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NAME

       environ - user environment

SYNOPSIS

       extern char **environ;

DESCRIPTION

       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.   When  a  child
       process is created via fork(2), it inherits a copy of its parent's environment.

       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 programs).

       LOGNAME
              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 variables such as LC_COLLATE,  LC_CTYPE,  LC_MESSAGES,  LC_MONETARY,  LC_NUMERIC,  and
              LC_TIME (see locale(7) for further details of the LC_* environment 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.

       EDITOR/VISUAL
              The user's preferred utility to edit text files.

       Names may be placed in the shell's environment by the export command in sh(1), or by the  setenv  command
       if you use csh(1).

       The  initial  environment  of  the  shell  is  populated  in  various  ways,  such  as  definitions  from
       /etc/environment that are processed by pam_env(8) for all users at login time  (on  systems  that  employ
       pam(8)).   In addition, various shell initialization scripts, such as the system-wide /etc/profile script
       and per-user initializations script may include commands that add variables to the  shell's  environment;
       see the manual page of your preferred shell for details.

       Bourne-style shells support the syntax

           NAME=value command

       to  create  an  environment  variable  definition only in the scope of the process that executes command.
       Multiple variable definitions, separated by white space, may precede command.

       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.  Examples include the following:

       *  The variables LANG, LANGUAGE, NLSPATH, LOCPATH,  LC_ALL,  LC_MESSAGES,  and  so  on  influence  locale
          handling; see catopen(3), gettext(3), and locale(7).

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

       *  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).

NOTES

       The  prctl(2) PR_SET_MM_ENV_START and PR_SET_MM_ENV_END operations can be used to control the location of
       the process's environment.

BUGS

       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.

       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.

SEE ALSO

       bash(1),  csh(1),  env(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), pam_env(8)

COLOPHON

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