Use $ANDROID_SERIAL if no selector is specified

Like adb, read the ANDROID_SERIAL environment variable to select a
device by serial if no explicit selection (-s, -d, -e or --tcpip=<addr>)
is provided via the command line.

Fixes #3111 <https://github.com/Genymobile/scrcpy/issues/3111>
PR #3113 <https://github.com/Genymobile/scrcpy/pull/3113>

README.md

@@ -448,6 +448,9 @@ scrcpy --serial 0123456789abcdef
 scrcpy -s 0123456789abcdef  # short version
 ```
 
+The serial may also be provided via the environment variable `ANDROID_SERIAL`
+(also used by `adb`).
+
 If the device is connected over TCP/IP:
 
 ```bash

app/scrcpy.1

@@ -477,6 +477,10 @@ Push file to device (see \fB\-\-push\-target\fR)
 .B ADB
 Path to adb.
 
+.TP
+.B ANDROID_SERIAL
+Device serial to use if no selector (-s, -d, -e or --tcpip=<addr>) is specified.
+
 .TP
 .B SCRCPY_ICON_PATH
 Path to the program icon.

app/src/cli.c

@@ -80,6 +80,11 @@ struct sc_envvar {
     const char *text;
 };
 
+struct sc_exit_status {
+    unsigned value;
+    const char *text;
+};
+
 struct sc_getopt_adapter {
     char *optstring;
     struct option *longopts;
@@ -655,6 +660,11 @@ static const struct sc_envvar envvars[] = {
         .name = "ADB",
         .text = "Path to adb executable",
     },
+    {
+        .name = "ANDROID_SERIAL",
+        .text = "Device serial to use if no selector (-s, -d, -e or "
+                "--tcpip=<addr>) is specified",
+    },
     {
         .name = "SCRCPY_ICON_PATH",
         .text = "Path to the program icon",
@@ -662,7 +672,22 @@ static const struct sc_envvar envvars[] = {
     {
         .name = "SCRCPY_SERVER_PATH",
         .text = "Path to the server binary",
-    }
+    },
+};
+
+static const struct sc_exit_status exit_statuses[] = {
+    {
+        .value = 0,
+        .text = "Normal program termination",
+    },
+    {
+        .value = 1,
+        .text = "Start failure",
+    },
+    {
+        .value = 2,
+        .text = "Device disconnected while running",
+    },
 };
 
 static char *
@@ -901,6 +926,25 @@ print_envvar(const struct sc_envvar *envvar, unsigned cols) {
     free(text);
 }
 
+static void
+print_exit_status(const struct sc_exit_status *status, unsigned cols) {
+    assert(cols > 8); // sc_str_wrap_lines() requires indent < columns
+    assert(status->text);
+
+    // The text starts at 9: 4 ident spaces, 3 chars for numeric value, 2 spaces
+    char *text = sc_str_wrap_lines(status->text, cols, 9);
+    if (!text) {
+        printf("<ERROR>\n");
+        return;
+    }
+
+    assert(strlen(text) >= 9); // Contains at least the initial identation
+
+    // text + 9 to remove the initial indentation
+    printf("    %3d  %s\n", status->value, text + 9);
+    free(text);
+}
+
 void
 scrcpy_print_usage(const char *arg0) {
 #define SC_TERM_COLS_DEFAULT 80
@@ -939,6 +983,11 @@ scrcpy_print_usage(const char *arg0) {
     for (size_t i = 0; i < ARRAY_LEN(envvars); ++i) {
         print_envvar(&envvars[i], cols);
     }
+
+    printf("\nExit status:\n\n");
+    for (size_t i = 0; i < ARRAY_LEN(exit_statuses); ++i) {
+        print_exit_status(&exit_statuses[i], cols);
+    }
 }
 
 static bool

app/src/server.c

@@ -707,7 +707,15 @@ run_server(void *data) {
         } else if (params->select_tcpip) {
             selector.type = SC_ADB_DEVICE_SELECT_TCPIP;
         } else {
-            selector.type = SC_ADB_DEVICE_SELECT_ALL;
+            // No explicit selection, check $ANDROID_SERIAL
+            const char *env_serial = getenv("ANDROID_SERIAL");
+            if (env_serial) {
+                LOGI("Using ANDROID_SERIAL: %s", env_serial);
+                selector.type = SC_ADB_DEVICE_SELECT_SERIAL;
+                selector.serial = env_serial;
+            } else {
+                selector.type = SC_ADB_DEVICE_SELECT_ALL;
+            }
         }
         struct sc_adb_device device;
         ok = sc_adb_select_device(&server->intr, &selector, 0, &device);