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, ‘declare -x’, and unset
commands modify the environment by
adding and deleting parameters and functions.
If the value of a parameter in the environment is modified,
the new value automatically 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.
If any parameter assignment statements, as described in Shell Parameters, appear before a simple command, the variable assignments are part of that command’s environment for as long as it executes. These assignment statements affect only the environment seen by that command. If these assignments precede a call to a shell function, the variables are local to the function and exported to that function’s children.
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.