Adding directories to PATH

kyle-shaggy · 11-17-2008, 09:41 PM

If anyone is familiar with exec maybe you could help me out. I have a basic shell that I am trying to personalize. One of the features I want to include is that when the user inputs the command PATH they are then prompted for a directory which then is cat'ed to the end of linux's PATH variable. Here is a snippet of what I have so far:

Code:

char *t_PATH = (char *)malloc(sizeof(char) * 100);
char c;
	if (strcmp("PATH", *g_argv) == 0)
	{
		printf("Please enter the directory you wish to add to PATH\n");
		while(c != '\n') 		             
		{
			c = getchar();
			strncat(t_PATH, c, 1);
		}
		execve ("/bin/set PATH=$PATH:", t_PATH, "");
	}

It compiles fine and when I enter PATH it allows me to enter a variable but upon pressing return, I get a segmentation fault. I believe I am incorrectly using execve somehow, but I am still new to it so I am quite confused as to how to fix it.

Anyone have any ideas?

**Salem** · 11-18-2008, 12:29 AM

> while(c != '\n')
c isn't initialised, so who knows whether this will pass the first time around.

> strncat(t_PATH, c, 1);
- t_PATH isn't initialised, so who knows where in the string your char is appended.
- c should have been passed as &c
- strncat does NOT always append a \0, so you could still end up with a broken string.

> execve ("/bin/set PATH=$PATH:", t_PATH, "");
Variable length exec calls end with a NULL parameter, not a "" parameter.

I suppose it's worth noting that exec is the wrong thing to be calling to set the path. But you should try anyway, you'll learn something in the process.

**CornedBee** · 11-18-2008, 03:11 AM

Writing any progrem to append to the path is wrong. But that's another good thing to learn.

Here's a hint. Write a shell script that sets an environment variable, call it, and then look at the environment.
Then source the shell script instead of calling it.

kyle-shaggy · 11-18-2008, 09:46 AM

Yah I could easily make a shell script to change the PATH env variable, but that is not how I want to go about doing it. I wanted to see if I would be able to change it with C code.

Essentially I was asking is there a C equivalent to:

Code:

 PATH = PATH=$PATH:/myfolder/

Researching more about setting environment variables in C I found this piece of code to help me out substantially:

Code:

   
if (-1 == putenv("PATH=/:/home/userid")) 
{
      printf("putenv failed \n");
      return EXIT_FAILURE;
}

cpjust · 11-18-2008, 10:41 AM

Why not use the setenv() function?

brewbuck · 11-18-2008, 02:40 PM

Quote:

Originally Posted by cpjust

Why not use the setenv() function?

That won't work either. Changing the parent's environment is impossible. The only way to do this is with a shell script that gets evaluated in the environment which needs to be modified.

There's no way to affect a different process's PATH.

11-17-2008, 09:41 PM	#1
kyle-shaggy Registered User Join Date: Nov 2008 Posts: 2	Adding directories to PATH If anyone is familiar with exec maybe you could help me out. I have a basic shell that I am trying to personalize. One of the features I want to include is that when the user inputs the command PATH they are then prompted for a directory which then is cat'ed to the end of linux's PATH variable. Here is a snippet of what I have so far: Code: char t_PATH = (char )malloc(sizeof(char) * 100); char c; if (strcmp("PATH", *g_argv) == 0) { printf("Please enter the directory you wish to add to PATH\n"); while(c != '\n') { c = getchar(); strncat(t_PATH, c, 1); } execve ("/bin/set PATH=$PATH:", t_PATH, ""); } It compiles fine and when I enter PATH it allows me to enter a variable but upon pressing return, I get a segmentation fault. I believe I am incorrectly using execve somehow, but I am still new to it so I am quite confused as to how to fix it. Anyone have any ideas?
kyle-shaggy is offline

11-18-2008, 12:29 AM	#2
Salem and the hat of Jobseeking Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,676	> while(c != '\n') c isn't initialised, so who knows whether this will pass the first time around. > strncat(t_PATH, c, 1); - t_PATH isn't initialised, so who knows where in the string your char is appended. - c should have been passed as &c - strncat does NOT always append a \0, so you could still end up with a broken string. > execve ("/bin/set PATH=$PATH:", t_PATH, ""); Variable length exec calls end with a NULL parameter, not a "" parameter. I suppose it's worth noting that exec is the wrong thing to be calling to set the path. But you should try anyway, you'll learn something in the process. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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11-18-2008, 03:11 AM	#3
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,492	Writing any progrem to append to the path is wrong. But that's another good thing to learn. Here's a hint. Write a shell script that sets an environment variable, call it, and then look at the environment. Then source the shell script instead of calling it. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
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11-18-2008, 09:46 AM	#4
kyle-shaggy Registered User Join Date: Nov 2008 Posts: 2	Yah I could easily make a shell script to change the PATH env variable, but that is not how I want to go about doing it. I wanted to see if I would be able to change it with C code. Essentially I was asking is there a C equivalent to: Code: PATH = PATH=$PATH:/myfolder/ Researching more about setting environment variables in C I found this piece of code to help me out substantially: Code: if (-1 == putenv("PATH=/:/home/userid")) { printf("putenv failed \n"); return EXIT_FAILURE; }
kyle-shaggy is offline

11-18-2008, 10:41 AM	#5
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,278	Why not use the setenv() function? __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008 "the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010
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