lib/getusershell.c

   1 /* getusershell.c -- Return names of valid user shells.
   2
   3    Copyright (C) 1991, 1997, 2000-2001, 2003-2006, 2008-2024 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 <string.h>
  38 #include <ctype.h>
  39
  40 #include "stdio--.h"
  41 #include "filename.h"
  42 #include "xalloc.h"
  43
  44 #if ! defined ADDITIONAL_DEFAULT_SHELLS && defined __MSDOS__
  45 # define ADDITIONAL_DEFAULT_SHELLS \
  46   "c:/dos/command.com", "c:/windows/command.com", "c:/command.com",
  47 #else
  48 # define ADDITIONAL_DEFAULT_SHELLS /* empty */
  49 #endif
  50
  51 /* List of shells to use if the shells file is missing. */
  52 static char const* const default_shells[] =
  53 {
  54   ADDITIONAL_DEFAULT_SHELLS
  55   "/bin/sh", "/bin/csh", "/usr/bin/sh", "/usr/bin/csh", NULL
  56 };
  57
  58 /* Index of the next shell in 'default_shells' to return.
  59    0 means we are not using 'default_shells'. */
  60 static size_t default_index = 0;
  61
  62 /* Input stream from the shells file. */
  63 static FILE *shellstream = NULL;
  64
  65 /* Line of input from the shells file. */
  66 static char *line = NULL;
  67
  68 /* Number of bytes allocated for 'line'. */
  69 static size_t line_size = 0;
  70 \f
  71 /* Return an entry from the shells file, ignoring comment lines.
  72    If the file doesn't exist, use the list in DEFAULT_SHELLS (above).
  73    In any case, the returned string is in memory allocated through malloc.
  74    Return NULL if there are no more entries.  */
  75
  76 char *
  77 getusershell (void)
  78 {
  79   if (default_index > 0)
  80     {
  81       if (default_shells[default_index])
  82         /* Not at the end of the list yet.  */
  83         return xstrdup (default_shells[default_index++]);
  84       return NULL;
  85     }
  86
  87   if (shellstream == NULL)
  88     {
  89       shellstream = fopen (SHELLS_FILE, "r");
  90       if (shellstream == NULL)
  91         {
  92           /* No shells file.  Use the default list.  */
  93           default_index = 1;
  94           return xstrdup (default_shells[0]);
  95         }
  96     }
  97
  98   for (;;)
  99     {
 100       ssize_t nread = getline (&line, &line_size, shellstream);
 101
 102       /* End of file.  */
 103       if (nread == -1)
 104         return NULL;
 105       /* Skip empty lines. */
 106       else if (nread > 1)
 107         {
 108           char *start = line;
 109           char *comment = strchr (start, '#');
 110           char *end;
 111
 112           if (comment != NULL)
 113             {
 114               /* Trim the comment mark.  */
 115               comment = '\0';
 116               end = comment;
 117             }
 118           else
 119             {
 120               /* Trim the newline.  */
 121               end = start + nread;
 122               if (end[-1] == '\n')
 123                 *--end = '\0';
 124             }
 125
 126           /* Skip leading whitespace.  */
 127           while (start < end && isspace ((unsigned char) start[0]))
 128             start++;
 129           /* Trim trailing whitespace.  */
 130           while (start < end && isspace ((unsigned char) end[-1]))
 131             *--end = '\0';
 132
 133           /* Only return absolute file names. */
 134           if (start < end && IS_ABSOLUTE_FILE_NAME (start))
 135             return start;
 136         }
 137     }
 138 }
 139
 140 /* Rewind the shells file. */
 141
 142 void
 143 setusershell (void)
 144 {
 145   default_index = 0;
 146   if (shellstream)
 147     rewind (shellstream);
 148 }
 149
 150 /* Close the shells file. */
 151
 152 void
 153 endusershell (void)
 154 {
 155   if (shellstream)
 156     {
 157       fclose (shellstream);
 158       shellstream = NULL;
 159     }
 160 }
 161
 162 #ifdef TEST
 163 int
 164 main (void)
 165 {
 166   char *s;
 167
 168   while (s = getusershell ())
 169     puts (s);
 170   exit (0);
 171 }
 172 #endif