Users and Files: who is first

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Reminder of scripting project due Thursday
• Today's schedule
  • Return Short quiz #7, range 4-10, average=6.3
  • Return homework #4, range 4.5-9.5, average=8.5
  • Start Understanding UNIX Programming
    • who
    • cp

Files and Users

• Today we will write a version of the who utility.
• In the process, we will learn about files, users, time, and buffering.

How does who work?

• What does it do?
  • Purpose:
  • Output:
• How does it do it?
• How can we learn about the details?

How does who work?

• What does it do?
  • Purpose: list users currently logged on
  • Output: logname, terminal, time, from where
• We can see what it does by running the command.
• ... and by using the system manual pages.

How does who work?

• How does who do it?
• Where do we start?
  • By reading the man page (and follow the "see also" links)
  • Searching the manual for relevant keywords
  • Read relevant .h files in /usr/include

How does who work?

• Answer (for Solaris): calls library functions that
  • open wtmp (or equivalent) file
  • reads a record
  • closes wtmp
• All who has to do is
  • call library functions
  • display information from structure provided
• Now we can write who.c

Close, but still needs work

• How to we get it to look good?
  • Suppress old entries
  • Format time
• Need to look some more in the manual and header files

Fixes for who2.c

• Suppress blank entries
  • check ut_type (se utmp.h>
• Format time
  • ctime() converts to string
  • Unix stores time as seconds seince the beginning of the epoch
• Result: who2.c

cp (read and write)

% cp sourcefile copyfile

• What does cp do?
  • Answer:
• How does it do so?
  • Answer:

cp (read and write)

% cp sourcefile copyfile

• What does cp do?
  • Answer: creates or truncates a file, then writes data into it
• How does it do so?
  • Answer: let's find out by searching the manual pages