ENVIRON(7)                 Linux Programmer's Manual                ENVIRON(7)

       environ - user environment

       extern char **environ;

       The  variable  environ points to an array of pointers to strings called
       the "environment".  The last pointer in this array has the value  NULL.
       (This variable must be declared in the user program, but is declared in
       the header file <unistd.h> if the _GNU_SOURCE  feature  test  macro  is
       defined.)   This  array  of strings is made available to the process by
       the exec(3) call that started the process.

       By convention the strings in environ have the form "name=value".   Com-
       mon examples are:

       USER   The  name  of  the logged-in user (used by some BSD-derived pro-

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

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

       LANG   The name of a locale to use for locale categories when not over-
              ridden  by LC_ALL or more specific environment variables such as
              LC_TIME  (see locale(7) for further details of the LC_* environ-
              ment variables).

       PATH   The sequence of directory prefixes that  sh(1)  and  many  other
              programs  apply  in  searching for a file known by an incomplete
              pathname.  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,
              and so on)

       PWD    The current working directory.  Set by some shells.

       SHELL  The pathname of the user's login shell.

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

       PAGER  The user's preferred utility to display text files.

              The user's preferred utility to edit text files.

       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(3).  A C program can manipulate its environment  using  the  func-
       tions getenv(3), putenv(3), setenv(3), and unsetenv(3).

       Note  that the behavior of many programs and library routines is influ-
       enced by the presence or value of  certain  environment  variables.   A
       random collection:

       and so on influence locale handling; see  catopen(3),  gettext(3),  and

       TMPDIR  influences  the  path  prefix of names created by tmpnam(3) and
       other routines, and the temporary directory used by sort(1)  and  other

       LD_LIBRARY_PATH,  LD_PRELOAD  and  other  LD_*  variables influence the
       behavior of the dynamic loader/linker.

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

       The behavior of malloc(3) is influenced by MALLOC_* variables.

       The variable HOSTALIASES gives the name of a file containing aliases to
       be used with gethostbyname(3).

       TZ and TZDIR give timezone information used  by  tzset(3)  and  through
       that  by functions like ctime(3), localtime(3), mktime(3), strftime(3).
       See also tzselect(8).

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

       COLUMNS  and  LINES  tell  applications about the window size, possibly
       overriding the actual size.

       PRINTER or LPDEST may specify the desired printer to use.  See lpr(1).


       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

       There is also the risk of name space pollution.  Programs like make and
       autoconf allow overriding of default utility names from the environment
       with similarly named variables in all caps.  Thus one uses CC to select
       the  desired  C  compiler (and similarly MAKE, AR, AS, FC, LD, LEX, RM,
       YACC, etc.).  However, in some traditional  uses  such  an  environment
       variable  gives  options  for the program instead of a pathname.  Thus,
       one has MORE, LESS, and GZIP.  Such usage is considered  mistaken,  and
       to  be  avoided  in  new programs.  The authors of gzip should consider
       renaming their option to GZIP_OPT.

       env(1),  bash(1),  csh(1),  login(1),  printenv(1),   sh(1),   tcsh(1),
       execve(2),   clearenv(3),  exec(3),  getenv(3),  putenv(3),  setenv(3),
       unsetenv(3), locale(7), ld.so(8)

       This page is part of release 4.04 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

Linux                             2014-08-19                        ENVIRON(7)
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