Environment variable

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:

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:

VARIABLE=value program_name [arguments]

The persistence of an environment variable can be session-wide or system-wide.

unset is a builtin command implemented by both the Bourne shell family (sh, ksh, bash, etc.) and the C shell family (csh, tcsh, etc.) of Unix command line shells. 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. Read-only shell variables cannot be unset. If one tries to unset a read-only variable, the unset command will print an error message and return a non-zero exit code.

In DOS, OS/2 and Windows command-line interpreters such as COMMAND.COM and CMD.EXE, the SET command is used to assign environment variables and values using the following arguments:

In PowerShell, the assignment follows a syntax similar to Unix:

and 32767 bytes. 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. Some command processors limit the maximum length of a variable name to 80 characters. While principally only limited by the size of the ,