commands: Add export command

The `export` command allows one to set environment variables
easily within their configuration. It additionally allows
one to get an environment variable using the `$ENV:<name>`
syntax.

With the configuration snippet listed below, which MUST appear
before any `exec` commands -- so that environment variables are
all available to these newly launched programs -- set a number
of variables used by GTK & Qt for forcing Wayland rendering.

```
export GDK_BACKEND wayland
export CLUTTER_BACKEND wayland
export MOZ_ENABLE_WAYLAND 1
export QT_QPA_PLATFORM wayland-egl
export QT_WAYLAND_DISABLE_WINDOWDECORATION 1
export ELM_DISPLAY wl
export _JAVA_AWT_WM_NONREPARENTING 0
export SDL_VIDEODRIVER wayland
```

While the above does not demonstrate reading variables, the
example below does demonstrates its use:

```
export PATH $ENV:HOME/bin:$ENV:PATH
```

Signed-off-by: Joseph Benden <joe@benden.us>

include/sway/commands.h

@@ -123,6 +123,7 @@ sway_cmd cmd_default_floating_border;
 sway_cmd cmd_default_orientation;
 sway_cmd cmd_exec;
 sway_cmd cmd_exec_always;
+sway_cmd cmd_export;
 sway_cmd cmd_exit;
 sway_cmd cmd_floating;
 sway_cmd cmd_floating_maximum_size;

sway/commands.c

@@ -58,6 +58,7 @@ static struct cmd_handler handlers[] = {
 	{ "default_floating_border", cmd_default_floating_border },
 	{ "exec", cmd_exec },
 	{ "exec_always", cmd_exec_always },
+	{ "export", cmd_export },
 	{ "floating_maximum_size", cmd_floating_maximum_size },
 	{ "floating_minimum_size", cmd_floating_minimum_size },
 	{ "floating_modifier", cmd_floating_modifier },

sway/commands/export.c

@@ -0,0 +1,58 @@
+#define _POSIX_C_SOURCE 200809L
+#define _GNU_SOURCE 1
+#define __BSD_VISIBLE 1
+#include <stdio.h>
+#include <string.h>
+#include <strings.h>
+#include "sway/commands.h"
+#include "sway/config.h"
+#include "list.h"
+#include "log.h"
+#include "stringop.h"
+
+// sort in order of longest->shortest
+static int compare_set_qsort(const void *_l, const void *_r) {
+	struct sway_variable const *l = *(void **)_l;
+	struct sway_variable const *r = *(void **)_r;
+	return strlen(r->name) - strlen(l->name);
+}
+
+// export TERM xterm-256color
+struct cmd_results *cmd_export(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "export", EXPECTED_AT_LEAST, 2))) {
+		return error;
+	}
+
+	char *search_var;
+	if (asprintf(&search_var, "$ENV:%s", argv[0]) < 0) {
+		return cmd_results_new(CMD_FAILURE, "Unable to allocate search variable");
+	}
+
+	struct sway_variable *var = NULL;
+	// Find old variable if it exists
+	int i;
+	for (i = 0; i < config->symbols->length; ++i) {
+		var = config->symbols->items[i];
+		if (strcmp(var->name, search_var) == 0) {
+			break;
+		}
+		var = NULL;
+	}
+	if (var) {
+		free(var->value);
+	} else {
+		var = malloc(sizeof(struct sway_variable));
+		if (!var) {
+			return cmd_results_new(CMD_FAILURE, "Unable to allocate variable");
+		}
+		var->name = strdup(search_var);
+		list_add(config->symbols, var);
+		list_qsort(config->symbols, compare_set_qsort);
+	}
+	var->value = join_args(argv + 1, argc - 1);
+	// NOTE(jbenden): Should an empty string mean to unsetenv?
+	if (setenv(argv[0], var->value, true) != 0)
+		return cmd_results_new(CMD_FAILURE, "Unable to set environment variable");
+	return cmd_results_new(CMD_SUCCESS, NULL);
+}

sway/config.c

@@ -919,6 +919,33 @@ char *do_var_replacement(char *str) {
 			++find;
 			continue;
 		}
+		// Is an environment variable requested?
+		if (strncmp(find, "$ENV:", strlen("$ENV:")) == 0) {
+			char *var_name = &find[5];
+			int vnlen = strlen(var_name);
+			if (var_name[0] == '\0') continue;
+			char *env_value = getenv(var_name);
+			if (env_value) {
+				int vvlen = strlen(env_value);
+				char *newstr = malloc(strlen(str) - (strlen("$ENV:") + vnlen) + vvlen + 1);
+				if (!newstr) {
+					sway_log(SWAY_ERROR,
+							"Unable to allocate replacement "
+							"during variable expansion");
+					continue;
+				}
+				char *newptr = newstr;
+				int offset = find - str;
+				strncpy(newptr, str, offset);
+				newptr += offset;
+				strncpy(newptr, env_value, vvlen);
+				newptr += vvlen;
+				strcpy(newptr, find + vnlen + strlen("$ENV:"));
+				free(str);
+				str = newstr;
+				find = str + offset + vvlen;
+			}
+		}
 		// Find matching variable
 		for (i = 0; i < config->symbols->length; ++i) {
 			struct sway_variable *var = config->symbols->items[i];

sway/meson.build

@@ -51,6 +51,7 @@ sway_sources = files(
 	'commands/exit.c',
 	'commands/exec.c',
 	'commands/exec_always.c',
+	'commands/export.c',
 	'commands/floating.c',
 	'commands/floating_minmax_size.c',
 	'commands/floating_modifier.c',

sway/sway.5.scd

@@ -586,6 +586,21 @@ The default colors are:
 *exec_always* <shell command>
 	Like *exec*, but the shell command will be executed _again_ after *reload*.
 
+*export* <name> <value>
+	Sets the Wayland session environment variable _name_ to a specific
+	_value_. All environment variables are available in the config
+	using the _$ENV:<name>_ syntax.
+
+	Example:
+```
+    	export CLUTTER_BACKEND wayland
+    	export GDK_BACKEND wayland
+    	export MOZ_ENABLE_WAYLAND 1
+    	export PATH $ENV:HOME/bin:$ENV:PATH
+    	export QT_QPA_PLATFORM wayland-egl
+    	export QT_WAYLAND_DISABLE_WINDOWDECORATION 1
+```
+
 *floating_maximum_size* <width> x <height>
 	Specifies the maximum size of floating windows. -1 x -1 removes the upper
 	limit. The default is 0 x 0, which will use the width and height of the