environ - user environment


     extern char **environ;


     The variable environ points to an array  of  strings  called
     the  `environment'.   (This variable must be declared in the
     user program, but is declared in the header file unistd.h in
     case  the header files came from libc4 or libc5, and in case
     they came from glibc and  _GNU_SOURCE  was  defined.)   This
     array  of  strings  is  made available to the process by the
     exec(2) call that started the process.  By convention  these
     strings have the form `name=value'.  Common examples are:

     USER The name of the  logged-in  user  (used  by  some  BSD-
          derived programs).

          The name of the logged-in user (used by  some  System-V
          derived programs).

     HOME A user's login directory,  set  by  login(1)  from  the
          password file passwd(5).

     LANG The name of a locale to use for locale categories  when
          not  overridden  by LC_ALL or more specific environment

     PATH The sequence of directory prefixes that sh(1) and  many
          other  programs  apply in searching for a file known by
          an incomplete path name.  The prefixes are separated by
          `:'.   (Similarly one has CDPATH used by some shells to
          find the target of a change directory command,  MANPATH
          used by man(1) to find manual pages, etc.)

     PWD  The current working directory. Set by some shells.

          The file name of the user's login shell.

     TERM The terminal type for which output is to be prepared.

     Further names may be placed in the environment by the export
     command  and `name=value' in sh(1), or by the setenv command
     if you use csh(1).  Arguments may  also  be  placed  in  the
     environment  at  the  point  of an exec(2).  A C program can
     manipulate its environment  using  the  functions  getenv(),
     putenv(), setenv() and unsetenv().

     Note  that  the  behaviour  of  many  programs  and  library
     routines  is  influenced by the presence or value of certain
     environment variables.  A random collection:

     The variables  LANG,  LANGUAGE,  NLSPATH,  LOCPATH,  LC_ALL,
     LC_MESSAGES etc. influence locale handling.

     TMPDIR influences  the  path  prefix  of  names  created  by
     tmpnam() and other routines, the temporary directory used by
     sort(1) and other programs, etc.

     LD_LIBRARY_PATH, LD_PRELOAD and other LD_* variables  influ-
     ence the behaviour of the dynamic loader/linker.

     POSIXLY_CORRECT makes certain programs and library  routines
     follow the prescriptions of POSIX.

     The behaviour of malloc() is influenced  by  MALLOC_*  vari-

     The variable HOSTALIASES gives the name of a file containing
     aliases to be used with gethostbyname().

     TZ and TZDIR give time zone information.

     TERMCAP gives information on how to address a given terminal
     (or gives the name of a file containing such information).

     Etc. etc.

     Clearly there is a security risk here. Many a system command
     has  been  tricked  into  mischief  by  a user who specified
     unusual values for IFS or LD_LIBRARY_PATH.


     login(1),  sh(1),   bash(1),   csh(1),   tcsh(1),   exec(2),
     getenv(3), putenv(3), setenv(3), unsetenv(3).