File System, Part 4: Working with directories

How do I find out if file (an inode) is a regular file or directory?

Use the S_ISDIR macro to check the mode bits in the stat struct:

struct stat s;
stat("/tmp", &s );
if( S_ISDIR(s.st_mode) ) { ... 

Note, later we will write robust code to verify that the stat call succeeds (returns 0); if the stat call fails, we should assume the stat struct content is arbitrary.

How do I recurse into subdirectories?

First a puzzle - how many bugs can you find in the following code?

void dirlist(char*path) {
  
  struct dirent* dp;
  DIR* dirp = opendir(path);
  while ((dp = readdir(dirp)) != NULL) {
     char newpath[strlen(path) + strlen(dp->d_name) + 1];
     sprintf(newpath,"%s/%s", newpath, dp->d_name);
     printf("%s\n", dp->d_name);
     dirlist(newpath);
  }
}

int main(int argc, char**argv) { dirlist(argv[1]);return 0; }

Did you find all 4 bugs?

// Check opendir result (perhaps user gave us a path that can not be opened as a directory
if(!dirp) { perror("Could not open directory"); return }
// +2 as we need space for the / and the terminating 0
char newpath[strlen(path) + strlen(dp->d_name) + 2]; 
// Correct parameter
sprintf(newpath,"%s/%s", path, dp->d_name); 
// Perform stat test (and verify) before recursing
if( 0==stat(newpath,&s) && S_ISDIR(s.st_mode)) dirlist(newpath)

What are symbolic links? How do they work? How do I make one?

symlink(const char *target, const char *symlink);

To create a symbolic link in the shell use ln -s

To read the contents of the link as just a file use readlink

> readlink myfile.txt
../../dir1/notes.txt

To read the meta-(stat) information of a symbolic link use lstat not stat

struct stat s;
stat("myfile.txt", &s1); // stat info about  the notes.txt file
lstat("myfile.txt", &s2); // stat info about the symbolic link

Advantages of symbolic links

• Can refer to a files that don't exist yet
• Unlike hard links, can refer to directories as well as regular files
• Can refer to files (and directories) that exist outside of the current file system

Main disadvantage: Slower than regular files and directories. When the links contents are read, they must be interpreted as a new path to the target file.

What is /dev/null and when is it used?

The file /dev/null is a great place to store bits that you never need to read! Bytes sent to /dev/null/ are never stored - they are simply discarded. A common use of /dev/null is to discard standard output. For example,

ls . >/dev/null

Why would I want to set a directory's sticky bit?

When a directory's sticky bit is set only the file's owner, the directory's owner, and the root user can rename (or delete) the file. This is useful when multiple users have write access to a common directory

A common use of the sticky bit is for the shared and writable /tmp directory.

Why do shell and script programs start with #!/usr/bin/env python ?

Ans: For portability! While it is possible to write the fully qualified path to a python or perl interpreter, this approach is not portable because you may have installed python in a different directory me.

To overcome this use the env utility is used to find and execute the program on the user's path. The env utility itself has historically been stored in /usr/bin - and it must be specified with an absolute path.

How do I make 'hidden' files i.e. not listed by "ls"? How do I list them?

Easy! Create files (or directories) that start with a "." - then (by default) they are not displayed by standard tools and utilities.

This is often used to hide configuration files inside the user's home directory. For example ssh stores its preferences inside a directory called .sshd

To list all files including the normally hidden entries use ls with -a option

>ls -a
.			a.c			myls
..			a.out			other.txt
.secret	

What happens if I turn off the execute bit on directories?

The execute bit for a directory is used to control whether the directory contents is listable.

>chmod ugo-x dir1
>ls -l
drw-r--r--   3 angrave  staff   102 Nov 10 11:22 dir1

However when attempting to list the contents of the directory,

>ls dir1
ls: dir1: Permission denied

In other words, the directory itself is discoverable but its contents cannot be listed.

What is file globbing (and who does it)?

Before executing the program the shell expands parameters into matching filenames. For example, if the current directory has three filenames that start with my ( my1.txt mytext.txt myomy), then

echo my*

Expands to

echo my1.txt mytext.txt myomy

This is known as file globbing and is processed before the command is executed. ie the command's parameters are identical to manually typing every matching filename.

Creating secure directories

Suppose you created your own directory in /tmp and then set the permissions so that only you can use the directory (see below). Is this secure?

mkdir /tmp/mystuff
chmod 700 /tmp/mystuff

There is a window of opportunity between when the directory is created and when it's permissions are changed. This leads to several vulnerabilities that are based on a race condition (where an attacker modifies the directory in some way before the privileges are removed). Some examples include:

Another user replaces mystuff with a hardlink to an existing file or directory owned by the second user, then they would be able to read and control the contents of the mystuff directory. Oh no - our secrets are no longer secret!

However in this specific example the /tmp directory has the sticky bit set, so other users may not delete the mystuff directory, and the simple attack scenario described above is impossible. This does not mean that creating the directory and then later making the directory private is secure! A better version is to atomically create the directory with the correct permissions from its inception -

mkdir -m 700 /tmp/mystuff

How do I automatically create parent directories?

mkdir -p d1/d2/d3

Will automatically create d1 and d2 if they don't exist.

My default umask 022; what does this mean?

The umask subtracts (reduces) permission bits from 777 and is used when new files and new directories are created by open,mkdir etc. Thus 022 (octal) means that group and other privileges will not include the writable bit . Each process (including the shell) has a current umask value. When forking, the child inherits the parent's umask value.

For example, by setting the umask to 077 in the shell, ensures that future file and directory creation will only be accessible to the current user,

umask 077
mkdir secretdir

As a code example, suppose a new file is created with open() and mode bits 666 (write and read bits for user,group and other):

open("myfile", O_CREAT ,  S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)

If umask is octal 022, then the permissions of the created file will be 0666 & ~022 ie.

           S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH

How can I copy bytes from one file to another?

Use the versatile dd command. For example, the following command copies 1mb of data from the file /dev/urandom to the file /dev/null. The data is copied as 1024 blocks of blocksize 1024 bytes. dd if=/dev/urandom of=/dev/null bs=1k count=1024 Both the input and output files in the example above are virtual - they don't exist on a disk. Instead they are part of the dev filesystem, which is virtual filesystem provided by the kernel. The virtual file /dev/urandom provides an infinite stream of random bytes, while the virtal file /dev/null ignores all bytes written to it. A common use of /dev/null is to discard the output of a command,

myverboseexecutable > /dev/null

Another commonly used /dev virtual file is /dev/zero which provides an infinite stream of zero bytes. For example, we can benchmark the operating system performance of reading stream zero bytes in the kernel into a process memory and writing the bytes back to the kernel without any disk I/O. Note the throughput (~20GB/s) is strongly dependent on blocksize. For small block sizes the overhead of additional read and write system calls will dominate.

> dd if=/dev/zero of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.0539153 s, 19.9 GB/s

What happens when I touch a file?

The touch executable creates file if it does not exist and also updates the file's last modified time to be the current time. For example, we can make a new private file with the current time:

> umask 077       # all future new files will maskout all r,w,x bits for group and other access
> touch file123   # create a file if it does not exist, and update its modified time
> stat file123
  File: `file123'
  Size: 0         	Blocks: 0          IO Block: 65536  regular empty file
Device: 21h/33d	Inode: 226148      Links: 1
Access: (0600/-rw-------)  Uid: (395606/ angrave)   Gid: (61019/     ews)
Access: 2014-11-12 13:42:06.000000000 -0600
Modify: 2014-11-12 13:42:06.001787000 -0600
Change: 2014-11-12 13:42:06.001787000 -0600

An example use of touch is to force make to recompile a file that is unchanged after modifying the compiler options inside the makefile. Remeber that make is 'lazy' - it will compare the modified time of the source file with the corresponding output file to see if the file needs to be recompiled

touch myprogram.c   # force my source file to be recompiled
make