Environment variable

In all Unix and Unix-like systems, as well as on Windows, each process has its own separate set of environment variables. By default, when a process is created, it inherits a duplicate run-time 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. A running program can access the values of environment variables for configuration purposes. 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, non-exported variables are preferred for this as they do not leak outside the process. 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. Similarly, changing or removing a variable's value inside a DOS or Windows batch file will change the variable for the duration of COMMAND.COMor CMD.EXE's existence, respectively. In Unix, the environment variables are normally initialized during system startup by the system init startup 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. In general, the collection of environment variables function as an associative array where both the keys and values are strings. The interpretation of characters in either string differs among systems. When data structures such as lists need to be represented, it is common to use a colon (common on Unix and Unix-like) or semicolon-delineated (common on Windows and DOS) list. == Syntax ==

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. By convention, names of environment variables are normally expressed in all capital letters. This helps keep environment variables distinctly different from other variables and identifiers used in programming codes. Nevertheless, note that case sensitivity in environment variable names differs between operating systems. That is, Unix-like operating systems are case-sensitive with respect to environment variable names, while DOS, OS/2, and Windows are not case-sensitive. Unix In most Unix and Unix-like command-line shells, an environment variable's value is retrieved by placing a $ sign before the variable's name. If necessary, the name can also be surrounded by braces. To display the user home directory, the user may type: echo $HOME In Unix and Unix-like systems, the names of environment variables are case-sensitive. The command env displays all environment variables and their values. The command printenv can also be used to print a single variable by giving that variable name as the sole argument to the command. DOS, OS/2 and Windows In DOS, OS/2 and Windows command-line interpreters such as COMMAND.COM and CMD.EXE, an environment variable is retrieved by placing a % sign before and after it. In DOS, OS/2 and Windows command-line interpreters as well as their API, upper or lower case is not distinguished for environment variable names. The environment variable named HOMEDRIVE contains the drive letter (plus its trailing : colon) of the user's home directory, whilst HOMEPATH contains the full path of the user's home directory within that drive. So to see the home drive and path, the user may type this: ECHO %HOMEDRIVE%%HOMEPATH% The command SET (with no arguments) displays all environment variables and their values. In Windows NT and later set can also be used to print all variables whose name begins with a given prefix by giving the prefix as the sole argument to the command. In Windows PowerShell, the user may type the following: "$Env:HomeDrive$Env:HomePath" or one of the following redundant equivalents: Write-Output "$Env:HomeDrive$Env:HomePath" echo "$Env:HomeDrive$Env:HomePath" write "$Env:HomeDrive$Env:HomePath" return "$Env:HomeDrive$Env:HomePath" Write-Host "$Env:HomeDrive$Env:HomePath" In PowerShell, upper or lower case is not distinguished for environment variable names. The following command displays all environment variables and their values: Get-ChildItem env: Assignment: Unix The commands env and set can be used to set environment variables and are often incorporated directly into the shell. The following commands can also be used, but are often dependent on a certain shell. VARIABLE=value # (there must be no spaces around the equals sign) export VARIABLE # for Bourne and related shells export VARIABLE=value # for ksh, 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 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 are unknown to all other processes. The printenv command will not display them, and child processes do not inherit them. VARIABLE=value The prefix syntax exports a "true" environment variable to a child process without affecting the current process: putenv("VARIABLE_NAME"="VALUE"); == Examples ==

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 (for roaming application settings shared across multiple devices), LOCALAPPDATA (for local application settings) or PROGRAMDATA (for application settings shared between multiple OS users) should be 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. == True environment variables ==

Unix ;$PATH: Contains a colon-separated list of directories that the shell searches for commands that do not contain a slash in their name (commands with slashes are interpreted as file names to execute, and the shell attempts to execute the files directly). It is equivalent to the DOS, OS/2 and Windows %PATH% variable. ;$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. ;$LIBPATH or $SHLIB_PATH: Alternatives to $LD_LIBRARY_PATH typically used on older Unix versions. ;$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 time zone itself or referencing a file (in /usr/share/zoneinfo). ;$BROWSER: Contains a colon-separated list of a user's web browser preferences, for use by programs that need to allow the user to view content at a URL. The browsers in the list are intended to be attempted from first to last, stopping after the first one that succeeds. This arrangement allows for fallback behavior in different environments, e.g., in an X11 environment, a graphical browser (such as Firefox) can be used, but in a console environment a terminal-base browser (such as Lynx) can be used. A %s token may be present to specify where the URL should be placed; otherwise the browser should be launched with the URL as the first argument. ;%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 local data files of Applications. Its uses include storing of desktop themes, Windows error reporting, caching and profiles of web browsers. ;%ComSpec%/%COMSPEC%:The %ComSpec% variable contains the full path to the command processor; on the Windows NT family of operating systems, this is cmd.exe, while on Windows 9x, %COMSPEC% is COMMAND.COM. ;%OS%:The %OS% variable contains a symbolic name of the operating system family to distinguish between differing feature sets in batchjobs. It resembles an identically named environment variable %OS% found in all DOS-related operating systems of Digital Research-origin like Concurrent DOS, Multiuser DOS, REAL/32, DOS Plus, DR DOS, Novell DOS and OpenDOS. %OS% always holds the string "Windows_NT" on the Windows NT family.). ;%CommonProgramFiles%, %CommonProgramFiles(x86)%, %CommonProgramW6432%: This variable points to the Common Files subdirectory of the Program Files directory. The default on English-language systems is "C:\Program Files\Common 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). ;%OneDrive%: The %OneDrive% variable is a special system-wide environment variable found on Windows NT and its derivatives. Its value is the path of where (if installed and setup) the Onedrive directory is located. The value of %OneDrive% is in most cases "C:\Users\{Username}\OneDrive\". ;%SystemDrive%: The %SystemDrive% variable is a special system-wide environment variable found on 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 the Windows NT family of operating systems. 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 XP and newer versions use "\WINDOWS". :* Windows 2000, NT 4.0 and NT 3.1 use "\WINNT". :* Windows NT 3.5 and NT 3.51 uses "\WINNT35". :* Windows NT 4.0 Terminal Server uses "\WTSRV". ;%windir%:This variable points to the Windows directory. (On the Windows NT family of operating systems, it is identical to the %SystemRoot% variable). Windows 95–98 and Windows ME are, by default, installed in "C:\Windows". For other versions of Windows, see the %SystemRoot% entry above. 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% since Windows Vista): 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 login 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 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 behavior of certain built-in console commands. These variables also do not need to be explicitly specified as command line arguments. Default values The following tables shows typical default values of certain environment variables under English versions of Windows as they can be retrieved under CMD. (Some of these variables are also defined when running COMMAND.COM under Windows, but differ in certain important details: Under COMMAND.COM, the names of environment variable are always uppercased. Some, but not all variables contain short 8.3 rather than long file names. While some variables present in the CMD environment are missing, there are also some variables specific to the COMMAND environment.) 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. ==Pseudo-environment variables==

The command processors in DOS and Windows also support pseudo-environment variables. These are values that are fetched like environment variables, but are not truly stored in the environment but computed when requested. DOS Besides true environment variables, which are statically stored in the environment until changed or deleted, a number of pseudo-environment variables exist for batch processing. The so-called replacement parameters or replaceable parameters (Microsoft / IBM terminology) aka replacement variables (Digital Research / Novell / Caldera terminology) or batch file parameters (JP Software terminology) %1..%9 and %0 can be used to retrieve the calling parameters of a batchjob, see SHIFT. In batchjobs, they can be retrieved just like environment variables, but are not actually stored in the environment. Some command-line processors (like DR-DOS COMMAND.COM, Multiuser DOS MDOS.COM/TMP.EXE (Terminal Message Process), JP Software 4DOS, 4OS2, Take Command (formerly 4NT) and Windows cmd.exe) support a type of pseudo-environment variables named system information variables (Novell / Caldera terminology) or internal variables (JP Software terminology), which can be used to retrieve various possibly dynamic, but read-only information about the running system in batch jobs. The returned values represent the status of the system in the moment these variables are queried; that is, reading them multiple times in a row may return different values even within the same command; querying them has no direct effect on the system. Since they are not stored in the environment, they are not listed by SET and do not exist for external programs to retrieve. If a true environment variable of the same name is defined, it takes precedence over the corresponding variable until the environment variable is deleted again. They are not case-sensitive. While almost all such variables are prefixed with an underscore ("_") by 4DOS etc. by convention (f.e. %_SECOND%), they are not under DR-DOS COMMAND.COM (f.e. %OS_VERSION%). In addition, 4DOS, 4OS2, 4NT, and Take Command also support so called variable functions, including user-definable ones. They work just like internal variables, but can take optional parameters (f.e. %@EVAL[]%) and may even change the system status depending on their function. System information variables supported by DR-DOS COMMAND.COM: ;%AM_PM%: This pseudo-variable returns the ante- or post-midday status of the current time. The returned string depends on the locale-specific version of DR-DOS, f.e. "am" or "pm" in the English version. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%DAY%: This pseudo-variable returns the days of the current date in a 2-digit format with leading zeros, f.e. "01".."31". See also the similar pseudo-variable %_DAY%. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%DAY_OF_WEEK%: This pseudo-variable returns the day name of the week in a 3-character format. The returned string depends on the locale-specific version of DR-DOS, f.e. "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", or "Sat" in the English version. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%ERRORLEVEL%: In COMMAND.COM of DR-DOS 7.02 and higher, this pseudo-variable returns the last error level returned by an external program or the RETURN command, f.e. "0".."255". See also the identically named pseudo-variable %ERRORLEVEL% under Windows and the IF ERRORLEVEL conditional command. ;%ERRORLVL%: In DR-DOS 7.02 and higher, this pseudo-variable returns the last error level in a 3-digit format with leading zeros, f.e. "000".."255". Under Multiuser DOS, this is a true environment variable automatically updated by the shell to the return code of exiting programs. See also the related pseudo-variable %ERRORLEVEL% under DR-DOS and the IF ERRORLEVEL command. ;%GREETING_TIME%: This pseudo-variable returns the 3-level day greeting time. The returned string depends on the locale-specific version of DR-DOS, f.e. "morning", "afternoon", or "evening" in the English version. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%HOUR%: This pseudo-variable returns the hours of the current time in 12-hour format without leading zeros, f.e. "1".."12". It resembles an identically named identifier variable in Novell NetWare login scripts. ;%HOUR24%: This pseudo-variable returns the hours of the current time in 24-hour format in a 2-digit format with leading zeros, f.e. "00".."23". It resembles an identically named identifier variable in Novell NetWare login scripts. See also the similar pseudo-variable %_HOUR%. ;%MINUTE%: This pseudo-variable returns the minutes of the current time in a 2-digit format with leading zeros, f.e "00".."59". It resembles an identically named identifier variable in Novell NetWare login scripts. See also the similar pseudo-variable %_MINUTE%. ;%MONTH%: This pseudo-variable returns the months of the current date in a 2-digit format with leading zeros, f.e. "01".."12". It resembles an identically named identifier variable in Novell NetWare login scripts. See also the similar pseudo-variable %_MONTH%. ;%MONTH_NAME%: This pseudo-variable returns the month name of the current date. The returned string depends on the locale-specific version of DR-DOS, f.e. "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", or "December" in the English version. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%NDAY_OF_WEEK%: This pseudo-variable returns the number of day of the current week, f.e. "1".."7" (with "1" for Sunday). It resembles an identically named identifier variable in Novell NetWare login scripts. ;%OS_VERSION%: This pseudo-variable returns the version of the operating system depending on the current setting of the environment variable %VER%. If %VER% is not defined, %OS_VERSION% returns "off". It resembles an identically named identifier variable in Novell NetWare login scripts, which may return versions also for non-DR-DOS versions of DOS. ;%SECOND%: This pseudo-variable returns the seconds of the current time in a 2-digit format with leading zeros, f.e. "00".."59". It resembles an identically named identifier variable in Novell NetWare login scripts. See also the similar pseudo-variable %_SECOND%. ;%SHORT_YEAR%: This pseudo-variable returns the year of the current date in a 2-digit format with leading zeros, f.e. "93".."99", "00".."92". It resembles an identically named identifier variable in Novell NetWare login scripts. ;%YEAR% and %_YEAR%: Supported since Novell DOS 7, the %YEAR% pseudo-variable returns the year of the current date in a 4-digit format, f.e. "1980".."2099". It resembles an identically named identifier variable in Novell NetWare login scripts. DR-DOS 7.02 and higher added %_YEAR% for compatibility with 4DOS, returning the same value. ;%/%: In COMMAND.COM of DR-DOS 7.02 and higher, this pseudo-variable returns the current SwitChar setting of the system, either "/" (DOS style) or "-" (Unix style). See also the related CONFIG.SYS directive SWITCHAR and the environment variable %SWITCHAR%. ;%_CODEPAGE%: This pseudo-variable returns the systems' current code page ("1".."65533"), f.e. "437", "850", "858". This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the CHCP command. ;%_COLUMNS%: This pseudo-variable returns the current number of screen columns depending on the display mode, f.e. "40", "80", "132", etc. This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also a similar environment variable %$WIDTH% under DOS Plus. ;%_COUNTRY%: This pseudo-variable returns the systems' current country code ("1".."65534"), f.e. "1" for USA, "44" for UK, "49" for Germany, "20049" with ISO 8601, "21049"22049" --> with ISO 8601 and Euro support. This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the CONFIG.SYS directive COUNTRY. ;%_DAY%: This pseudo-variable returns the days of the current date without leading zeros, f.e. "1".."31". This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the similar pseudo-variable %DAY%. ;%_HOUR%: This pseudo-variable returns the hours of the current time in 24-hour format without leading zeros, f.e. "0".."23". This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the similar pseudo-variable %HOUR24%. ;%_MINUTE%: This pseudo-variable returns the minutes of the current time without leading zeros, f.e "0".."59". This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the similar pseudo-variable %MINUTE%. ;%_MONTH%: This pseudo-variable returns the months of the current date without leading zeros, f.e. "1".."12". This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the similar pseudo-variable %MONTH%. ;%_ROWS%: This pseudo-variable returns the current number of screen rows depending on the display mode, f.e. "25", "43", "50", etc. This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See a similar environment variable %$LENGTH% under DOS Plus. ;%_SECOND%: This pseudo-variable returns the seconds of the current time without leading zeros, f.e. "0".."59". This variable was originally introduced by 4DOS, but also became available with COMMAND.COM since DR-DOS 7.02. See also the similar pseudo-variable %SECOND%. System information variables supported by DR-DOS COMMAND.COM with networking loaded: ;%LOGIN_NAME%: This pseudo-variable returns the user name. This always worked with NETX, but it will also work with Personal NetWare's ODI/VLM if the current drive is a PNW-mapped drive (otherwise an empty string is returned). See also the similarly named environment variable %LOGINNAME%. ;%P_STATION%: This pseudo-variable returns the physical station number in a format "????????????". The value depends on the MAC address of the network adapter, but can be overridden. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%STATION%: This pseudo-variable returns the logical station number starting with "1" for the first client. The numbers are assigned by the file server and remain static for as long as the IPX connection remains established. It resembles an identically named identifier variable in Novell NetWare login scripts. ;%FULL_NAME%: This pseudo-variable returns the full name of the logged in user, if available. It resembles an identically named identifier variable in Novell NetWare login scripts. See also the related pseudo-variable %LOGIN_NAME%. Windows Dynamic environment variables (also named internal variables or system information variables under DOS) are pseudo-environment variables supported by CMD.EXE when command-line extensions are enabled, and they expand to various discrete values whenever queried, that is, their values can change when queried multiple times even within the same command. While they can be used in batch jobs and at the prompt, they are not stored in the environment. Consequently, they are neither listed by SET nor do they exist for external programs to read. They are not case-sensitive. Indirectly, they are also supported under Windows' COMMAND.COM, which has been modified to internally call CMD.EXE to execute the commands. ;%CD%: This pseudo-variable expands to the current directory equivalent to the output of the command CD when called without arguments. While a long filename can be returned under CMD.EXE depending on the current directory, the fact that the current directory will always be in 8.3 format under COMMAND.COM will cause it to return a short filename under COMMAND.COM, even when COMMAND internally calls CMD. ;%CMDCMDLINE%: This pseudo-variable expands to the original startup parameters of CMD.EXE, f.e. "C:\Windows\system32\cmd.exe". Under Windows' COMMAND.COM, this may return something like "C:\Windows\system32\cmd.exe /c ..." due to the fact that COMMAND.COM calls CMD.EXE internally. ;%CMDEXTVERSION%: This pseudo-variable expands to the version of the command-line extensions of CMD.EXE, if enabled (e.g. "1" under Windows NT, "2" under Windows 2000 and Windows XP). ;%DATE%: This pseudo-variable expands to the current date. The date is displayed according to the current user's date format preferences. ;%ERRORLEVEL%: This pseudo-variable expands to the last set error level, a value between "0" and "255" (without leading zeros). External commands and some internal commands set error levels upon execution. See also the identically named pseudo-variable %ERRORLEVEL% under DR-DOS and the IF ERRORLEVEL command. ;%HIGHESTNUMANODENUMBER%: This pseudo-variable returns the number of the highest NUMA node. ;%RANDOM%: This pseudo-variable returns a random number between "0" and "32767". ;%TIME%: This pseudo-variable returns the current time. The time is displayed according to the current user's time format preferences. If the %TIME% and %DATE% variables are both used, it is important to read them both in this particular order in rapid succession in order to avoid midnight-rollover problems. Other shells Unix-like shells have similar dynamically generated variables, bash's $RANDOM being a well-known example. However, since these shells have a concept of local variables, they are described as special local variables instead. == See also ==