Environment variable

Environment variables are a set of dynamic named values that can affect the way running processes will behave on a computer.

They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP environment variable to discover a suitable location to store temporary files, or the HOME or USERPROFILE variable to find the directory structure owned by the user running the process.

They were introduced in their modern form in 1979 with Version 7 Unix, so are included in all Unix operating system flavors and variants from that point onward including Linux and OS X. From PC DOS 2.0 in 1982, all succeeding Microsoft operating systems including Microsoft Windows, and OS/2 also have included them as a feature, although with somewhat different syntax, usage and standard variable names.

Details

In all Unix and Unix-like systems, each process has its own separate set of environment variables. By default, when a process is created, it inherits a duplicate environment of its parent process, except for explicit changes made by the parent when it creates the child. At the API level, these changes must be done between running fork and exec. Alternatively, from command shells such as bash, a user can change environment variables for a particular command invocation by indirectly invoking it via env or using the ENVIRONMENT_VARIABLE=VALUE <command> notation. All Unix operating system flavors, DOS, and Windows have environment variables; however, they do not all use the same variable names. A running program can access the values of environment variables for configuration purposes.

Temporary file

Temporary files, or foo files (.TMP), are files created to temporarily contain information while a new file is being made. It may be created by computer programs for a variety of purposes; principally when a program cannot allocate enough memory for its tasks, when the program is working on data bigger than the architecture's address space, or as a primitive form of inter-process communication.

Auxiliary memory

Modern operating systems employ virtual memory, however programs that use large amounts of data (e.g. video editing) may need to create temporary file(s).

Inter-process communication

Most operating systems offer primitives such as pipes, sockets or shared memory to pass data among programs, but often the simplest way (especially for programs that follow the Unix philosophy) is to write data into a temporary file and inform the receiving program of the location of the temporary file.

Creation

On POSIX systems, temporary files can be safely created with the mkstemp or tmpfile library functions. Some systems provide the former POSIX (now removed) mktemp(1) program. These files are typically located in the standard temporary directory, /tmp on Unix machines or %TEMP% (which is log-in specific) on Windows machines.

Unix filesystem

In Unix and operating systems inspired by it, the file system is considered a central component of the operating system. It was also one of the first parts of the system to be designed and implemented by Ken Thompson in the first experimental version of Unix, dated 1969.

Like in other operating systems, the filesystem provides information storage and retrieval, as well as interprocess communication, in the sense that the many small programs that traditionally comprise a Unix system can store information in files so that other programs can read these, although pipes complemented it in this role starting with the Third Edition. Additionally, the filesystem provides access to other resources through so-called device files that are entry points to terminals, printers, and mice.

The rest of this article uses "Unix" as a generic name to refer to both the original Unix operating system as well as its many workalikes.

Principles

The filesystem appears as a single rooted tree of directories. Instead of addressing separate volumes such as disk partitions, removable media, and network shares as separate trees (as done in MS-DOS and Windows: each "drive" has a drive letter that denotes the root of its file system tree), such volumes can be "mounted" on a directory, causing the volume's file system tree to appear as that directory in the larger tree. The root of the entire tree is denoted /.