happen if `.env` file is sourced. The `\` at the end of a value line means line continuation. Value can contain newline characters.

The `termux.env` file should be sourceable by `POSIX` compliant shells like `bash`, `zsh`, `sh`, android's `mksh`, etc. Other shells with require manual parsing of the file to export variables.

Related discussion #2565

termux-shared/src/main/java/com/termux/shared/shell/command/environment/ShellEnvironmentUtils.java

@@ -42,6 +42,60 @@ public class ShellEnvironmentUtils {
         return environmentList;
     }
 
+    /**
+     * Convert environment {@link HashMap} to {@link String} where each item equals "key=value".
+     *
+     */
+    @NonNull
+    public static String convertEnvironmentToDotEnvFile(@NonNull HashMap<String, String> environmentMap) {
+        return convertEnvironmentToDotEnvFile(convertEnvironmentMapToEnvironmentVariableList(environmentMap));
+    }
+
+    /**
+     * Convert environment {@link HashMap} to `.env` file {@link String}.
+     *
+     * The items in the `.env` file have the format `export name="value"`.
+     *
+     * If the {@link ShellEnvironmentVariable#escaped} is set to {@code true}, then
+     * {@link ShellEnvironmentVariable#value} will be considered to be a literal value that has
+     * already been escaped by the caller, otherwise all the `"`\$` in the value will be escaped
+     * with `a backslash `\`, like `\"`. Note that if `$` is escaped and if its part of variable,
+     * then variable expansion will not happen if `.env` file is sourced.
+     *
+     * The `\` at the end of a value line means line continuation. Value can contain newline characters.
+     *
+     * Check {@link #isValidEnvironmentVariableName(String)} and {@link #isValidEnvironmentVariableValue(String)}
+     * for valid variable names and values.
+     *
+     * https://github.com/ko1nksm/shdotenv#env-file-syntax
+     * https://github.com/ko1nksm/shdotenv/blob/main/docs/specification.md
+     */
+    @NonNull
+    public static String convertEnvironmentToDotEnvFile(@NonNull List<ShellEnvironmentVariable> environmentList) {
+        StringBuilder environment = new StringBuilder();
+        Collections.sort(environmentList);
+        for (ShellEnvironmentVariable variable : environmentList) {
+            if (isValidEnvironmentVariableNameValuePair(variable.name, variable.value, true) && variable.value != null) {
+                environment.append("export ").append(variable.name).append("=\"")
+                    .append(variable.escaped ? variable.value : variable.value.replaceAll("([\"`\\\\$])", "\\\\$1"))
+                    .append("\"\n");
+            }
+        }
+        return environment.toString();
+    }
+
+    /**
+     * Convert environment {@link HashMap} to {@link List< ShellEnvironmentVariable >}. Each item
+     * will have its {@link ShellEnvironmentVariable#escaped} set to {@code false}.
+     */
+    @NonNull
+    public static List<ShellEnvironmentVariable> convertEnvironmentMapToEnvironmentVariableList(@NonNull HashMap<String, String> environmentMap) {
+        List<ShellEnvironmentVariable> environmentList = new ArrayList<>();
+        for (String name :environmentMap.keySet()) {
+            environmentList.add(new ShellEnvironmentVariable(name, environmentMap.get(name), false));
+        }
+        return environmentList;
+    }
 
     /**
      * Check if environment variable name and value pair is valid. Errors will be logged if

termux-shared/src/main/java/com/termux/shared/shell/command/environment/ShellEnvironmentVariable.java

@@ -0,0 +1,28 @@
+package com.termux.shared.shell.command.environment;
+
+public class ShellEnvironmentVariable implements Comparable<ShellEnvironmentVariable> {
+
+    /** The name for environment variable */
+    public String name;
+
+    /** The value for environment variable */
+    public String value;
+
+    /** If environment variable {@link #value} is already escaped. */
+    public boolean escaped;
+
+    public ShellEnvironmentVariable(String name, String value) {
+        this(name, value, false);
+    }
+
+    public ShellEnvironmentVariable(String name, String value, boolean escaped) {
+        this.name = name;
+        this.value = value;
+        this.escaped = escaped;
+    }
+
+    @Override
+    public int compareTo(ShellEnvironmentVariable other) {
+        return this.name.compareTo(other.name);
+    }
+}