L337Hax0r wrote:
for my operating systems design class i am supposed to write a shell in c.
i have no idea where to start and our teacher hasn't given us much info.
anyone know of any good tutorials?
Here is an example shell (well, it's more of a skeleton, really), which does
not actually execute any internal commands - it just pretends to. On the
other hand, it /will/ execute external commands if they don't happen to
have the same name as the internal commands it understands, so use with
care!
Note that it is case-sensitive. If you're in Windows, MKDIR foo, for
example, will be treated as an external command and will actually create a
foo directory, whereas mkdir foo will be treated as internal, and will
merely /pretend/ to make a directory.
/* shelldemo.c
* Richard Heathfield, 2003
* Contact:
bi****@eton.powernet.co.uk
* Permission is granted to copy this file and to modify it,
* provided that credit to the author is given where appropriate
* and modifications are clearly marked as such. In other words,
* don't pretend you wrote my bits, and don't pretend I wrote
* your bits.
* This code is intended merely to be an expression of an idea for
* matching up requests to function calls, and was written very
* quickly. It was not intended to be considered a model of
* exemplary programming practice. Witness the large main(), and
* the use of "magic numbers".
* Retain this comment. Thank you.
* - rjh 8 Jan 2003.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int cmpstr(const void *vs1, const void *vs2)
{
char * const *s1 = vs1;
char * const *s2 = vs2;
return strcmp(*s1, *s2);
}
int rjhchdir(char *args)
{
if(args != NULL)
{
printf("You changed directory to %s.\n", args);
}
else
{
printf("You changed to your home directory.\n");
}
return 0;
}
int rjhclear(void)
{
printf("You cleared the display.\n");
return 0;
}
int rjhdf(void)
{
printf("You asked for disk usage info.\n");
return 0;
}
int rjhdu(char *args)
{
printf("You asked for directory usage info.\n");
if(args != NULL)
{
printf("Specifically, you wanted the option %s\n", args);
}
return 0;
}
int rjhls(char *args)
{
printf("You asked for a directory listing.\n");
if(args != NULL)
{
printf("Options: %s\n", args);
}
return 0;
}
int rjhmkdir(char *args)
{
if(args != NULL)
{
printf("You created a directory, %s\n", args);
}
else
{
printf("You have to say what "
"directory you want to create.\n");
}
return 0;
}
int rjhsu(char *args)
{
if(args != NULL)
{
printf("You switched user to %s\n", args);
}
else
{
printf("You are now root.\n");
}
return 0;
}
int main(void)
{
/* ensure this list is sorted! */
char *command[] =
{
"cd",
"clear",
"df",
"du",
"exit",
"ls",
"mkdir",
"su"
};
size_t numcommands = sizeof command / sizeof command[0];
char line[4096] = {0};
char safeline[4096] = {0};
int done = 0;
int i = 0;
char *s = line;
char **t = NULL;
char *prompt = ">";
char *args = NULL;
char *nl = NULL;
while(!done)
{
printf("%s", prompt);
fflush(stdout);
if(NULL == fgets(line, sizeof line, stdin))
{
t = NULL;
done = 1;
}
else
{
nl = strchr(line, '\n');
if(nl != NULL)
{
*nl = '\0';
strcpy(safeline, line);
}
else
{
int ch;
printf("Line too long! Ignored.\n");
while((ch = getchar()) != '\n' && ch != EOF)
{
continue;
}
if(ch == EOF)
{
done = 1;
}
}
args = strchr(line, ' ');
if(args != NULL)
{
*args++ = '\0';
}
t = bsearch(&s,
command,
numcommands,
sizeof command[0],
cmpstr);
}
if(!done && t != NULL)
{
i = (int)(t - command);
switch(i)
{
case 0: rjhchdir(args); break;
case 1: rjhclear(); break;
case 2: rjhdf(); break;
case 3: rjhdu(args); break;
case 4: done = 1; break;
case 5: rjhls(args); break;
case 6: rjhmkdir(args); break;
case 7: rjhsu(args); break;
default: break;
}
}
else
{
printf("%s is not an internal command. "
"Trying to execute it instead.\n", line);
system(safeline);
}
}
return 0;
}
--
Richard Heathfield :
bi****@eton.powernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ:
http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc:
http://users.powernet.co.uk/eton 
