Environment variable - Wikipedia, the free encyclopedia. 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 1. Version 7 Unix, so are included in all Unixoperating system flavors and variants from that point onward including Linux and OS X. From PC DOS 2. 0 in 1. 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. 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. After seeing reports of Nokia Video Manager wiping out the Set Your Default Programs list in Windows. Default Programs list is. This topic discusses the Set Program Access and Computer Defaults. Set Program Access and Computer Defaults (SPAD). All Unixoperating 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. Examples of environment variables include: PATH . When the user types a command into a system terminal 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) . Information Technology Services ITS Home. Change the Default Program for PDF Files. Click the Start button on your Windows 7 desktop and select Default Programs. Default Programs Editor makes it easy to fix file association. Autoplay settings & Upgrade for 'File Types' options from Windows XP. 27 Comments on 'To Change the Default E-mail Program in Windows'. Also from control Panel i entered default program -set default-choose outlook(desktop). Many applications incorrectly use USERPROFILE for application settings in Windows . 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 $LD. If a setuid program did this, it would be insecure, because its caller could get it to run arbitrary code and hence misuse its authority. For this reason, libc unsets these environment variables at startup in a setuid process. They are usually referenced by putting special symbols in front of or around the variable name. For instance, to display the user home directory, in most scripting environments, the user has to type: In DOS, OS/2 and Windows command- line interpreters such as COMMAND. COM and cmd. exe, the user has to type this: In Windows Power. Shell, the user has to type this: Write- Output$env: HOMEPATHThe commands env, set, and printenv display all environment variables and their values. 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. It unsets a shell variable, removing it from memory and the shell's exported environment. It is implemented as a shell builtin, because it directly manipulates the internals of the shell. If one tries to unset a read- only variable, the unset command will print an error message and return a non- zero exit code. DOS, OS/2 and Windows. It is equivalent to the DOS, OS/2 and Windows%PATH% variable. HOMEContains 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. PWDThis variable points to the current directory. Equivalent to the output of the command pwd when called without arguments. DISPLAYContains the identifier for the display that X1. LANG is used to set to the default locale. For example, if the locale values are pt. Different aspects of localization are controlled by individual $LC. TZRefers to time zone. It can be in several formats, either specifying the timezone itself or referencing a file (in /usr/share/zoneinfo). Under DOS the master environment is provided by the primary command processor, which inherits the pre- environment defined in CONFIG. SYS when first loaded. Its size can be configured through the COMMAND /E: n parameter between 1. Local environment segments inherited to child processes are typically reduced down to the size of the contents they hold. Some command- line processors (like 4. DOS) allow to define a minimum amount of free environment space that will be available when launching secondary shells. While the content of environment variables remains unchanged upon storage, their names (without the . Environment variable names containing lowercase letters are stored in the environment just like normal environment variables, but remain invisible to most DOS software, since they are written to expect uppercase variables only. DR- DOS COMMAND. COM supports environment variables up to 2. DOS even up to 5. It is usually changed via the APPEND /E command, which also ensures that the directory names are converted into uppercase. Some DOS software actually expects the names to be stored in uppercase and the length of the list not to exceed 1. Long filenames containing spaces or other special characters must not be quoted (. It is set by the DOS BIOS (IO. SYS, IBMBIO. COM, etc.) to the name defined by the corresponding CONFIG. SYS directive MENUITEM before launching the primary command processor. Its main purpose is to allow further special cases in AUTOEXEC. BAT and similar batchjobs depending on the selected option at boot time. This can be emulated under DR- DOS by utilizing the CONFIG. SYS directive SET like SET CONFIG=1.%CMDLINE% (introduced with 4. DOS, also supported since MS- DOS 7. This variable contains the fully expanded text of the currently executing command line. It can be read by applications to detect the usage of and retrieve long command lines, since the traditional method to retrieve the command line arguments through the PSP (or related API functions) is limited to 1. FCBs get expanded or the default DTA is used. While 4. DOS supports longer command lines, COMMAND. COM still only supports a maximum of 1. U: 1. 28. 2. 55 to specify the size of the command line buffer), but nevertheless internal command lines can become longer through f. In addition to the command- line length byte in the PSP, the PSP command line is normally limited by ASCII- 1. ASCII- 1. 3 inserted at position 1. The variable will be suppressed for external commands invoked with a preceding @- symbol like in @XCOPY .. Some beta versions of Microsoft Chicago used %CMDLINE% to store only the remainder of the command line excessing 1. It must not contain long filenames, but under DR- DOS it may contain file and directory passwords. It is set up by the primary command processor to point to itself (typically reflecting the settings of the CONFIG. SYS directive SHELL), so that the resident portion of the command processor can reload its transient portion from disk after the execution of larger programs. The value can be changed at runtime to reflect changes in the configuration, which would require the command processor to reload itself from other locations. The variable is also used when launching secondary shells.%COPYCMD% (supported since MS- DOS 6. PC DOS 6. 3)Allows a user to specify the /Y switch (to assume . 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% (supported since MS- DOS 5. PC DOS 5. 0)Allows a user to specify customized default parameters for the DIR command, including file specifications. 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 overridden by using DIR /- W at the command line.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
January 2017
Categories |