Environment Variables
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## Environment Variables

### Overview

All Windows environment variables are imported when Cygwin starts. Apart from that, you may wish to specify settings of several important environment variables that affect Cygwin's operation.

The `CYGWIN` variable is used to configure a few global settings for the Cygwin runtime system. Typically you can leave `CYGWIN` unset, but if you want to set one ore more options, you can set it using a syntax like this, depending on the shell in which you're setting it. Here is an example in CMD syntax:

    C:\> set CYGWIN=error_start:C:\cygwin\bin\gdb.exe glob

This is, of course, just an example. For the recognized settings of the `CYGWIN` environment variable, see the section called “The `CYGWIN` environment variable”.

Locale support is controlled by the `LANG` and `LC_xxx` environment variables. For a more detailed description see the section called “Internationalization”.

The `PATH` environment variable is used by Cygwin applications as a list of directories to search for executable files to run. This environment variable is converted from Windows format (e.g. `C:\Windows\system32;C:\Windows`) to UNIX format (e.g., `/cygdrive/c/Windows/system32:/cygdrive/c/Windows`) when a Cygwin process first starts. Set it so that it contains at least the `x:\cygwin\bin` directory where "`x:\cygwin` is the "root" of your cygwin installation if you wish to use cygwin tools outside of bash. This is usually done by the batch file you're starting your shell with.

The `HOME` environment variable is used by many programs to determine the location of your home directory. This environment variable, if it exists, is converted from Windows format when a Cygwin process first starts. However, it's usually set in the shell profile scripts in the /etc directory, and it's not recommended to set the variable in your Windows environment.

The `TERM` environment variable specifies your terminal type. It is automatically set to `cygwin` if you have not set it to something else.

The `LD_LIBRARY_PATH` environment variable is used by the Cygwin function `dlopen ()` as a list of directories to search for .dll files to load. This environment variable is converted from Windows format to UNIX format when a Cygwin process first starts. Most Cygwin applications do not make use of the `dlopen ()` call and do not need this variable.

The `GMON_OUT_PREFIX` environment variable is helpful in some situations when profiling Cygwin programs. For more information, see the section called “Profiling programs that fork”. Only software developers with a need to profile their programs have a use for this variable.

In addition to `PATH`, `HOME`, `LD_LIBRARY_PATH`, and `GMON_OUT_PREFIX`, there are three other environment variables which, if they exist in the Windows environment, are converted to UNIX format: `TMPDIR`, `TMP`, and `TEMP`. The first is not set by default in the Windows environment but the other two are, and they point to the default Windows temporary directory. If set, these variables will be used by some Cygwin applications, possibly with unexpected results. You may therefore want to unset them by adding the following two lines to your `~/.bashrc` file:

    unset TMP
    unset TEMP

This is done in the default `~/.bashrc` file. Alternatively, you could set `TMP` and `TEMP` to point to `/tmp` or to any other temporary directory of your choice. For example:

    export TMP=/tmp
    export TEMP=/tmp

### Restricted Win32 environment

There is a restriction when calling Win32 API functions which require a fully set up application environment. Cygwin maintains its own environment in POSIX style. The Win32 environment is usually stripped to a bare minimum and not at all kept in sync with the Cygwin POSIX environment.

If you need the full Win32 environment set up in a Cygwin process, you have to call

    #include <sys/cygwin.h>

    cygwin_internal (CW_SYNC_WINENV);

to synchronize the Win32 environment with the Cygwin environment. Note that this only synchronizes the Win32 environment once with the Cygwin environment. Later changes using the `setenv` or `putenv` calls are not reflected in the Win32 environment. In these cases, you have to call the aforementioned `cygwin_internal` call again.

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