Thread: umask

i am working on my own shell and am having trouble working with some of the built-in commands

does anyone have any suggestions on what would be the best way to implement the umask function

thanks

well, the man page gives you

SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>

mode_t umask(mode_t mask);

so play around with those headers and that struct.

Well you need to understand permission bits.
There are 3 general permission bytes.
user,group,world

Each has a different setting
with chmod you add the buts up to get what you want
like bit 1 == execute
bit 2 == write
bit 4 == read

So if you want to use make a file world readable and user writable,readable, and executable, with the group having no permisisons you would to this

chmod 701 $FILENAME

Now umask (or usermask) will actually change the default permissions, opposite of what you want.

a umake of 077 will make a new file have the chmod of 700.

It masks off the bits with new file.

The only difficulty is to rembemer that umask works opposite of chmod.

I would recomend that you read the chmod manpage, as it will explain how to use chmod to modify the permissions on a file that already exists. umask works on new files.

To demo, in a shell do this
#umask 057 && touch test1
#umask 077 && touch test2
# umake 500 && touch test3
# ls -l test*

and you can see the differences in the permission of each file and how the umake will change it.

HTH

Squid

okay..let me be a little more clear

below is the code for a function I am trying to write

the input will be from the command line

a string of the form "umask 123"

needless to say it is not working. If someone could point out the error in my ways i would appreciate it.

Code:

void my_umask( int argc, char * argv[] )
{
    char num[4];
    long len, n1,n2,n3, flag = 0,n, old_mask;
    
    if(argc > 2)
        my_error("too many arguments.");
    else if(argc == 2)
    {
        len = strlen(argv[1]);
        
        if( len > 3 )
        {
           my_error("unrecognized argument for umask.");
           return;
        }
        else
        {
           strcpy( num, (argv[1]) );
           n=atol(num);
                     
           n1 = n/100;
           n2 = ( n - (n1*100) )/ 10;
           n3 = ( n - ((n1*100) + (n2*10)) );
          
           if( n1>=0 && n1<=7)
              if( n2>=0 && n2<=7)
                 if( n3>=0 && n3<=7)  
                 {
                    flag = 1;
                    mask = n;
                    umask(n);
                 }//end if
           
           if(flag != 1)
           {
              my_error("illegal argument for umask.");
              return;
           }
        }
    }
    else  //argc==1  print the value of mask
        umask();    //not doing anything?????
    return;
}