environment variable

environment variable
environment

   <programming, operating system> A {variable} that is bound in
   the current environment.  When evaluating an expression in
   some environment, the evaluation of a variable consists of
   looking up its name in the environment and substituting its
   value.

   Most programming languages have some concept of an environment
   but in {Unix} {shell scripts} it has a specific meaning
   slightly different from other contexts.  In shell scripts,
   environment variables are one kind of {shell variable}.  They
   differ from {local variables} and {command line arguments} in
   that they are inheritted by a {child process}.  Examples are
   the PATH variable that tells the shell the {file system}
   {paths} to search to find command {executables} and the TZ
   variable which contains the local time zone.  The variable
   called "SHELL" specifies the type of shell being used.

   These variables are used by commands or {shell scripts} to
   discover things about the environment they are operating in.
   Environment variables can be changed or created by the {user}
   or a program.

   To see a list of environment variables type "setenv" at the
   {csh} or {tcsh} {prompt} or "set" at the {sh}, {bash}, {jsh}
   or {ksh} prompt.

   In other programming languages, e.g. {functional programming}
   languages, the environment is extended with new bindings when
   a {function}'s {parameters} are bound to its {actual
   arguments} or when new variables are declared.  In a
   {block-structured} {procedural} language, the environment
   usually consists of a {linked list} of {activation records}.

   (1999-01-26)