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
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.
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
'declare -x' commands.
The environment for any simple command
or function may be augmented temporarily by prefixing it with
parameter assignments, as described in section 3.4 Shell Parameters.
These assignment statements affect only the environment seen
by that command.
-k option is set (see section 4.3 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.