Check existence of a login Post: 302172149

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Top Forums UNIX for Advanced & Expert Users Check existence of a login Post 302172149 by fpmurphy on Sunday 2nd of March 2008 09:40:35 PM

03-02-2008

Registered User

Quote:

So the shell knows whether a user exists or not... the question is how?

No, in general, a shell does not know whether a particular user exists or not. The operating system may know that information.

As for tilde expansion, here is the relevant section from ksh93 s+.

Code:

/*
 * This routine is used to resolve ~ expansion.
 * A ~ by itself is replaced with the users login directory.
 * A ~- is replaced by the previous working directory in shell.
 * A ~+ is replaced by the present working directory in shell.
 * If ~name  is replaced with login directory of name.
 * If string doesn't start with ~ or ~... not found then 0 returned.
 */

static char *sh_tilde(register const char *string)
{
        register char           *cp;
        register int            c;
        register struct passwd  *pw;
        register Namval_t *np=0;
        static Dt_t *logins_tree;
        if(*string++!='~')
                return(NIL(char*));
        if((c = *string)==0)
        {
                if(!(cp=nv_getval(nv_scoped(HOME))))
                        cp = getlogin();
                return(cp);
        }
        if((c=='-' || c=='+') && string[1]==0)
        {
                if(c=='+')
                        cp = nv_getval(nv_scoped(PWDNOD));
                else
                        cp = nv_getval(nv_scoped(OLDPWDNOD));
                return(cp);
        }
        if(logins_tree && (np=nv_search(string,logins_tree,0)))
                return(nv_getval(np));
        if(!(pw = getpwnam(string)))
                return(NIL(char*));
        if(!logins_tree)
                logins_tree = dtopen(&_Nvdisc,Dtbag);
        if(np=nv_search(string,logins_tree,NV_ADD))
                nv_putval(np, pw->pw_dir,0);
        return(pw->pw_dir);
}

fpmurphy

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