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(bashref.info)Environment

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Environment
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   When a program is invoked it is given an array of strings called the
ENVIRONMENT.  This is a list of name-value pairs, of the form
`name=value'.

   Bash provides several ways to manipulate the environment.  On
invocation, the shell scans its own environment and creates a parameter
for each name found, automatically marking it for EXPORT to child
processes.  Executed commands inherit the environment.  The `export'
and `declare -x' commands allow parameters and functions to be added to
and deleted from the environment.  If the value of a parameter in the
environment is modified, the new value becomes part of the environment,
replacing the old.  The environment inherited by any executed command
consists of the shell's initial environment, whose values may be
modified in the shell, less any pairs removed by the `unset' and
`export -n' commands, plus any additions via the `export' and `declare
-x' commands.

   The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter assignments, as described in
Note: Shell Parameters.  These assignment statements affect only the
environment seen by that command.

   If the `-k' option is set (Note: The Set Builtin), then all
parameter assignments are placed in the environment for a command, not
just those that precede the command name.

   When Bash invokes an external command, the variable `$_' is set to
the full path name of the command and passed to that command in its
environment.