lib/getusershell.c

   1 /* getusershell.c -- Return names of valid user shells.
   2
   3    Copyright (C) 1991, 1997, 2000-2001, 2003-2006, 2008-2025 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 <stdio.h>
  37 #include <stdlib.h>
  38 #include <string.h>
  39 #include <ctype.h>
  40
  41 #include "filename.h"
  42 #include "xalloc.h"
  43
  44 #if GNULIB_FOPEN_SAFER
  45 # include "stdio--.h"
  46 #endif
  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   for (;;)
 103     {
 104       ssize_t nread = getline (&line, &line_size, shellstream);
 105
 106       /* End of file.  */
 107       if (nread == -1)
 108         return NULL;
 109       /* Skip empty lines. */
 110       else if (nread > 1)
 111         {
 112           char *start = line;
 113           char *comment = strchr (start, '#');
 114           char *end;
 115
 116           if (comment != NULL)
 117             {
 118               /* Trim the comment mark.  */
 119               *comment = '\0';
 120               end = comment;
 121             }
 122           else
 123             {
 124               /* Trim the newline.  */
 125               end = start + nread;
 126               if (end[-1] == '\n')
 127                 *--end = '\0';
 128             }
 129
 130           /* Skip leading whitespace.  */
 131           while (start < end && isspace ((unsigned char) start[0]))
 132             start++;
 133           /* Trim trailing whitespace.  */
 134           while (start < end && isspace ((unsigned char) end[-1]))
 135             *--end = '\0';
 136
 137           /* Only return absolute file names. */
 138           if (start < end && IS_ABSOLUTE_FILE_NAME (start))
 139             return start;
 140         }
 141     }
 142 }
 143
 144 /* Rewind the shells file. */
 145
 146 void
 147 setusershell (void)
 148 {
 149   default_index = 0;
 150   if (shellstream)
 151     rewind (shellstream);
 152 }
 153
 154 /* Close the shells file. */
 155
 156 void
 157 endusershell (void)
 158 {
 159   if (shellstream)
 160     {
 161       fclose (shellstream);
 162       shellstream = NULL;
 163     }
 164 }
 165
 166 #ifdef TEST
 167 int
 168 main (void)
 169 {
 170   char *s;
 171
 172   while (s = getusershell ())
 173     puts (s);
 174   exit (0);
 175 }
 176 #endif