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/* Compile with: g++ -Wall –Werror -o shell shell.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
/* The array below will hold the arguments: args[0] is the command. */
static char * args [512 ];
pid_t pid ;
int command_pipe [2 ];
#define READ 0
#define WRITE 1
/*
* Handle commands separatly
* input: return value from previous command (useful for pipe file descriptor)
* first: 1 if first command in pipe-sequence (no input from previous pipe)
* last: 1 if last command in pipe-sequence (no input from previous pipe)
*
* EXAMPLE: If you type "ls | grep shell | wc" in your shell:
* fd1 = command(0, 1, 0), with args[0] = "ls"
* fd2 = command(fd1, 0, 0), with args[0] = "grep" and args[1] = "shell"
* fd3 = command(fd2, 0, 1), with args[0] = "wc"
*
* So if 'command' returns a file descriptor, the next 'command' has this
* descriptor as its 'input'.
*/
static int command (int input , int first , int last )
{
int pipettes [2 ];
/* Invoke pipe */
pipe ( pipettes );
pid = fork ();
/*
SCHEME:
STDIN --> O --> O --> O --> STDOUT
*/
if (pid == 0 ) {
if (first == 1 && last == 0 && input == 0 ) {
// First command
dup2 ( pipettes [WRITE ], STDOUT_FILENO );
} else if (first == 0 && last == 0 && input != 0 ) {
// Middle command
dup2 (input , STDIN_FILENO );
dup2 (pipettes [WRITE ], STDOUT_FILENO );
} else {
// Last command
dup2 ( input , STDIN_FILENO );
}
if (execvp ( args [0 ], args ) == -1 )
_exit (EXIT_FAILURE ); // If child fails
}
if (input != 0 )
close (input );
// Nothing more needs to be written
close (pipettes [WRITE ]);
// If it's the last command, nothing more needs to be read
if (last == 1 )
close (pipettes [READ ]);
return pipettes [READ ];
}
/* Final cleanup, 'wait' for processes to terminate.
* n : Number of times 'command' was invoked.
*/
static void cleanup (int n )
{
int i ;
for (i = 0 ; i < n ; ++ i )
wait (NULL );
}
static int run (char * cmd , int input , int first , int last );
static char line [1024 ];
static int n = 0 ; /* number of calls to 'command' */
int main ()
{
printf ("SIMPLE SHELL: Type 'exit' or send EOF to exit.\n" );
while (1 ) {
/* Print the command prompt */
printf ("$> " );
fflush (NULL );
/* Read a command line */
if (!fgets (line , 1024 , stdin ))
return 0 ;
int input = 0 ;
int first = 1 ;
char * cmd = line ;
char * next = strchr (cmd , '|' ); /* Find first '|' */
while (next != NULL ) {
/* 'next' points to '|' */
* next = '\0' ;
input = run (cmd , input , first , 0 );
cmd = next + 1 ;
next = strchr (cmd , '|' ); /* Find next '|' */
first = 0 ;
}
input = run (cmd , input , first , 1 );
cleanup (n );
n = 0 ;
}
return 0 ;
}
static void split (char * cmd );
static int run (char * cmd , int input , int first , int last )
{
split (cmd );
if (args [0 ] != NULL ) {
if (strcmp (args [0 ], "exit" ) == 0 )
exit (0 );
n += 1 ;
return command (input , first , last );
}
return 0 ;
}
static char * skipwhite (char * s )
{
while (isspace (* s )) ++ s ;
return s ;
}
static void split (char * cmd )
{
cmd = skipwhite (cmd );
char * next = strchr (cmd , ' ' );
int i = 0 ;
while (next != NULL ) {
next [0 ] = '\0' ;
args [i ] = cmd ;
++ i ;
cmd = skipwhite (next + 1 );
next = strchr (cmd , ' ' );
}
if (cmd [0 ] != '\0' ) {
args [i ] = cmd ;
next = strchr (cmd , '\n' );
next [0 ] = '\0' ;
++ i ;
}
args [i ] = NULL ;
}
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