Files and Directories: ls and touch continued

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Friday is Exam #2. Topics include:
  • Coding style
  • Debugging tools and techniques
  • Bash scripting
  • UNIX programming
     
    
  • ... bring questions to Wednesday's class!
• Today's schedule
  • Collect homework #5
  • Return Short Quiz #8: Range 3-10, Average 6.7
  • Continue UNIX Programming with ls and touch

Collaborative Programming

• On Friday, many students participated in helping to write versions of ls and touch.
• We will continue that today, by finishing touch, and extending the simple ls that we wrote Friday.
• Please feel free to work and discuss today's labwork in teams (of any number of students)

First, recall the utility touch(1)

• What is touch? What does it do?
  
• How does it work?
  
• We started to implement it on Friday. The (incomplete) code that we wrote is here but I recommend starting over, as the current direction is incorrect.
• Your task: implement a correct version of touch(1)

Extending ls

• Now that touch is complete, let's revisit ls and extend it to work like ls -l works.
• The (working) version of ls that was developed in class is here.
• The following slides will help walk you through the necessary steps.
• Your goal, at the end, is to have an equivalent to ls -l.

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_nlink
  • st_mtime
• So we can modify ls to provide -l features using stat() on the filename

How well should your 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 (in a few slides)
• 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, followed by
  • three bits for suid, sgid, and sticky bit, followed by
  • 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 (&) with a value to MASK the rest
   
  
• So we can test permissions, e.g.

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

• Typical masks can be found in </sys/stat.h>

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
• First, search manual pages

Converting UIDs

• From man pages, we learn about /etc/passwd and ypcat passwd
• The getpwuid() function provides access to user information.
• A similar function is available to convert gid information.
  
• Your ls should now be able to generate correct permissions, types, owner, group, etc. just like ls -l