[GRASS5] Login Shells (Was: Re: GDAL binaries for OSX)

[Glynn Clements]

Hamish wrote:

> OSX's Terminal looks for .bash_profile but not .bashrc. Apple's X11 looks for
> .bashrc but not .bash_profile. Solution (kludge) follows:

The terminals don't look for these files; bash does. However, exactly
which files depends upon the argument list:

       A login shell is one whose  first  character  of  argument
       zero is a -, or one started with the -login flag.

       An  interactive shell is one whose standard input and out­
       put 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.

       Login shells:
         On login (subject to the -noprofile option):
               if /etc/profile exists, source it.

               if ~/.bash_profile exists, source it,
                 else if ~/.bash_login exists, source it,
                   else if ~/.profile exists, source it.

         On exit:
               if ~/.bash_logout exists, source it.

       Non-login interactive shells:
         On startup (subject to the -norc and -rcfile options):
               if ~/.bashrc exists, source it.

It appears that OSX' Terminal application starts bash as a login
shell. Whether or not xterm starts bash as a login shell depends upon
the [+/-]ls switch:

       -ls     This  option  indicates  that  the  shell  that is
               started in the xterm window will be a login  shell
               (i.e.,  the  first  character of argv[0] will be a
               dash, indicating to the shell that it should  read
               the user's .login or .profile).

               Note  that this is incompatible with -e, since the
               login program does not provide a  way  to  specify
               the command to run in the new shell.  If you spec­
               ify both, xterm uses -ls.

       +ls     This option  indicates  that  the  shell  that  is
               started  should not be a login shell (i.e. it will
               be a normal ``subshell'').

and the loginShell X resource:

       loginShell (class LoginShell)
               Specifies whether or not the shell to  be  run  in
               the  window  should  be  started as a login shell.
               The default is ``false.''

The general idea behind a "login" shell is that the "top level" shell
(the one which is started automatically when you log in on a terminal)
should be a login shell, but subsequent shells (e.g. those started by
typing "bash" in an existing shell, using the "spawn shell" feature in
a program, etc) shouldn't be.

The bash_profile files (/etc/bash_profile, ~/.bash_profile) are only
run for login shells, so you can do things like:

	PATH="$PATH:/usr/local/bin"
	export PATH

and $PATH won't end up getting ":/usr/local/bin" appended repeatedly.

Also, if you set environment variables in bash_profile, they will be
inherited by subshells; if you change them, the changes will be
inherited by subshells. OTOH, if you set environment variables in
~/.bashrc, those settings will replace any changed versions.

In general, environment variables should be set in bash_profile, while
aliases functions should be defined in bashrc.

Whether or not xterm should spawn a login shell depends to an extent
upon whether your X startup scripts (e.g. ~/.xsession or ~/.xinitrc)
source bash_profile. If they don't, then you probably need to use
"xterm -ls" or set loginShell to True, e.g. by adding:

	XTerm*vt100.loginShell: True

to ~/.Xdefaults.

Alternatively, you can have ~/.bashrc source ~/.bash_profile, but only
if $SHLVL is 1, e.g.

	if [ "$SHLVL" = 1 ] ; then
		. ~/.bash_profile
	fi

At startup, bash sets $SHLVL to one more than the value which it
inherited from its parent, or to 1 if SHLVL wasn't set. Consequently,
it will be 1 for a top-level shell and >1 for subshells.

-- 
Glynn Clements <glynn.clements at virgin.net>