A portable application (portable app), sometimes also called standalone, is a computer software program designed to run independently from an operating system. This type of application is stored on a removable storage device such as a CD, USB flash drive, flash card, or floppy disk – storing its program files, configuration information and data on the storage medium alone.
Portable applications can be run on any computer system with which they are compatible but typically require a specific operating system (such as Microsoft Windows XP or above, certain version of a Linux distro, etc.)
Depending on the operating system, portability is more or less complex to implement; to operating systems such as AmigaOS, all applications are by definition portable. Portable apps are distinct from software portability where software allows its source code to be compiled for different computing platforms.
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A portable application does not leave its files or settings on the host computer. Typically the application does not write to the Windows registry or store its configuration files (such as an INI file) in the user's profile; instead, it stores its configuration files in the program's directory. Another requirement, since file paths will often differ on changing computers due to variation in Windows drive letter assignments, is the need for applications to store them in a relative format. While some applications have options to support this behavior, many programs are not designed to do this. A common technique for such programs is the use of a launcher program to copy necessary settings and files to the host computer when the application starts and move them back to the application's directory when it closes.
An alternative strategy for achieving application portability within Windows, without requiring application source code changes, is application virtualization: An application is "sequenced" or "packaged" against a runtime layer that transparently intercepts its file system and registry calls, then redirects these to other persistent storage without the application's knowledge. This approach leaves the application itself unchanged, yet portable.
The same approach [1] is used for individual application components: run-time libraries, COM components or ActiveX, not only for the entire application. As a result, when individual components are ported in such manner they are able to be: integrated into original portable applications, repeatedly instantiated (virtually installed) with different configurations/settings on the same operating system (OS) without mutual conflicts. As the ported components do not affect the OS-protected related entities (registry and files), the components will not require administrative privileges for installation and management.
Microsoft saw the need for an application-specific registry [2] for its Windows operating system as far back as 2005. It eventually incorporated some of this technology, using the techniques mentioned above, via its Application Compatibility Database [3] using its Detours [4] code library, into Windows XP. It did not, however, make any of this technology available via one of its system APIs.
Programs written with a Unix-like base in mind often do not make any assumptions. Whereas many Windows programs assume the user is an administrator—something very prevalent in the days of Windows 95/98/ME (and to some degree in Windows XP/2000, though not in Windows Vista)—such would quickly result in "Permission denied" errors in Unix-like environments since users will be in an unprivileged state much more often. Programs are therefore generally designed around using the HOME environment variable to store settings (e.g. $HOME/.w3m for the w3m browser). The dynamic linker provides an environment variable LD_LIBRARY_PATH that programs can use to load libraries from non-standard directories. Assuming /mnt contains the portable programs and configuration, a command line may look like:
HOME=/mnt/home/user LD_LIBRARY_PATH=/mnt/usr/lib /mnt/usr/bin/w3m www.example.com
A linux application without need for a user-interaction (e.g. adapting a script or environment variable) on varying directory paths can be achieved with the GCC Linker option $ORIGIN which allows a relative library search path.[5]
Not all programs honor this – some completely ignore $HOME and instead do a user lookup in /etc/passwd to find the home directory, therefore thwarting portability.
Some Linux distributions already have native support for portable apps (Super OS, with RUNZ files).
There are also cross-distro package formats that don't require installation or admin rights to run.[6]