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

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".  Com-
     mon examples are:

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

     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 over-
            ridden 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_*  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.

     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 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:

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), 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

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.