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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> 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".  Com-
       mon examples are:

       USER   The name of the logged-in user (used by  some  BSD-derived  pro-

	      The  name	 of  the logged-in user (used by some System-V derived

       HOME   A user's login directory, set by login(1) from the password file

       LANG   The name of a locale to use for locale categories when not over-
	      ridden by LC_ALL or more specific environment variables such  as
	      LC_TIME (see locale(7) for further details of the LC_*  environ-
	      ment 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.

	      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 addi-
       tion, 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,	 sepa-
       rated 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	 func-
       tions getenv(3), putenv(3), setenv(3), and unsetenv(3).

       Note  that the behavior of many programs and library routines is influ-
       enced by the presence or value of certain environment variables.	 Exam-
       ples include the following:

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

       *  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), strf-
	  time(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

       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.

       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

       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),  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)

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Linux				  2017-03-13			    ENVIRON(7)