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ENVIRON(7)                 Linux Programmer's Manual                ENVIRON(7)

       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 programs).

              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 envi-
              ronment  variables  like  LC_COLLATE,  LC_CTYPE,  LC_MESSAGES,  LC_MONETARY,  LC_NUMERIC,  LC_TIME,  cf.

       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, etc.)

       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

       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.

       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),  local-
       time(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

       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 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.

       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)

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

Linux                             2009-07-25                        ENVIRON(7)