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 . Locale support is controlled by the LANG and LC_xxx environment variables. For a more detailed description see . 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 . 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 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.