Environment variables are a set of dynamic named values that can affect the way running processes will behave on a computer.
They can be said in some sense to create the operating environment in which a process runs. For example, an environment variable with a standard name can store the location that a particular computer system uses to store temporary files—this may vary from one computer system to another. A process which invokes the environment variable by (standard) name can be sure that it is storing temporary information in a directory that exists and is expected to have sufficient space.
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In all Unix and Unix-like systems, each process has its own private 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 API level, these changes must be done between fork and exec. Alternatively, from shells such as bash, you 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, MS-DOS, and Microsoft Windows have environment variables; however, they do not all use the same variable names. Running programs can access the values of environment variables for configuration purposes.
Examples of environment variables include:
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 the script, although 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 shell they are using. In Microsoft Windows, environment variables defaults are stored in the windows registry or set in autoexec.bat.
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 program search path, in most scripting environments, the user has to type:
echo $PATH
On DOS or Windows systems, the user has to type this:
echo %PATH%
The commands env, set, and printenv display all environment variables and their values. 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 BY INSTALLING, achieve their effect.
Environment variables are local to the process in which they were set. That means if we open two terminal windows (Two different processes running shell) and change value of environment variable in one window, that change will not be seen by other window.
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.
Environment variables persistence can be session-wide or system-wide.
In DOS and Windows, the set command without any arguments displays all environment variables along with their values.
To set a variable to a particular value, use:
set VARIABLE=value
However, this is temporary. Permanent change to the environment variable can be achieved through editing the registry (not recommended for novices) and using the Windows Resource Kit application setx.exe. With the introduction of Windows Vista, the setx command became part of Windows.
Users of the Windows GUI can manipulate variables via <Control Panel:System:Advanced:Environment Variables>; through the Windows Registry this is done changing the values under HKCU\Environment (for user specific variables) and HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment (for System variables).
To set a variable whose name is in another variable, you can do:
set %VARNAME%=value
This feature allows certain interesting applications. For example, you may create a uni-dimensional array of elements (vector) this way:
set VECTOR[%I%]=value of element subscript %I% :MkVec set VECNAME=%1 set i=0 :loop shift if "%1" == "" goto exitloop set /a i+=1 set %VECNAME%[%i%]=%1 goto loop :exitloop exit /B %i% call :MkVec DOWNAME=Monday Tuesday Wednesday Thursday Friday Saturday Sunday
To see the current value of a particular variable, use:
echo %VARIABLE%
or
set VARIABLE
Note: Please take note that doing so will print out all variables beginning with 'VARIABLE'. Another example is:
C:\> set p
Path=c:\.. ..
PATHEXT=.COM;.EXE;.BAT;
PROCESSOR_ARCHITECTURE=.. ..
PROCESSOR_IDENTIFIER=x8..
PROCESSOR_LEVEL=6..
PROCESSOR_REVISION=1706..
ProgramFiles=C:\Program.. .
PROMPT=$P$G
To see the value of an array element a double expansion is required: one for the subscript value and an additional expansion for the array element. This may be achieved via Delayed !VARIABLE! Expansion this way:
set DOW=value of Day of Week (1..7) echo !DOWNAME[%DOW%]!
To delete a variable, the following command is used:
set VARIABLE=
In Windows the names of environment variables are case insensitive.
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]
The tool that gives closest parallel in Windows is the SETLOCAL/ENDLOCAL commands that prevent variables from being set globally.
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.
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). Equivalent to the Windows %PATH% variable. See Path (computing)
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.
This variable points to the current directory. Equivalent to the output of the command pwd when called without arguments.
Contains the identifier for the display that X11 programs should use by default.
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 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.
Refers to Time zone. It can be in several formats, either specifying the timezone itself or referencing a file (in /usr/share/zoneinfo).
This variable contains the full path to the command processor, command.com.
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). Note that %PATH% can also be set like this PATH=c:\dos; where SET isn't required.
These variables contain the path to the directory where temporary files should be stored.
These 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.
This variable points to the current directory. Equivalent to the output of the command cd when called without arguments.
This variable expands to the current date. The date is displayed according to the current user's date format preferences.
The following is a way of reformatting the date and time for use in file copies. The example assumes UK format of day month year and the time is set for a 24 hour clock.
@echo off echo %DATE% %TIME% for /F "tokens=1-3 delims=/" %%a in ("%DATE%") do set MTH=%%a& set DAY=%%b& set YR=%%c for /F "tokens=1-3 delims=:." %%a in ("%TIME%") do set HR=%%a& set MIN=%%b& set SEC=%%c if "%HR:~0,1%"==" " set HR=0%HR:~1,1% set MYDATE=%YR%%MTH%%DAY%-%HR%%MIN%%SEC% echo %MYDATE%
This variable points to the current error level. If there was an error in the previous command, this is what you need to check against to find out about that.
This variable returns a random number between 0 and 32767
This variable points to the current time. The time is displayed according to the current user's time format preferences.
These variables refer to locations of critical operating system resources, and as such generally are not user-dependent.
Contains the full path to the Application Data folder of the logged-in user. Does not work on Windows NT 4.0 SP6 UK.
This variable is the temporary files of Applications. Its uses include storing of Desktop Themes, Windows Error Reporting, Caching and profiles of web browsers.
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.
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. Equivalent to the Unix $PATH variable.
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).
This variable points to Common Files directory. The default is C:\Program Files\Common Files.
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 folder was placed. Also see next item.
The value of %SystemDrive% is in most cases C:.
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 folder, 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, on a clean installation:
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:
Note that Windows NT 4 Terminal Server Edition by default installs to C:\WTSRV.
These 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.
The %AllUsersProfile%(%PROGRAMDATA%) 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 folders will appear in every user's Start menu or Desktop, respectively.
The variable holds 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).
The %UserProfile% variable is 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.
In certain cases it is not possible to create file paths containing environment variables using the Windows GUI, and it is necessary to fight with the user interface to make things work as intended.
| Variable | Windows XP | Windows Vista/7 |
|---|---|---|
| %ALLUSERSPROFILE% and %PROGRAMDATA% | C:\Documents and Settings\All Users | C:\ProgramData |
| %APPDATA% | C:\Documents and Settings\{username}\Application Data | C:\Users\{username}\AppData\Roaming |
| %COMPUTERNAME% | {computername} | {computername} |
| %COMMONPROGRAMFILES% | C:\Program Files\Common Files | C:\Program Files\Common Files |
| %COMMONPROGRAMFILES(x86)% | C:\Program Files (x86)\Common Files | C:\Program Files (x86)\Common Files |
| %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 |
| %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 |
| {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} |
| %USERDATA% | %SystemDrive%\Documents and Settings\{username} | %SystemDrive%\Users\{username} |
| %USERNAME% | {username} | {username} |
| %USERPROFILE% | %SystemDrive%\Documents and Settings\{username} | %SystemDrive%\Users\{username} |
| %WINDIR% | C:\Windows | C:\Windows |
| %PUBLIC% | %SystemDrive%\Users\Public | |
| %PROGRAMDATA% | %SystemDrive%\ProgramData | |
| %PSModulePath% | %SystemRoot%\system32\WindowsPowerShell\v1.0\Modules\ |
In this list, there is no environment variable that refers to the location of the user's My Documents folder, so there is no standard method for setting a program's home directory to be the My Documents folder.