Just "write" command without any parameters writes conf to file.
diff --git a/vtysh/vtysh.c b/vtysh/vtysh.c
index 6b65826..6000f81 100644
--- a/vtysh/vtysh.c
+++ b/vtysh/vtysh.c
@@ -1382,6 +1382,11 @@
"Write running configuration to memory, network, or terminal\n"
"Write configuration to the file (same as write memory)\n")
+ALIAS (vtysh_write_memory,
+ vtysh_write_cmd,
+ "write",
+ "Write running configuration to memory, network, or terminal\n")
+
ALIAS (vtysh_write_terminal,
vtysh_show_running_config_cmd,
"show running-config",
@@ -1849,6 +1854,7 @@
install_element (ENABLE_NODE, &vtysh_show_running_config_cmd);
install_element (ENABLE_NODE, &vtysh_copy_runningconfig_startupconfig_cmd);
install_element (ENABLE_NODE, &vtysh_write_file_cmd);
+ install_element (ENABLE_NODE, &vtysh_write_cmd);
/* write terminal command */
install_element (ENABLE_NODE, &vtysh_write_terminal_cmd);