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       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> 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(3)
       call that started the process.

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

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

              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
              overridden by LC_ALL or more specific environment variables like
              LC_TIME, cf.  locale(5).

       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,

       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
       functions getenv(3), putenv(3), setenv(3), and unsetenv(3).

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

       etc. influence locale handling, cf.  locale(5).

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

       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.


       bash(1), csh(1),  login(1),  sh(1),  tcsh(1),  execve(2),  clearenv(3),
       exec(3), getenv(3), putenv(3), setenv(3), unsetenv(3), locale(5)


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