basic CLI program in C

Question

Okay so overall im trying to complete a basic CLI C program which will complete functions such as clear, quit, cd, ls, help (bring up the unix man) etc.. i altered my code and so far i have this, im getting segmination error when trying to execute the cd command part of the program, (im very new to c btw);

    #include<stdio.h>
    #include<stdlib.h>
    #include<string.h>
    #include<unistd.h>

    int main (int argc, char *argv[])
    {
      char input[] = " ";
      char *argument;
     while(strcmp(input, "quit")!= 0)
      {


      printf("$");
      scanf ("%s", input);

     if(strcmp(input,"clear") == 0)
     {
       printf("\e[1;1H\e[2J");
      }

    else if(strcmp(argv[1],"cd") == 0)
     {


      if(chdir(argv[2]) == -1)
      {
         printf("\n directory does not exists");
       }

     }



    else if(strcmp(input, "echo") == 0)
     {
    char str[50];
    scanf("%[^\n]+", str);

    printf(" %s", str);
     }





  }

 }

Where did `input` come from? You've declared it, then you're checking its value. But you didn't write any code that sets its value or obtains it from anywhere. Also, when processing command line arguments, you need to find out if they exist as arguments. Check `argc` for how many valid values are in `argv[]`. — lurker, Sep 03 '17 at 09:16
Why do you `chdir` twice? The second call will either fail or be irrelevant. Also, use `perror` or call `strerror` to get a reasonable error message. — William Pursell, Sep 03 '17 at 09:34
Compile with all warnings and debug info (`gcc -Wall -Wextra -g`) then learn to use the debugger (`gdb`) and [valgrind](http://valgrind.org/) — Basile Starynkevitch, Sep 03 '17 at 11:08
Please [edit] your question to show us what kind of debugging you've done. I expect you have run your [mcve] within Valgrind or a similar checker, and to have investigated with a debugger such as GDB, for example. Ensure you've enabled a full set of compiler warnings, too. What did the tools tell you, and what information are they missing? And read Eric Lippert's [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). — Toby Speight, Sep 20 '17 at 13:48
Hint: what does `scanf("%s", input)` do? And how big is the `input` array? — Toby Speight, Sep 20 '17 at 13:50

Brian Zammit · Answer 1 · 2017-09-03T09:23:51.333

input is declared as a ' ' (space) character. It will never match 'cd'.

This is probably more along the lines of what you want to achieve, where the first parameter is the command (cd), and the second will be the directory:

int main (int argc, char *argv[])
  {

    char *argument;

  if(strcmp(argv[1],"cd") == 0)
 {


    if(chdir(argv[2]) == -1)
    {
       printf("\n directory does not exists");
    }

  }

Edit Also please note that there is no need for the else satement. If chdir does not return an error, it will change the directory, thus no need to call it again in an else.

Additionally, another tip for using system calls in general, it would be of great help if you print the error number returned by the system upon a failure in system call. This will make things easier when things start going wrong. To do this simply include <errno.h>' and modify the printf to printerrno` which gives specific details about the error:

printf("Chdir error: %d", errno);

For instance chdir() does not only return an error when the directory does not exist, but also for example if you do not have permissions to view the contents of the directory. See the man page for a list of possible errors.

Should really be `printf("chdir error: %s\n", strerror(errno));` — Basile Starynkevitch, Sep 03 '17 at 11:07

score -1 · Answer 2 · answered Sep 03 '17 at 09:29

-1

To implement your own shell, you need to take input directly from stdin, not from command-line arguments (argv) from another shell. The basic pattern is like this:

Read input
Execute command
Print results
Loop back to step 1

answered Sep 03 '17 at 09:29

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basic CLI program in C

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