# environ

## 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 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 _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
           functions getenv(3), putenv(3), setenv(3), and unsetenv(3).
    
           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.
    
           **LOGNAME**
                  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** Absolute path to the current working directory; required to
                  be partially canonical (no _._ or _.._ components).
    
           **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).
    
           **EDITOR** /**VISUAL**
                  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 ld.so(8).
    
           •  **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), 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).
    


## NOTES



           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.)
    


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


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