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 ?

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 than I.

#!/usr/bin/python

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 - so 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. If between these two lines, another user replaced 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, so the simple attack described above is impossible.

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?

Todo