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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> 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 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 like LC_COLLATE, LC_CTYPE, LC_MESSAGES,  LC_MONETARY,  LC_NUMERIC,  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, 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.

       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:

       The variables LANG, LANGUAGE, NLSPATH, LOCPATH, LC_ALL, LC_MESSAGES, 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).

       Etc.

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

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.