Files and Directories: ls and touch

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Reminder: Homework 5 online (from book), due Monday
• Monday's class will be in the lab
• Today's schedule
  • Short Quiz #8: Scripting and Programming UNIX Utilities
  • Review of some slides from Wednesday
  • Continue Understanding UNIX Programming
    • ls
    • touch

Writing ls (using same approach)

• So writing ls should be easy...
  • Open directory
  • Repeatedly read directory entry and display entry
  • Close directory
• Much like the who command

Directories

• Q: What is a directory?
• A: A special kind of file that contains a list of files and/or other directories
• Note: Every directory contains special entries . and ..
• Q: Can we use file operators on directories? (e.g., open, read, etc.)
• A: Once, there was no other way...

Reading Directories

• A2: Even if you could read all directories as files, you don't want to -- particularly since UNIX supports a variety of directory formats
  
• Q: So how do we read directories?
  man -k direct
  man -k direct | grep read
• A: opendir(), readdir(), closedir()

Using Directory Functions

#include <dirent.h>

opendir(name)
• Creates a connection, returns a DIR *
readdir(DIR *)
• Gets next record from directory, returns struct dirent *
closedir(DIR *)
• Closes the connection

Writing first version of ls.c

main()
    opendir()
    while ( readdir() )
       print d_name

    closedir()
    return 0

• Let's compile and run the code

Something a little different

• Each student will help write ls, five minutes at a time!

How well does our ls work?

• No columns -- we can fix that, might be interesting
• Not sorted -- solution is to use qsort() on array
• No -a option (to list all files) -- solution is simple
• No -l option -- hmm.... let's come back to that.

First, let's consider the utility touch(1)

• What is touch? What does it do?
  
• How does it work?
  
• Can we implement it? Let's try.

How do we add -l to our ls?

• Q: What does -l display?
• A: modtime, size, owner, group, links, type, permissions
• Q: Where can we learn about these?
• A: As always, search the manual
  
• Result: the stat() system call

How does stat() work?

• int stat(char *name, struct stat *buf)
  
• struct contains
  • st_mode
  • st_uid
  • st_gid
  • st_size
  • st_links
  • st_mtime
• So we can modify ls to provide -l features using stat() on the filename

How well does our ls -l work?

• Perfect: filenames, filesizes
• We can use ctime() to convert the mod time
• Owner and group info are stored as numbers, so we need to map them to names
• What about type? permission?

Where are type and permissions stored?

• st_mode is a 16 bit value. Within it, we have
  • four bits for the file type
  • three bits for suid, sgid, and sticky bit
  • nine bits for owner, group and world rwx permissions
• So we will use subfield coding
  • Much like telephone numbers, IP addresses, etc.

Bits and subfields

• Q: How to examine a bit or subfield?
• A: Use bitwise AND (&) to MASK the rest
  
  
• So we can test permissions, e.g.

  if (st_mode & 004)
     printf("Readable by others\n");
  

UIDs and GIDs

• UIDs and GIDs from stat() are just numbers
• How can we convert them to names?
  
• Then we can finish our ls -l