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       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(3) call that started the process.  By convention
       these strings have the form `name=value'.  Common examples

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

              The name of the logged-in user (used by  some  Sys­
              tem-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 variables  like  LC_COLLATE,  LC_CTYPE,

       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.

       SHELL  The file name of the user's login shell.

       TERM   The  terminal  type  for which output is to be pre­

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

              The user's preferred utility to edit text files.

              The   user's  preferred  utility  to  browse  URLs.

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

       TMPDIR influences the path prefix of names created by tmp­
       nam(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 behaviour of the dynamic loader/linker.

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

       The behaviour of malloc(3) is influenced by MALLOC_* vari­

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

       TZ and TZDIR give time zone information used  by  tzset(3)
       and  through  that by functions like ctime(), localtime(),
       mktime(), strftime().  See also tzselect(1).

       TERMCAP gives information on how to address a given termi­
       nal  (or gives the name of a file containing such informa­

       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  com­
       mand  has  been tricked into mischief by a user who speci­
       fied unusual values for IFS or LD_LIBRARY_PATH.

       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.
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