A login shell is one whose first character of argument zero is a -, or 
        one started with the --login option.
        
        An interactive shell is one started without non-option arguments and
        without the -c option whose standard input and error are both connected
        to terminals (as determined by isatty(3)), or one started with the -i
        option.  PS1 is set and $- includes i if bash is interactive,
        allowing a shell script or a startup file to test this state.

        The following paragraphs describe how bash executes its startup
        files.  If any of the files exist but cannot be read, bash
        reports an error.  Tildes are expanded in file names as described below
        under Tilde Expansion in the EXPANSION section.

        When bash is invoked as an interactive login shell, or as a non-
        interactive shell with the --login option, it first reads and
        executes commands from the file /etc/profile, if that file exists.
        After reading that file, it looks for ~/.bash_profile, ~/.bash_login,
        and ~/.profile, in that order, and reads and executes commands from the
        first one that exists and is readable. The --noprofile option may be
        used when the shell is started to inhibit this behavior.

        When a login shell exits, bash reads and executes commands from the
        files ~/.bash_logout and /etc/bash.bash_logout, if the files
        exists.

        When an interactive shell that is not a login shell is started, bash
        reads and executes commands from ~/.bashrc, if that file
        exists.  This may be inhibited by using the --norc option. The --rcfile 
        file option will force bash to read and execute com- mands from
        file instead of ~/.bashrc.

        When bash is started non-interactively, to run a shell script, for
        example, it looks for the variable BASH_ENV in the environment, expands
        its value if it appears there, and uses the expanded value as the name 
        of a file to read and execute.  Bash behaves as if the following command 
        were executed: if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi but the value 
        of the PATH variable is not used to search for the file name.

        If bash is invoked with the name sh, it tries to mimic the startup
        behavior of historical versions of sh as closely as possible,
        while conforming to the POSIX standard as well.  When invoked as an
        interactive login shell, or a non-interactive shell with the --login
        option, it first attempts to read and execute commands from /etc/profile
        and ~/.profile, in that order.  The --noprofile option may be used to
        inhibit this behavior.  When invoked as an interactive shell with the
        name sh, bash looks for the variable ENV, expands its value if it is
        defined, and uses the expanded value as the name of a file to read and
        execute.  Since a shell invoked as sh does not attempt to read and
        execute commands from any other startup files, the --rcfile option has
        no effect.  A non-interactive shell invoked with the name sh does not
        attempt to read any other startup files.  When invoked as sh, bash
        enters posix mode after the startup files are read.

        When bash is started in posix mode, as with the --posix command line
        option, it follows the POSIX standard for startup files.  In this
        mode, interactive shells expand the ENV variable and commands are read
        and executed from the file whose name is the expanded value. No
        other startup files are read.

        Bash attempts to determine when it is being run with its standard input
        connected to a a network connection, as if by the remote shell
        daemon, usually rshd, or the secure shell daemon sshd.  If bash
        determines it is being run in this fashion, it reads and executes
        commands from ~/.bashrc, if that file exists and is readable.  It will
        not do this if invoked as sh.  The --norc option may be used to inhibit
        this behavior, and the --rcfile option may be used to force another file
        to be read, but rshd does not generally invoke the shell with those
        options or allow them to be specified.

        If the shell is started with the effective user (group) id not equal to
        the real user (group) id, and the -p option is not supplied,
        no startup files are read, shell functions are not inherited from the
        environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
        variables, if they appear in the environment, are ignored, and the
        effective user id is set to the real user id.  If the -p option is 
        supplied at invocation, the startup behavior is the same, but the 
        effective user id is not reset.