"Failed to load platform plugin "xcb" " while launching qt5 app on linux without qt installed

Use ldd (man ldd) to show shared library dependencies. Running this on libqxcb.so

.../platforms$ ldd libqxcb.so

shows that xcb depends on libQt5DBus.so.5 in addition to libQt5Core.so.5 and libQt5Gui.so.5 (and many other system libs). Add libQt5DBus.so.5 to your collection of shared libs and you should be ready to move on.

As was posted earlier, you need to make sure you install the platform plugins when you deploy your application. Depending on how you want to deploy things, there are two methods to tell your application where the platform plugins (e.g. platforms/plugins/libqxcb.so) are at runtime which may work for you.

The first is to export the path to the directory through the QT_QPA_PLATFORM_PLUGIN_PATH variable.

QT_QPA_PLATFORM_PLUGIN_PATH=path/to/plugins ./my_qt_app

or

export QT_QPA_PLATFORM_PLUGIN_PATH=path/to/plugins./my_qt_app

The other option, which I prefer is to create a qt.conf file in the same directory as your executable. The contents of which would be:

[Paths]Plugins=/path/to/plugins

More information regarding this can be found here and at using qt.conf

I tried to start my binary, compiled with Qt 5.7, on Ubuntu 16.04 LTS where Qt 5.5 is preinstalled. It didn't work.

At first, I inspected the binary itself with ldd as was suggested here, and satisfied all "not found" dependencies. Then this notorious This application failed to start because it could not find or load the Qt platform plugin "xcb" error was thrown.

How to resolve this in Linux

Firstly you should create platforms directory where your binary is, because it is the place where Qt looks for XCB library. Copy libqxcb.so there. I wonder why authors of other answers didn't mention this.

Then you may want to run your binary with QT_DEBUG_PLUGINS=1 environment variable set to check which dependencies of libqxcb.so are not satisfied. (You may also use ldd for this as suggested in the accepted answer).

The command output may look like this:

me@xerus:/media/sf_Qt/Package$ LD_LIBRARY_PATH=. QT_DEBUG_PLUGINS=1 ./BinaryQFactoryLoader::QFactoryLoader() checking directory path "/media/sf_Qt/Package/platforms" ...QFactoryLoader::QFactoryLoader() looking at "/media/sf_Qt/Package/platforms/libqxcb.so"Found metadata in lib /media/sf_Qt/Package/platforms/libqxcb.so, metadata={    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",    "MetaData": {        "Keys": [            "xcb"        ]    },    "className": "QXcbIntegrationPlugin",    "debug": false,    "version": 329472}Got keys from plugin meta data ("xcb")loaded library "/media/sf_Qt/Package/platforms/libqxcb.so"QLibraryPrivate::loadPlugin failed on "/media/sf_Qt/Package/platforms/libqxcb.so" : "Cannot load library /media/sf_Qt/Package/platforms/libqxcb.so: (/usr/lib/x86_64-linux-gnu/libQt5DBus.so.5: version `Qt_5' not found (required by ./libQt5XcbQpa.so.5))"This application failed to start because it could not find or load the Qt platform plugin "xcb"in "".Available platform plugins are: xcb.Reinstalling the application may fix this problem.Aborted (core dumped)

Note the failing libQt5DBus.so.5 library. Copy it to your libraries path, in my case it was the same directory where my binary is (hence LD_LIBRARY_PATH=.). Repeat this process until all dependencies are satisfied.

P.S. thanks to the author of this answer for QT_DEBUG_PLUGINS=1.