xenial (7) environ.7.gz

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.

       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.

       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, and so on influence locale handling;
       see catopen(3), gettext(3), and locale(7).

       TMPDIR influences the path prefix of names created by tmpnam(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).

       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

       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)

COLOPHON

       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
       http://www.kernel.org/doc/man-pages/.