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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 Shell Parameters. These assignment statements affect only the environment seen by that command.
If the -k option is set (see 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 pathname of the command and passed to that command in its environment.