Environment variable

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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 operating 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[edit]

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 Microsoft 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.

Examples of environment variables include:

  • PATH - a list of directory paths. When the user types a command without providing the full path, this list is checked to see whether it contains a path that leads to the command.
  • HOME (Unix-like) and USERPROFILE (Microsoft Windows) - indicate where a user's home directory is located in the file system.
  • HOME/{.AppName} (Unix-like) and APPDATA\{DeveloperName\AppName} (Microsoft Windows) - for storing application settings. Many applications incorrectly use USERPROFILE for application settings in Windows - USERPROFILE should only be used in dialogs that allow user to choose between paths like Documents/Pictures/Downloads/Music, for programmatic purposes APPDATA (roaming), LOCALAPPDATA or PROGRAMDATA (shared between users) is used.
  • TERM (Unix-like) - specifies the type of computer terminal or terminal emulator being used (e.g., vt100 or dumb).
  • PS1 (Unix-like) - specifies how the prompt is displayed in the Bourne shell and variants.
  • MAIL (Unix-like) - used to indicate where a user's mail is to be found.
  • TEMP - location where processes can store temporary files

Shell scripts and batch files use environment variables to communicate data and preferences to child processes. They can also be used to store temporary values for reference later in a shell script. However, in Unix, other variables are usually used for this.

In Unix, an environment variable that is changed in a script or compiled program will only affect that process and possibly child processes. The parent process and any unrelated processes will not be affected. In MS-DOS, changing or removing a variable's value inside a batch file will change the variable for the duration of COMMAND.COM's existence.

In Unix, the environment variables are normally initialized during system startup by the system init scripts, and hence inherited by all other processes in the system. Users can, and often do, augment them in the profile script for the command shell they are using. In Microsoft Windows, each environment variable's default value is stored in the Windows registry or set in the AUTOEXEC.BAT file.

On Unix, a setuid program is given an environment chosen by its caller, but it runs with different authority from its caller. The dynamic linker will usually load code from locations specified by the environment variables LD_LIBRARY_PATH and LD_PRELOAD and run it with the process's authority. If a setuid program did this, it would be insecure, because its caller could get it to run arbitrary code and hence misuse its authority. For this reason, libc unsets these environment variables at startup in a setuid process. setuid programs usually unset unknown environment variables and check others or set them to reasonable values.

Use and Display[edit]

The variables can be used both in scripts and on the command line. They are usually referenced by putting special symbols in front of or around the variable name. For instance, to display the user home directory, in most scripting environments, the user has to type:

echo $HOME

On DOS, OS/2 or Windows systems, the user has to type this:

ECHO %HOME%

In Windows PowerShell, the user has to type this:

Write-Output $HOME

The commands env, set, and printenv display all environment variables and their values.

Assignment[edit]

env and set are also used to set environment variables and are often incorporated directly into the shell. printenv can also be used to print a single variable by giving that variable name as the sole argument to the command.

In Unix, the following commands can also be used, but are often dependent on a certain shell.

export VARIABLE=value  # for Bourne, bash, and related shells
setenv VARIABLE value  # for csh and related shells

A few simple principles govern how environment variables achieve their effect.

Environment variables are local to the process in which they were set. If two shell processes are spawned and the value of an environment variable is changed in one, that change will not be seen by the other.

When a child process is created, it inherits all the environment variables and their values from the parent process. Usually, when a program calls another program, it first creates a child process by forking, then the child adjusts the environment as needed and lastly the child replaces itself with the program to be called. This procedure gives the calling program control over the environment of the called program.

In Unix and Unix-like systems, the names of environment variables are case-sensitive.

In Unix shells, variables may be assigned without the export keyword. Variables defined in this way are displayed by the set command, but are not true environment variables, as they are stored only by the shell and not recognized by the kernel. The printenv command will not display them, and child processes do not inherit them.

VARIABLE=value

However, if used in front of a program to run, the variables will be exported to the environment and thus appear as real environment variables to the program:

VARIABLE=value program_name [arguments]

Environment variables persistence can be session-wide or system-wide.

In DOS, OS/2 and Windows, the SET command without any arguments displays all environment variables along with their values.

Common environment variables[edit]

Unix[edit]

$PATH
Contains a colon-separated list of directories that the shell searches for commands that do not contain a slash in their name (commands with slashes are interpreted as file names to execute, and the shell attempts to execute the files directly). It is equivalent to the Windows %PATH% variable.
$HOME
Contains the location of the user's home directory. Although the current user's home directory can also be found out through the C functions getpwuid and getuid, $HOME is often used for convenience in various shell scripts (and other contexts). Using the environment variable also gives the user the possibility to point to another directory.
$PWD
This variable points to the current directory. Equivalent to the output of the command pwd when called without arguments.
$DISPLAY
Contains the identifier for the display that X11 programs should use by default.
$LD_LIBRARY_PATH
On many Unix systems with a dynamic linker, contains a colon-separated list of directories that the dynamic linker should search for shared objects when building a process image after exec, before searching in any other directories.
$LANG, $LC_ALL, $LC_...
LANG is used to set to the default locale. For example, if the locale values are pt_BR, then the language is set to (Brazilian) Portuguese and Brazilian practice is used where relevant. Different aspects of localization are controlled by individual LC_-variables (LC_CTYPE, LC_COLLATE, LC_DATE etc.). LC_ALL can be used to force the same locale for all aspects.
$TZ
Refers to Time zone. It can be in several formats, either specifying the timezone itself or referencing a file (in /usr/share/zoneinfo).

DOS[edit]

%COMSPEC%
This variable contains the full path to the command processor, COMMAND.COM.
%PATH%
This variable contains a semicolon-delimited list of directories in which the command interpreter will search for executable files. Equivalent to the Unix $PATH variable (although note that PATH on Windows additionally performs the same task as LD_LIBRARY_PATH on Unix-like systems).
%TEMP% and %TMP%
These variables contain the path to the directory where temporary files should be stored.

Microsoft Windows[edit]

Discrete value variables generally expand to discrete values, such as the current working directory, the current date, or a random number. Some of these are true environment variables and will be expanded by all functions that handle environment variables. Others, like %CD% simply look like environment variables and will only be expanded by some functions and shells. They are not case-sensitive.

%CD%
This variable points to the current directory. Equivalent to the output of the command cd when called without arguments.
%DATE%
This variable expands to the current date. The date is displayed according to the current user's date format preferences.
%ERRORLEVEL%
This variable points to the current error level. If there was an error in the previous command, it is checked against this.
%RANDOM%
This variable returns a random number between 0 and 32767.
%TIME%
This variable points to the current time. The time is displayed according to the current user's time format preferences.

System path variables refer to locations of critical operating system resources, and as such generally are not user-dependent.

%AppData%
Contains the full path to the Application Data directory of the logged-in user. Does not work on Windows NT 4.0 SP6 UK.
%LOCALAPPDATA%
This variable is the temporary files of Applications. Its uses include storing of Desktop Themes, Windows Error Reporting, Caching and profiles of web browsers.
%ComSpec%
This variable contains the full path to the command processor; on Windows NT based operating systems this is cmd.exe, while on Windows 9x and ME it is the DOS command processor, COMMAND.COM.
%PATH%
This variable contains a semicolon-delimited (do not put spaces in between) list of directories in which the command interpreter will search for an executable file that matches the given command. Environment variables that represent paths may be nested within the PATH variable but only at one level of indirection. If this subpath environment variable itself contains an environment variable representing a path, PATH will not expand properly in the variable substitution. Equivalent to the Unix $PATH variable.
%ProgramFiles%
This variable points to Program Files directory, which stores all the installed program of Windows and others. The default on English-language systems is C:\Program Files. In 64-bit editions of Windows (XP, 2003, Vista), there are also %ProgramFiles(x86)% which defaults to C:\Program Files (x86) and %ProgramW6432% which defaults to C:\Program Files. The %ProgramFiles% itself depends on whether the process requesting the environment variable is itself 32-bit or 64-bit (this is caused by Windows-on-Windows 64-bit redirection).
%CommonProgramFiles%
This variable points to Common Files directory. The default is C:\Program Files\Common Files.
%SystemDrive%
The %SystemDrive% variable is a special system-wide environment variable found on Microsoft Windows NT and its derivatives. Its value is the drive upon which the system directory was placed. The value of %SystemDrive% is in most cases C:.
%SystemRoot%
The %SystemRoot% variable is a special system-wide environment variable found on Microsoft Windows NT and its derivatives. Its value is the location of the system directory, including the drive and path. The drive is the same as %SystemDrive% and the default path on a clean installation depends upon the version of the operating system. By default, Windows NT 5.1 (Windows XP) and newer versions use \WINDOWS, Windows NT 5.0 (Windows 2000), Windows NT 4.0 and Windows NT 3.1 use \WINNT, Windows NT 3.5x uses \WINNT35, and Windows NT 4.0 Terminal Server uses \WTSRV
%windir%
This variable points to the Windows directory (on Windows NT-based operating systems it is identical to the %SystemRoot% variable, above). If the System is on drive C: then the default values are C:\WINDOWS on Windows 95, Windows 98, Windows Me, Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008 and Windows 7 and C:\WINNT for Windows NT 4, and Windows 2000. Windows NT 4 Terminal Server Edition by default installs to C:\WTSRV.

User management variables store information related to resources and settings owned by various user profiles within the system. As a general rule, these variables do not refer to critical system resources or locations that are necessary for the OS to run.

%AllUsersProfile% (%PROGRAMDATA% for Windows Vista, Windows 7)
This variable expands to the full path to the All Users profile directory. This profile contains resources and settings that are used by all system accounts. Shortcut links copied to the All Users' Start menu or Desktop directories will appear in every user's Start menu or Desktop, respectively.
%UserDomain%
The name of the Workgroup or Windows Domain to which the current user belongs. The related variable, %LOGONSERVER%, holds the hostname of the server that authenticated the current user's logon credentials (name and password). For Home PCs, and PCs in a Workgroup, the authenticating server is usually the PC itself. For PCs in a Windows Domain, the authenticating server is a domain controller (a primary domain controller, or PDC, in Windows NT 4-based domains).
%UserProfile%
A special system-wide environment variable found on Microsoft Windows NT and its derivatives. Its value is the location of the current user's profile directory, in which is found that user's HKCU registry hive (NTUSER). Users can also use the %USERNAME% variable to determine the active users login identification.

Optional System variables are not explicitly specified by default but can be used to modify the default behaviour of certain built-in console commands. These variables also do not need to be explicitly specified as command line arguments.

%CopyCmd%
Allows a user to specify the /Y switch as the default for the COPY, XCOPY, and MOVE console commands. A default of /Y can be overridden by supplying the /-Y switch on the command line. The /Y switch instructs the command to replace existing files without prompting for confirmation.
%DirCmd%
Allows a user to specify customized default switches for the DIR console command. Preset default switches can be overridden by providing the negative switch on the command line. For example, if DIRCMD contains the /W switch, then it can be disabled by using DIR /-W.

Default Values on Microsoft Windows[edit]

Variable Windows XP Windows Vista/7
 %ALLUSERSPROFILE% C:\Documents and Settings\All Users C:\ProgramData
 %APPDATA% C:\Documents and Settings\{username}\Application Data C:\Users\{username}\AppData\Roaming
 %COMMONPROGRAMFILES% C:\Program Files\Common Files C:\Program Files\Common Files
 %COMMONPROGRAMFILES(x86)% C:\Program Files (x86)\Common Files (only in 64-bit version) C:\Program Files (x86)\Common Files (only in 64-bit version)
 %COMPUTERNAME% {computername} {computername}
 %COMSPEC% C:\Windows\System32\cmd.exe C:\Windows\System32\cmd.exe
 %HOMEDRIVE% C: C:
 %HOMEPATH% \Documents and Settings\{username} \Users\{username}
 %LOCALAPPDATA% C:\Users\{username}\AppData\Local
 %LOGONSERVER% \\{domain_logon_server} \\{domain_logon_server}
 %PATH% C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;{plus program paths} C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;{plus program paths}
 %PATHEXT% .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.WSF;.WSH .com;.exe;.bat;.cmd;.vbs;.vbe;.js;.jse;.wsf;.wsh;.msc
 %PROGRAMDATA%  %SystemDrive%\ProgramData
 %PROGRAMFILES%  %SystemDrive%\Program Files  %SystemDrive%\Program Files
 %PROGRAMFILES(X86)%  %SystemDrive%\Program Files (x86) (only in 64-bit version)  %SystemDrive%\Program Files (x86) (only in 64-bit version)
 %PROMPT% Code for current command prompt format. Code is usually $P$G Code for current command prompt format. Code is usually $P$G
 %PSModulePath%  %SystemRoot%\system32\WindowsPowerShell\v1.0\Modules\
 %PUBLIC%  %SystemDrive%\Users\Public
{Drive}:\$Recycle.Bin C:\Recycle.Bin C:\$Recycle.Bin
 %SystemDrive% C: C:
 %SystemRoot% The Windows directory, usually C:\Windows, formerly C:\WINNT  %SystemDrive%\Windows
 %TEMP% and %TMP%  %SystemDrive%\Documents and Settings\{username}\Local Settings\Temp  %SystemDrive%\Users\{username}\AppData\Local\Temp
 %USERDOMAIN% {userdomain} {userdomain}
 %USERNAME% {username} {username}
 %USERPROFILE%  %SystemDrive%\Documents and Settings\{username}  %SystemDrive%\Users\{username}
 %windir%  %SystemDrive%\WINDOWS  %SystemDrive%\WINDOWS

In this list, there is no environment variable that refers to the location of the user's My Documents directory, so there is no standard method for setting a program's home directory to be the My Documents directory.

Cautions against overuse[edit]

Some critics warn against overuse of environment variables, because of differences between shell languages, that they are ephemeral and easy to overlook, are specific to a user and not to a program. The recommended alternative is configuration files.[1][2]

See also[edit]

References[edit]

  1. ^ http://peterlyons.com/problog/2010/02/environment-variables-considered-harmful
  2. ^ http://www.insectnation.org/howto/environment-variables-considered-harmful

External links[edit]