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 de-
       fined.)   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 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

       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 re-
       naming their option to GZIP_OPT.

       bash(1), csh(1), env(1), login(1),  printenv(1),  sh(1),  tcsh(1),  ex-
       ecve(2),  clearenv(3),  exec(3),  getenv(3),  putenv(3), setenv(3), un-
       setenv(3), locale(7), ld.so(8), pam_env(8)

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Linux                             2017-09-15                        ENVIRON(7)
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