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       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  array  of  strings  is  made
       available  to  the  process  by  the execve(2) call when a new program is started.  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".   The  name  is  case-
       sensitive  and  may  not contain the character "=".  The value can be anything that can be
       represented as a string.  The name and the value may not contain  an  embedded  null  byte
       ('\0'), since this is assumed to terminate the string.

       Environment  variables  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 addition, 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, separated by white  space,  may  precede

       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

       What  follows is a list of environment variables typically seen on a system.  This list is
       incomplete and includes only common variables seen by average users  in  their  day-to-day
       routine.   Environment  variables specific to a particular program or library function are
       documented in the ENVIRONMENT section of the appropriate manual page.

       USER   The name of the logged-in user (used by some BSD-derived programs).  Set  at  login
              time, see section NOTES below.

              The  name  of  the logged-in user (used by some System-V derived programs).  Set at
              login time, see section NOTES below.

       HOME   A user's login directory.  Set at login time, see section NOTES below.

       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 employ when
              searching for an executable file that is specified as a simple  filename  (i.a.,  a
              pathname that contains no slashes).  The prefixes are separated by colons (:).  The
              list of prefixes is searched from beginning to end, by checking the pathname formed
              by  concatenating  a  prefix,  a slash, and the filename, until a file with execute
              permission is found.

              As a legacy feature, a zero-length prefix (specified as two adjacent colons, or  an
              initial or terminating colon) is interpreted to mean the current working directory.
              However, use of this feature is deprecated,  and  POSIX  notes  that  a  conforming
              application  shall  use  an  explicit  pathname  (e.g.,  .)  to specify the current
              working directory.

              Analogously to PATH, 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  absolute  pathname  of the user's login shell.  Set at login time, see section
              NOTES below.

       TERM   The terminal type for which output is to be prepared.

       PAGER  The user's preferred utility to display text files.  Any  string  acceptable  as  a
              command-string operand to the sh -c command shall be valid.  If PAGER is null or is
              not set, then applications that launch a pager will default to a  program  such  as
              less(1) or more(1).

              The  user's  preferred  utility  to  edit  text  files.  Any string acceptable as a
              command_string operand to the sh -c command shall be valid.

       Note that the behavior of many programs and library routines is influenced by the presence
       or value of certain environment variables.  Examples 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.  See also

       *  POSIXLY_CORRECT makes certain programs and library routines follow the prescriptions of

       *  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

       *  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).


       Historically and by standard, environ must be declared in the user program.  However, as a
       (nonstandard) programmer convenience, environ is declared in the header file <unistd.h> if
       the _GNU_SOURCE feature test macro is defined (see feature_test_macros(7)).

       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.

       The HOME, LOGNAME, SHELL, and USER variables are set  when  the  user  is  changed  via  a
       session management interface, typically by a program such as login(1) from a user database
       (such as passwd(5)).  (Switching to the root user  using  su(1)  may  result  in  a  mixed
       environment where LOGNAME and USER are retained from old user; see the su(1) manual page.)


       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  and
       LESS.  Such usage is considered mistaken, and to be avoided in new programs.


       bash(1),   csh(1),  env(1),  login(1),  printenv(1),  sh(1),  su(1),  tcsh(1),  execve(2),
       clearenv(3), exec(3), getenv(3), putenv(3), setenv(3), unsetenv(3),  locale(7),,


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