How to keep my shell running after a command - c++

Question

I am running this basic shell program in another shell. I am unable to figure out why my shell doesn't keep running after "ls" executes. I dont have an exit for it but it goes back to original shell. I have to run my shell program every time if want to use it. i figured thats what the fork() is supposed to do. I only want my shell to exit using the exit command which i coded with the if else statement. Any suggestions would be much appreciated. Oh and disregard the gettoks() parser function, i couldn't figure out how to use it for input so i wrote if else statements for the string input cmSTR rather then using the gettoks() parser. Mainly because i couldn't figure how to pass the input into it
#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <errno.h>
#include <signal.h>
#include <cstdlib>
#include <sys/wait.h>
using namespace std;
// Initializing counters for trapping
static int cc_counter = 0;
static int cz_counter = 0;
static int cq_counter = 0;
//Functions for trap signal handler
void cc_handler( int signo )
{
++cc_counter;
}
void cz_handler( int signo )
{
++cz_counter;
}
void cq_handler( int signo )
{
++cq_counter;
}
//*********************************************************
//
// Extern Declarations
//
//*********************************************************
using namespace std;
extern "C"
{
extern char **gettoks();
}
//*********************************************************
//
// Main Function
//
//*********************************************************
int main( int argc, char *argv[] )
{
// local variables
int ii;
char **toks;
int retval;
// initialize local variables
ii = 0;
toks = NULL;
retval = 0;
char buf[1000];//Initialize of size for current working directory
string cmSTR;//String to hold input
int status;//Initialization of status for fork()
pid_t pid;//Declaration of pid
// main (infinite) loop
while( true )
{
signal( SIGINT, cc_handler );// Traps Ctrl+C
signal( SIGTSTP, cz_handler);// Traps Ctrl+Z
signal( SIGQUIT, cq_handler);// Traps Ctrl+\
//prompt and show current working directory
cout <<("RS_SHELL:") << getcwd(buf,1000) << "\t";
getline(cin ,cmSTR);//read input from keyboard
// if else loop to switch based on command input
if(cmSTR == "ls")// if ls, then execute arguement
{
execl( "/bin/ls", "ls", NULL );//System call to execute ls
}
else if(cmSTR == "exit")//if exit, then execute block of code
{
cout << "Ctrl C entered: " << ++cc_counter << "times"<< endl;
cout << "Ctrl Z entered: " << ++cz_counter << "times"<< endl;
cout << "Ctrl Back Slash entered: " << ++cq_counter << "times"<< endl;
exit(1);
}
else if(cmSTR == "guish")// if guish, execute guish shell
{
execvp("guish", NULL);
}
//if input is not any of previous commands then fork()
else if(cmSTR != "ls" && cmSTR != "exit" && cmSTR != "guish" && cmSTR != "\n")
{
pid = fork();
if (pid < 0)//Loop to fork parent and child process
{
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0)//Child process
{
execvp("guish", NULL);//system call to execute guish shell
}
else //Parent process
{
waitpid( -1, &status,0);
exit(0);
}
}
// get arguments
toks = gettoks();
if( toks[0] != NULL )
{
// simple loop to echo all arguments
for( ii=0; toks[ii] != NULL; ii++ )
{
cout << "Argument " << ii << ": " << toks[ii] << endl;
}
if( !strcmp( toks[0], "exit" ))
break;
}
}
// return to calling environment
return( retval );
}

Answer 1

As you suspected, execl and its related functions overlay the current process with a new process. Thus, after the execl call that starts ls, your program won't exist any more to keep running.
If you want your shell program to stay around after running ls, you'll need to fork() before the call execl( "/bin/ls", "ls", NULL );.
Also, if you want the output from ls to appear in the same console as your shell, as I think you might be intending, you will need to pipe the output from ls back to your shell and then write that output onto your shell's console. See Writing my own shell… stuck on pipes?, for instance.