lib/getusershell.c

   1 /* getusershell.c -- Return names of valid user shells.
   2
   3    Copyright (C) 1991, 1997, 2000-2001, 2003-2006, 2008-2020 Free Software
   4    Foundation, Inc.
   5
   6    This program is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3 of the License, or
   9    (at your option) any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  18
  19 /* Written by David MacKenzie <djm@gnu.ai.mit.edu> */
  20
  21 #include <config.h>
  22
  23 /* Specification.  */
  24 #include <unistd.h>
  25
  26 #ifndef SHELLS_FILE
  27 # ifndef __DJGPP__
  28 /* File containing a list of nonrestricted shells, one per line. */
  29 #  define SHELLS_FILE "/etc/shells"
  30 # else
  31 /* This is a horrible kludge.  Isn't there a better way?  */
  32 #  define SHELLS_FILE "/dev/env/DJDIR/etc/shells"
  33 # endif
  34 #endif
  35
  36 #include <stdlib.h>
  37 #include <ctype.h>
  38
  39 #include "stdio--.h"
  40 #include "xalloc.h"
  41
  42 #if USE_UNLOCKED_IO
  43 # include "unlocked-io.h"
  44 #endif
  45
  46 static size_t readname (char **, size_t *, FILE *);
  47
  48 #if ! defined ADDITIONAL_DEFAULT_SHELLS && defined __MSDOS__
  49 # define ADDITIONAL_DEFAULT_SHELLS \
  50   "c:/dos/command.com", "c:/windows/command.com", "c:/command.com",
  51 #else
  52 # define ADDITIONAL_DEFAULT_SHELLS /* empty */
  53 #endif
  54
  55 /* List of shells to use if the shells file is missing. */
  56 static char const* const default_shells[] =
  57 {
  58   ADDITIONAL_DEFAULT_SHELLS
  59   "/bin/sh", "/bin/csh", "/usr/bin/sh", "/usr/bin/csh", NULL
  60 };
  61
  62 /* Index of the next shell in 'default_shells' to return.
  63    0 means we are not using 'default_shells'. */
  64 static size_t default_index = 0;
  65
  66 /* Input stream from the shells file. */
  67 static FILE *shellstream = NULL;
  68
  69 /* Line of input from the shells file. */
  70 static char *line = NULL;
  71
  72 /* Number of bytes allocated for 'line'. */
  73 static size_t line_size = 0;
  74 \f
  75 /* Return an entry from the shells file, ignoring comment lines.
  76    If the file doesn't exist, use the list in DEFAULT_SHELLS (above).
  77    In any case, the returned string is in memory allocated through malloc.
  78    Return NULL if there are no more entries.  */
  79
  80 char *
  81 getusershell (void)
  82 {
  83   if (default_index > 0)
  84     {
  85       if (default_shells[default_index])
  86         /* Not at the end of the list yet.  */
  87         return xstrdup (default_shells[default_index++]);
  88       return NULL;
  89     }
  90
  91   if (shellstream == NULL)
  92     {
  93       shellstream = fopen (SHELLS_FILE, "r");
  94       if (shellstream == NULL)
  95         {
  96           /* No shells file.  Use the default list.  */
  97           default_index = 1;
  98           return xstrdup (default_shells[0]);
  99         }
 100     }
 101
 102   while (readname (&line, &line_size, shellstream))
 103     {
 104       if (*line != '#')
 105         return line;
 106     }
 107   return NULL;                  /* End of file. */
 108 }
 109
 110 /* Rewind the shells file. */
 111
 112 void
 113 setusershell (void)
 114 {
 115   default_index = 0;
 116   if (shellstream)
 117     rewind (shellstream);
 118 }
 119
 120 /* Close the shells file. */
 121
 122 void
 123 endusershell (void)
 124 {
 125   if (shellstream)
 126     {
 127       fclose (shellstream);
 128       shellstream = NULL;
 129     }
 130 }
 131
 132 /* Read a line from STREAM, removing any newline at the end.
 133    Place the result in *NAME, which is malloc'd
 134    and/or realloc'd as necessary and can start out NULL,
 135    and whose size is passed and returned in *SIZE.
 136
 137    Return the number of bytes placed in *NAME
 138    if some nonempty sequence was found, otherwise 0.  */
 139
 140 static size_t
 141 readname (char **name, size_t *size, FILE *stream)
 142 {
 143   int c;
 144   size_t name_index = 0;
 145
 146   /* Skip blank space.  */
 147   while ((c = getc (stream)) != EOF && isspace (c))
 148     /* Do nothing. */ ;
 149
 150   for (;;)
 151     {
 152       if (*size <= name_index)
 153         *name = x2nrealloc (*name, size, sizeof **name);
 154       if (c == EOF || isspace (c))
 155         break;
 156       (*name)[name_index++] = c;
 157       c = getc (stream);
 158     }
 159   (*name)[name_index] = '\0';
 160   return name_index;
 161 }
 162
 163 #ifdef TEST
 164 int
 165 main (void)
 166 {
 167   char *s;
 168
 169   while (s = getusershell ())
 170     puts (s);
 171   exit (0);
 172 }
 173 #endif