UNIX Processes & Redirection

Last time: Using UNIX

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Collect HW#2
• Why are man and info better than other alternatives?
• Comments about labs?

Processes

• A process is the memory, open files, and other system resources needed while a program runs. The OS (kernel) creates a process when you run a program.
• For example, try:
  io:~% more /etc/termcap (e.g., view a large file with more)
  
  Then, press CTRL-Z to suspend the process.
  [1]+ Stopped more /etc/termcap
  io:~%
  SysV ps has different options than BSD-derived ps. Use whereis to find the other.

Process information

• ps reports information about processes (owner, cpu/memory usage, etc.)
  [1]+ Stopped more /etc/termcap
  io:~% ps
     PID TTY   TIME CMD
    2556 pts/7 0:00 more
    2540 pts/7 0:00 bash
  io:~%

  By default, it only shows the processes associated with the current terminal.

• When installed, the top command can also be used to see what programs are running, their memory and cpu usage, etc.

Managing Processes

• Normally, running a program in a shell will stop the shell while the other program runs.
• To allow both to run, start the program and put it into the background.
io:~% emacs &
io:~%
• Pressing CTRL-Z in a running program will suspend that process.

Managing Processes

• The jobs command will show you what processes are running or stopped from this shell:
  io:~% jobs
  [1]+ Stopped more /etc/termcap
  
• The fg and bg commands move the currently stopped program into the foreground or background (and change status of other jobs currently stopped or running).

Killing Processes

• Sometimes, you get a program that you can't stop with CTRL-C.
• To kill your process that won't otherwise stop, type
  kill <processid>

  or if needed,
  kill -9 <processid>

• The killall command can also be used to kill a process using its program name (instead of process identifier).

Redirection

• One of the great features of UNIX is the ability to connect multiple processes that read from stdin and stdout.
• cat, for example, writes to standard out
• In UNIX, stdin and stdout (and stderr) can be redirected.
  • "ls -l > ls-l.txt" will take the output of "ls -l" and write it to a new file called ls-l.txt
  • "more < file1" will use file1 as the standard input to more
  • "prog1 >> file1" will append prog1 stdout to file1 (not replace)
  • "prog1 2> errorfile" will only write stderr messages to errorfile
  • "prog1 &> bothfile" will write both stdout and stderr to bothfile
  • "sort < numfile > sortednums" redirects both stdin and stdout

Pipes

• Programs can be chained together using "pipes"
  • "cat longfile | wc" sends the output of "cat longfile" to the input of "wc"
  • "cat numfile | sort > sortednums" sends the output of "cat numfile" to the input of "sort", whose output is written to "sortednums"
  (wc is a program that counts the number of lines, words, and characters sent to it.)

Example of Pipes

• A more complex (realistic) example:

  cat airweb-access_log airweb-access_log.2005Jan* | grep -v airweb | 
     cut -d' ' -f11 | sort | uniq -c | sort -nr | more

  takes web server logs with entries such as:

  192.114.107.4 - - [23/Jan/2005:09:20:10 -0500] "GET / HTTP/1.1" 200 3515 
  "http://www2005.org/tutorials/accept.html" "Mozilla/4.0 (compatible; MSIE 
  6.0; Windows NT 5.0; .NET CLR 1.0.3705)"

  and generates reports like:
  

     1410 "-"
       24 "http://www.cse.lehigh.edu/~brian/"
       21 "http://www2005.org/tutorials/"
       18 "http://timconverse.com/blog/"
       17 "http://www2005.org/tutorials/workshops.html"
       16 "http://nike.psu.edu/dbevents/"
        8 "http://www.kdnuggets.com/news/2004/n24/28i.html"
        8 "http://blog.searchenginewatch.com/blog/050121-124259"
  

If sufficient time...

• Continue with mycat today (if time)
  • We can extend mycat to incorporate line numbers (like cat -n)