The search paths for header files and libraries can also be controlled through environment variables in the shell. These may be set automatically for each session using the appropriate login file, such as .bash_profile in the case of GNU Bash. Additional directories can be added to the include path using the environment variable C_INCLUDE_PATH (for C header files) or CPLUS_INCLUDE_PATH (for C++ header files). For example, the following commands will add /opt/gdbm-1.8.3/include to the include path when compiling C programs:
$ C_INCLUDE_PATH=/opt/gdbm-1.8.3/include $ export C_INCLUDE_PATH
and similarly for C++ programs:
$ CPLUS_INCLUDE_PATH=/opt/gdbm-1.8.3/include $ export CPLUS_INCLUDE_PATH
This directory will be searched after any
directories specified on the command line with the option -I,
and before the standard default directories (such as /usr/local/include
and /usr/include). The shell command export
is needed to
make the environment variable available to programs outside the shell
itself, such as the compiler—it is only needed once for each variable
in each shell session, and can also be set in the appropriate login
file.1
Similarly, additional directories can be added to the link path using the environment variable LIBRARY_PATH. For example, the following commands will add /opt/gdbm-1.8.3/lib to the link path:
$ LIBRARY_PATH=/opt/gdbm-1.8.3/lib $ export LIBRARY_PATH
This directory will be searched after any directories specified on the command line with the option -L, and before the standard default directories (such as /usr/local/lib and /usr/lib).
With the environment variable settings given above the program dbmain.c can be compiled without the -I and -L options,
$ gcc -Wall dbmain.c -lgdbm
because the default paths now use the directories specified in the
environment variables C_INCLUDE_PATH and LIBRARY_PATH. The
same compilation command with g++
would use the environment
variables CPLUS_INCLUDE_PATH and LIBRARY_PATH.