Date: Sat, 10 Apr 1999 23:15:00 +0100 From: Ben Smithurst <ben@scientia.demon.co.uk> To: Sue Blake <sue@welearn.com.au> Cc: david mankins <dm@k12-nis-2.bbn.com>, freebsd-questions@FreeBSD.ORG Subject: Re: login shell selection prompt Message-ID: <19990410231500.A81602@scientia.demon.co.uk> In-Reply-To: <19990410222804.05084@welearn.com.au> References: <19990410045140.50124@welearn.com.au> <199904092352.TAA12362@k12-nis-2.bbn.com> <19990410222804.05084@welearn.com.au>
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Sue Blake wrote:
> Yep, thanks, but a login is required before chsh can be run, and that
> login will be with the default bloated shell. That's no good:
> If the machine did have enough life left in it to run the normal bloated
> shell and chsh, then there wouldn't be any need to temporarily change the
> shell to something tiny.
> If the machine did not have enough life left in it to run the bloated
> shell and chsh, it'd be dead before I got a second chance to login with
> the lighter shell.
Why can't root's shell be /bin/sh? That's surely pretty light shell?
> The user will most likely be me or root. Not switch to a shell, but
> elect to use the substitute shell _before_ the regular bloated shell
> has a chance to load up and suck the last bit of life from the machine.
Hmm... you might try a small C program as your login shell.
> chsh is not a solution, and exec isn't either. Neither can prevent a
> superfluous few hundred Kb being compulsarily gobbled up just to get to
> a command prompt of any kind. Those are already part of the current
> range of alternatives that do not offer any particular benefits in time
> of crisis.
>
> I'm looking for a way to be asked for a shell decision at login BEFORE
> any shell is run. Yes, this is an unusual question and I don't imagine
> it'll budge with the usual answers.
Even with a shell script, as Crist suggested, or a small C program, you're
going to need to load something. Anyway, here's a C program you might try.
Disclaimer: I am no C guru, etc, etc, but this seems to work. Nitpick as
much as you like, I'm still learning C, so feedback on anything I'm doing
wrong would be useful so I don't get into bad habits :-)
/*
* shell.c: choose a shell at login
*/
#include <sys/types.h>
#include <err.h>
#include <pwd.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
void do_shell(char *);
int
main() {
struct passwd *user;
char newshell[128];
int tried_defaults = 0;
if ((user = getpwuid(getuid())) == NULL) {
/* eek! getpwuid failed, try default shells, fall through
to choose another if they fail */
warn("getpwuid");
execl("/bin/sh", "-sh", NULL);
warn("exec: sh");
execl("/bin/csh", "-csh", NULL);
warn("exec: csh");
printf("neither sh or csh worked, "
"you got any bright ideas?\n");
tried_defaults = 1;
}
printf("Please choose a shell [%s]: ",
user ? user->pw_shell : "no default");
fflush(stdout);
if (fgets(newshell, sizeof newshell, stdin) == NULL ||
strcmp(newshell, "\n") == 0)
{
/* EOF or empty line enteted */
if (user) {
do_shell(user->pw_shell);
warn("exec: %s", user->pw_shell);
}
if (!tried_defaults) {
execl("/bin/sh", "-sh", NULL);
warn("exec: sh");
execl("/bin/csh", "-csh", NULL);
warn("exec: csh");
tried_defaults = 1;
}
}
newshell[strlen(newshell)-1] = '\0';
do_shell(newshell);
if (!tried_defaults) {
execl("/bin/sh", "-sh", NULL);
warn("exec: sh");
execl("/bin/csh", "-csh", NULL);
warn("exec: csh");
}
return (0);
}
void
do_shell(char *shell) {
char argv0[128];
char *slash, *slash_prev = NULL;
/* argv[0] for login shells start with "-" */
argv0[0] = '-';
/* find the start of the last componenet of the path */
for (slash = strchr(shell, '/');
slash != NULL;
slash = strchr(slash + 1, '/'))
slash_prev = slash;
strncpy(argv0 + 1, slash_prev ? (slash_prev + 1) : shell, 126);
execl(shell, argv0, NULL);
warn("exec: %s", shell);
}
/* END */
--
Ben Smithurst
ben@scientia.demon.co.uk
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